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School of Natural Resource Sciences Queensland University of Technology MAGMATIC EVOLUTION OF THE SHIRA VOLCANICS, MT KILIMANJARO, TANZANIA By Stephen John Hayes B.App.Sc. (QUT) 2004 Supervisor: Associate Professor David A. Gust Queensland University of Technology A Thesis submitted for the degree of Master of Applied Science (Queensland University of Technology)

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Page 1: MAGMATIC EVOLUTION OF THE SHIRA VOLCANICS, MT KILIMANJARO ...eprints.qut.edu.au/15963/1/Stephen_Hayes_Thesis.pdf · MAGMATIC EVOLUTION OF THE SHIRA VOLCANICS, MT KILIMANJARO, TANZANIA

School of Natural Resource Sciences

Queensland University of Technology

MAGMATIC EVOLUTION OF THE

SHIRA VOLCANICS, MT

KILIMANJARO, TANZANIA

By

Stephen John Hayes

B.App.Sc. (QUT)

2004

Supervisor:

Associate Professor David A. Gust

Queensland University of Technology

A Thesis submitted for the degree of Master of Applied Science

(Queensland University of Technology)

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KEYWORDS

Kilimanjaro, East African Rift, alkalic magmatism, petrogenesis, magma

evolution, fractional crystallisation

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I

ABSTRACT

Mt Kilimanjaro, Africa’s highest mountain (5895m), is a large, young (<1.6Ma)

stratovolcano at the southern end of the East African Rift, in northern Tanzania.

Consisting of three distinct volcanic centres, Shira, Mawenzi and Kibo, Shira

contains the highest proportion of mafic rocks. Shira samples are strongly silica

under-saturated rocks, ranging from picro-basalt, to nephelinite and hawaiite

(Mg numbers (Mg #) ranging from 77.2–35.5). Phenocrysts constitute up to

55% of some samples, and include aluminous augite (often containing

abundant fluid and/or melt inclusions), olivine (Fo92-Fo49), plagioclase (An75-

An42), nepheline (Ne77-Ne68), magnesiochromite and ulvöspinel. Groups

identified on the basis of phenocryst assemblages and textures correlate with

location. East Shira Hill samples contain olivine and clinopyroxene phenocrysts

+ microphenocrysts of plagioclase (Group 1), or plagioclase and clinopyroxene

phenocrysts + microphenocrysts of olivine (Group 2). Samples with high Mg #’s

contain abundant cumulate clinopyroxene and olivine (Fo92-Fo85). Group 3

samples (Shira Ridge) contain nepheline phenocrysts and Group 4 samples

(Platzkegel) have distinct intergranular textures. Chondrite normalised REE

patterns are steep, with light REE-enrichment up to 400x chondrite. Spider

diagrams, normalised to OIB for primitive Shira samples have strong K

depletions and Pb enrichments.

The source of the Shira volcanic rocks is most likely an amphibole-bearing

spinel lherzolite, in which amphibole remains residual. Similarities in spider

diagram patterns and trace element ratios suggest a source similar to average

OIB. The Shira volcanic centre is a polygenetic volcano, in which multiple small

volume, low degree (4-10%) partial melts from a metasomatised subcontinental

lithospheric mantle follow pre-existing structural weaknesses, before ponding in

the lithosphere. Evolution of these small volume melts is dominated by shallow

fractional crystallisation of clinopyroxene, olivine ± spinel, with plagioclase also

fractionating from Group 4 (Platzkegel) samples. A magma mixing origin is

suggested for some samples and supported by complex zonation patterns in

major and trace element chemistry of clinopyroxene phenocrysts as well as

linear mixing arrays. The Shira volcanic centre has since ceased activity, and

collapsed to form the present day Shira Ridge and caldera before being

overlain by various Kibo and parasitic lavas to the east and northwest of the

Shira region.

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II

TABLE OF CONTENTS

INTRODUCTION...................................................................................................................................I

GEOGRAPHIC LOCATION ........................................................................................................................... 2 EAST AFRICA ............................................................................................................................................ 5 KILIMANJARO ........................................................................................................................................... 9 SHIRA...................................................................................................................................................... 11

METHODS ........................................................................................................................................... 13

FIELD INVESTIGATIONS........................................................................................................................... 13 SAMPLE PREPARATION ........................................................................................................................... 13 Petrography, Microprobe and Laser Ablation....................................................................................... 13 Geochemistry / Analytical Techniques ................................................................................................... 15

RESULTS ............................................................................................................................................. 18

PETROGRAPHY ........................................................................................................................................ 18 PHASE CHEMISTRY ................................................................................................................................. 19 Olivine.................................................................................................................................................... 21 Clinopyroxene ........................................................................................................................................ 22 Feldspar ................................................................................................................................................. 22 Spinel...................................................................................................................................................... 24 Feldspathoid........................................................................................................................................... 25 LASER ABLATION RESULTS .................................................................................................................... 26 Olivine.................................................................................................................................................... 26 Clinopyroxene ........................................................................................................................................ 27 Feldspar ................................................................................................................................................. 29 Spinel...................................................................................................................................................... 29 GEOCHEMICAL RESULTS......................................................................................................................... 31

DISCUSSION ....................................................................................................................................... 43

FRACTIONAL CRYSTALLISATION MODELS .............................................................................................. 43 Groups 1 and 2....................................................................................................................................... 48 Group 3 (K813-K820-K825) .................................................................................................................. 53 Group 4 (K361-K897-K894) .................................................................................................................. 53 Summary................................................................................................................................................. 56 CRUSTAL CONTAMINATION / MAGMA MIXING MODELS ........................................................................ 56 PRIMITIVE MAGMAS, MELTING AND SOURCES ....................................................................................... 62 Partial Melting Models .......................................................................................................................... 71 Source Characteristics and Formation .................................................................................................. 76

CONCLUSION..................................................................................................................................... 80

REFERENCE LIST............................................................................................................................. 82

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III

LIST OF FIGURES

Figure 1. East African Rift and location of Mt Kilimanjaro 3 Figure 2. Kilimanjaro regional geology, lava correlation and Shira cross section 4 Figure 3. Active versus passive rifting models 7 Figure 4. Hypothetical East African Rift model 7 Figure 5. Principle igneous centres of the East Africa Rift 8 Figure 6. Kilimanjaro geology, topography and sample locations 10 Figure 7. Shira geology, topography and sample locations 12 Figure 8. Sr and Ba comparisons for ICP-AES and LA-ICP-MS 16 Figure 9. Olivine microprobe results 20 Figure 10. Clinopyroxene microprobe results 22 Figure 11. Feldspar microprobe results 23 Figure 12. Spinel microprobe results 24 Figure 13. Feldspathoid microprobe results 25 Figure 14. Olivine LA-ICP-MS results 27 Figure 15. Clinopyroxene LA-ICP-MS results 28 Figure 16. Feldspar LA-ICP-MS results 30 Figure 17. Spinel LA-ICP-MS results 31 Figure 18. Major element analysis results 38 Figure 19. Trace element analysis results 39 Figure 20. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams 40 Figure 21. Total alkalis silica and silica saturation diagrams 41 Figure 22. Mg number versus CaO/Al2O3 and K2O versus P2O5 42 Figure 23. Normative plots distinguishing groups 42 Figure 24. Fractional crystallisation paths of Shira samples 44 Figure 25. Fractionation vectors produced from the removal of olivine,

clinopyroxene, spinel and plagioclase 45 Figure 26. KSH08-KSH03-K679-KSH02 fractionation model results 50 Figure 27. K2225-K803 fractionation model results 52 Figure 28. KSH01-K802 fractionation model results 52 Figure 29. K813-K820-K825 fractionation model results 54 Figure 30. K361-K897-K894 fractionation model results 55 Figure 31. Zr/Hf and Nb/Ta versus Mg number diagrams 57 Figure 32. Magma mixing model path 58 Figure 33. Backscanned image of KSH05 clinopyroxene 1, with LA-ICP-MS

results showing oscillatory zonation 59

Figure 34. Magma mixing results normalised to KSH11 60 Figure 35. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams for magma mixing model 61 Figure 36. Paths produced from addition of equilibrium olivine 65 Figure 37. Chondrite normalised REE and primitive mantle normalised

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IV

multi-element spider diagrams for equilibrium olivine addition 65

Figure 38. Compilation of primitive samples plotted on CaO versus Mg number 66 Figure 39. Paths produced from addition of clinopyroxene and olivine 67 Figure 40. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams for addition of clinopyroxene and olivine 67 Figure 41. MELTS models of ‘primary’ fractionation corrected magmas

using pressure = 0.5kb, H2O = 0.2% and fO2 = QFM. 69 Figure 42. REE and primitive mantle normalised multi-element spider diagrams

of reverse modal equilibrium batch melting models 73 Figure 43. REE and primitive mantle normalised multi-element spider diagrams

of reverse non-modal equilibrium batch melting models 73 Figure 44. Primitive mantle normalised multi-element spider diagram of forward

modal equilibrium batch melting models 74 Figure 45. OIB normalised multi-element spider diagram of fractionation

corrected samples 76

Figure 46. Model of the genesis and evolution of Mt Kilimanjaro and the Shira region 77

LIST OF TABLES

Table 1. Analytical precision of EDS microprobe results 15 Table 2. Analytical precision of ICP-AES major element results 16 Table 3. Shira volcanic rock group classification 18 Table 4. Samples analysed by EDS microprobe and LA-ICP-MS 18 Table 5. Representative microprobe analyses 19 Table 6. Group 1 geochemical results 34 Table 7. Group 2 geochemical results 35 Table 8. Group 3 geochemical results 36 Table 9. Group 4 geochemical results 37 Table 10. Partition coefficients used in modelling 46 Table 11. Microprobe results used in modelling 47 Table 12. Fractional crystallisation models 49 Table 13. Magma mixing model 60 Table 14. Compositions of ‘primary’ fractionation corrected magmas 68

LIST OF APPENDICES

Appendix A. Fractional crystallisation models 92 Appendix B. Magma mixing model 97 Appendix C. Primary magma compositions 99 Appendix D. Reverse partial melting models 101 Appendix E. Forward partial melting models 103

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V

STATEMENT OF ORIGINAL AUTHORSHIP

The work contained in this thesis has not been previously submitted for a degree or

diploma at any other higher education institution. To the best of my knowledge, this

contains no material previously published or written by another person except where

due reference is made.

Signed:…………………………………..

Date:…………………………………..

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VI

ACKNOWLEDGMENTS

Several people have provided invaluable assistance through the duration of my

project which I would sincerely like to thank. Firstly, I would like to gratefully

acknowledge the time, work, fieldwork assistance and financial assistance (through

numerous jobs) of my supervisor Associate Professor David Gust.

I would also like to acknowledge Dr Michael Carpenter and Dr Sally Gibson

(Cambridge University) for providing access to samples from the Sheffield University

Kilimanjaro rock collection. Furthermore, thankyou to Professor Richard Arculus

(Australian National University) for performing trace element analyses of all

Kilimanjaro samples and for his assistance when I went to Canberra for LA-ICP-MS

analysis.

Thankyou to the QUT technical staff, in particular Bill Kwiecien and Loc Duong, and

to Dave Purdy for showing me the ropes on all the machines at QUT. Thanks must

also go to Franco (Kilimanjaro guide) and our porters for not leaving us stranded on

the mountain or telling the national parks about our “souvenir rocks”, and also to

Luke for his endless supply of music and entertainment.

Finally, special thanks must go to my family, and Therese for their support and

encouragement and for tolerating me over the last two years.

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INTRODUCTION

The petrogenetic modelling of primitive mafic, alkalic rocks provides valuable

information on large parts of the earth’s interior which are otherwise

inaccessible. When combined with geophysical studies, the geochemical

studies of alkalic rocks may hold the key to understanding the composition and

evolution of the Earth’s mantle (Spath et al., 2001). Geochemical and

mineralogical studies yield valuable information concerning the magmatic

evolution and magma chamber dynamics of melts once segregated from their

source. Extensive geochemical and mineralogical studies have been

performed on numerous tectonic settings, including arc volcanics, ocean island

volcanics and continental flood basalts (e.g. MacDonald et al., 2001). However,

the source, production and evolution of large mixed-association, off rift axis

stratovolcanoes remains enigmatic within the studies of continental rifting and

has only recently been addressed (e.g. Spath et al., 2001). This thesis

investigates the geochemistry and mineralogy of the Pliocene to Pleistocene

Shira Volcanics, Mt Kilimanjaro to determine the processes responsible for their

evolution as well as speculate on their source and conditions of partial melting.

Continental rifting, in which voluminous alkalic magmatism is commonly

associated, has been the subject of geochemical investigations for decades

(e.g. Williams, 1970; 1971; Bailey, 1974; Baker, 1987; MacDonald et al., 2001).

Problems addressed by these studies include source region characteristics,

partial melting process, and magmatic evolution with respect to time and space.

Rifting processes are divided into “active” (plume driven) or “passive”

(lithospheric extension driven) (Keen, 1985; Wilson, 1989). In both models,

rising asthenosphere results in decompression melting of the asthenosphere,

metasomatism, and partial melting of the lithosphere due to conductive heating

(Turner et al., 1996). Proposed sources for rift magmas include the

subcontinental lithospheric mantle (McKenzie & Bickel, 1988; White &

McKenzie, 1989; Arndt & Christensen, 1992) and the asthenosphere / upwelling

mantle. The enriched incompatible element signatures observed in alkalic

rocks (e.g. Kay & Gast, 1973; Irving, 1980; Frey & Prinz, 1978; Frey et al.,

1978; Wass, 1980; Kempton et al., 1987) is attributed to either extremely low

degrees of melting (Green & Ringwood, 1967a; Green, 1969; 1973) or slightly

higher degrees of melting of a metasomatised/enriched source (e.g. Frey et al.,

1978; Bailey, 1987; Morris et al., 1987; Spath et al., 2001).

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The East African Rift (EAR) as a classic example of a continental rift, has been

studied for decades (e.g. Gregory, 1921; Willis, 1936; Williams, 1970; 1971;

Girdler, 1972; Rosendahl, 1987; Kampunzu & Mohr, 1991; Morley, 1999;

Morley et al., 1999; Rogers et al., 2000; MacDonald et al., 2001). In recent

times, plume-related models of rifting and rift magmatism dominate EAR

literature with one or more mantle plumes being postulated beneath the Kenya

Rift Valley or nearby Tanzanian Craton (e.g. Karson & Curtis, 1989; Ebinger et

al., 1997; Mechie et al., 1997; Simiyu & Keller, 1997; Rogers et al., 1999; 2000).

Upwelling mantle is believed to be responsible for lithospheric extension,

metasomatism of the lithosphere and partial melts of both the asthenosphere

and subcontinental lithospheric mantle within the EAR.

Upon segregation from their source, melts are subject to numerous magma

chamber processes including fractional crystallisation, assimilation of the

country rock, magma recharge and magma mixing. The role that these

processes play in the evolution of rift stratovolcanoes remains unaddressed,

and when better understood, will contribute to understanding the development

of continental rifts.

Mt Kilimanjaro is a large, young (<1.6Ma) stratovolcano located near the flank

of the propagating end of the EAR. The last significant geological studies on Mt

Kilimanjaro undertaken in 1953 and 1957 by Sheffield University and Geological

Survey of Tanganyika (Tanzania) recognised a wide variety of rocks that range

from strongly alkalic to tholeiitic compositions (Downie & Wilkinson, 1972). Mt

Kilimanjaro is composed of three distinct volcanic centres, Shira, Mawenzi and

Kibo (currently active); Shira contains the highest proportion of alkalic rocks.

Geographic Location

Mt Kilimanjaro is an active stratovolcano located near the eastern flank at the

southern end of the eastern branch of the EAR, in northern Tanzania (Figure 1).

Mt Kilimanjaro forms a shield shape, approximately 95km in length by 65km

width, trending WNW, with the summit, Uhuru peak, in the Kibo region reaching

an elevation of 5895m. The Shira volcanic centre and rocks are situated

between 8km and 40km west and south west of Kibo (Figure 2), and are

geographically made up of several distinct landmarks, including the Shira

Plateau, Shira Ridge, Shira Cathedral, East Shira Hill and Platzkegel (cone

place) lying dominantly between 3000m and 4200m elevation.

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Mt Kilimanjaro

DODOMA

300km

N

Kivu Lake

Mobutu Lake

Tanganyika Lake

Victoria Lake

Rukwa Lake

Malawi Lake

WESTE

RN

BRA

NC

H

EA

STE

RN

BRA

NC

H

ADEN GULF

INDIAN OCEAN

Volcanic Provinces

Lake

Fault

Mt Kilimanjaro - 5895m

NAIROBI

Major City

ADDIS ABABA

ASMARA

DJIBOUTI TOWN

LILONGWE

LUSAKA

Unguja/Zanzibar Island

KIGALI

BUJUMBURA

MOGADISHU

LEGEND

EAST AFRICAN RIFT

See Figures 2 and 6

Tanzanian craton

Figure 1. Location of Mt Kilimanjaro on the East African Rift. Also shown are the

eastern and western branches, major faults, extent of volcanic activity and location

of Tanzanian craton (after Kampunzu & Mohr, 1991).

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GEOLOGY

East Africa

The East African Rift (EAR) originates at the Afar triple junction and forms a

3500km SSW-trending branch from the Red Sea and Gulf of Aden spreading

zones (Figure 1). Rifting began about 45Ma in Ethiopia and has propagated in

a southerly direction at a rate of between 2.5cm/year (Oxburgh & Turcotte,

1974) to 5cm/year (Kampunzu & Mohr, 1991). Rifting occurs in three distinct

pulses (Baker, 1987) with an initial early Eocene (44-38Ma) period followed by

a middle Miocene (16-11Ma) episode, and a final and current rifting in the

Pliocene-Pleistocene (5-0Ma).

This style of rifting reflects an active rifting process (Kampunzu & Mohr, 1991;

Spath et al., 2001), driven by upwelling mantle (Figure 3), rather than a passive

rifting process where differential stresses in the lithosphere result in extension

and mantle plumes. Evidence for active rifting includes synchronous

magmatism with rift initiation, enriched magma sources in the early stages of

rifting, and a decrease in the lithospheric mantle component of the mafic lava

geochemistry (Kampunzu & Mohr, 1991).

The EAR is located above at least two mantle plumes (Rogers et al., 2000).

These plumes include the Afar plume (Kampunzu & Mohr, 1991), and the

southern Kenya Plume (e.g. Mechie et al., 1997; Simiyu & Keller, 1997; Rogers

et al., 1999; 2000). The Kenya Plume has different isotopic and trace element

characteristics to that of Afar (Rogers et al., 2000).

The EAR divides into an eastern and western branch in southern Ethiopia

(Figure 1), with the eastern branch having more profuse volcanic activity

(Kampunzu & Mohr, 1991). The western branch is highly potassic and extends

into Malawi and Mozambique, and the eastern branch is highly sodic. It extends

into northern Tanzania where it terminates into a diffuse (approximately 300km

wide) zone of normal faults. The potassic magmas of the western branch are

believed to be generated at greater depths than those of the eastern branch

(Girdler, 1983; Wilson, 1989) and are thought to be the result of a smaller,

related shoulder plume. The most popular hypothesis for the formation of this

shoulder plume is that the Archaen Tanzanian Craton (located between the

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eastern and western rift branches) has deflected a portion of the rising

asthenosphere (Kampunzu & Mohr, 1991; Zeyen et al., 1997; Winter, 2001).

EAR magmatism is diverse with ultra-alkalic / carbonatitic, alkalic, transitional

and tholeiitic suites all identified (Kampunzu & Mohr, 1991). Two contrasting

views of the evolution of magmatism in rift zones are proposed (Kampunzu &

Mohr, 1991). The first suggests that the diverse range of rock suites show a

progressive decrease in alkalinity with rift development, from ultra-alkalic

magmas associated with pre-rift regional uplift, to alkalic magmatism associated

with graben development, followed by tholeiitic magmatism upon initiation of

seafloor spreading (Gass, 1970; 1972; Baker et al., 1978, Lippard & Truckle,

1978; Baker, 1987). The second view is that magma alkalinity does not

significantly evolve, and continental and oceanic rifting magmatism can not be

correlated (Le Bas, 1971; Bailey, 1974). The complexity of correlating lava

flows and the close temporal and spatial association of alkalic, transitional and

tholeiitic rocks makes it extremely difficult to distinguish between these

hypotheses.

Transitional and tholeiitic rocks have occurred before, during and after rifting

within the EAR, and can predate, postdate or occur concurrently with alkalic

rocks of the same region (Kampunzu & Mohr, 1991). This indicates that

structural setting exerts an important, though not always predictable control on

magma composition within the EAR (Gass, 1970; Mohr, 1970). Volcanic

products have been shown to vary transversely across rifts, with the magmas

erupted on the flanks tending to be more alkalic and less voluminous than those

lavas erupted in the axial graben (Wilson, 1989). This variation probably results

from differing degrees of melting due to depth of magma production increasing

with distance from the axial graben.

Individual eruptive centres, occurring predominatly on the flanks and

propagating end of the EAR produce either mixtures of rock suites, or only one

rock suite (Figure 5). This off-rift volcanism can be explained as the result of

individual mantle plumes (e.g. Burke, 1996) or diapirs of plume material

deflected from the main plume along pre-existing structures (e.g. Bosworth,

1987, 1989; Mechie et al., 1997; Ritter & Kaspar, 1997; Spath et al., 2000;).

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Figure 3.

A)

B)

Active vs. Passive rifting models for continental rifting (Wilson, 1989 after Keen, 1985).

Active rifting, whereby mantle upwelling has caused lithospheric extension and regional uplift,

compared with Passive rifting where differential stresses in the lithosphere have resulted in

lithospheric extension, causing the mantle to plume in the area of thinned crust.

Figure 4. Hypothetical cross section (no vertical exaggeration) showing a proposed model for the current

stage of development of the East African rift system. This is the intermediate stage between initial

asthensopheric diapir rising and sea floor spreading (asthenospheic material reaching crustal levels).

Decompression melting results from the ascent of an asthenospheric diapir, which in turn can cause

metasomatism of the sub-continental lithospheric mantle (SCLM), and partial melting resulting in variably

alkaline melts. The reversed decollement (D1) provides room for the rising asthenosphere which can in

turn result in crustal anatexis. Eruption of alkaline lavas, mostly from a deep asthenospheric source fills

the rift valley with volcanic and volcaniclastic material. (Winter, 2001 after Kampunzu & Mohr, 1991)

grasso
This figure is not available online. Please consult the hardcopy thesis available from the QUT library
grasso
This image is not available online. Please consult the hardcopy thesis available from the QUT library
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0km 100km

Mixed association central volcanoes

Nephelinite-phonolite central volcanoes

Basalt-trachyte, trachyte-rhyolite and trachyphonolite shields

Quaternary basalts

Volcanic province

Ngorongoro Crater

Lake

Mt Kenya

Mt KilimanjaroNgorognoro

Oldoinyo Lengai

Mt Meru

Chyulu Hills

Napak

Kadam

Elgon

Moroto

Yelele

LakeVictoria

Emuruangogolak

Silali

Paka

Menengai

LongonotSuswa

OlorgesailieOlesakut

LakeTurkana

34° 35° 37° 38°36°

-1°

-2°

-3°Shira

KiboMawenzi

Figure 5. A map showing the distribution, alignment and eruption types of the principle

igneous centres of the East African Rift (after Kampunzu & Mohr, 1991 and Baker, 1987).

Note that there are a variety of rocks erupted, with some centers producing only one rock

suite, whilst others produce a mix.

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Kilimanjaro

Mt Kilimanjaro forms a shield approximately 96km in length by 64km width, with

the long axis trending WNW. The summit “Kibo” is the only volcanic centre

currently regarded as active, and reaches an elevation of 5895m (Uhuru Peak)

at the coordinates 3°05’S, 37°20’E (Figure 6).

Magmatic activity of Mt Kilimanjaro began in the lower Pleistocene, with several

eruptive centres creating a mix of alkalic, transitional, tholeiitic and pyroclastic

rock suites (Figures 2 & 6). Initial volcanic activity produced olivine basalts of

the Ol Molog, Kibongoto and Kilema regions (Figure 2a) approximately 1 million

years ago (Downie & Wilkinson, 1972). Faulting controlled the location of

magmatic activity, building a low complex shield. In the lower Pleistocene,

activity became focused at three main volcanic centres (Kibo, Shira and

Mawenzi). Initially, all three centres operated simultaneously producing basalts

of similar composition. Towards the end of the lower Pleistocene these centres

developed their individual characteristics. Shira produced silica-undersaturated

lavas, ankaramites and nephelinites followed by strongly silica under-saturated

lavas, ijolites and associated lavas from a smaller unknown centre to the east of

Shira. Mawenzi lavas changed from basalts to trachybasalts to trachyandesite,

with activity moving from the Neumann Tower to the main Mawenzi centre

before becoming extinct. The activity of Kibo is similar to that of Mawenzi, with

the production of trachyandesites long after the cessation of the Mawenzi

volcanic centre (Downie & Wilkinson, 1972). The final stage of Mt Kilimanjaro’s

evolution involved the production of aegerine phonolite flows, and creation of

the present caldera and ash pit. Kilimanjaro has remained dormant through the

Holocene, with only fumarolic activity taking place (Downie & Wilkinson, 1972).

Petrographic studies on Kilimanjaro by Abdullah (1963), Saggerson (1964),

Wilkinson and Downie (1965), Wilkinson (1967), Sahu (1969), Williams (1969),

and Downie and Wilkinson (1972) result in a correlation of the many lavas of

Kilimanjaro (Figure 2c).

Glaciation occurred episodically throughout the late Pleistocene and Holocene,

between periods of volcanic activity. The current glaciers of Kilimanjaro are

rapidily disappearing, exposing many previously unseen rock surfaces

(Hastenrath & Greischar, 1997; Irion, 2001).

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Shira

Shira consists of many distinct landmarks including the Shira Ridge, Platzkegel

(German for ‘cone place’), East Shira Hill, Shira Cathedral and the Shira

Plateau (Figure 7). Flows from parasitic cones obscure the Shira lavas to the

north and south, whilst vegetation covers a great deal of the flanks below the

ridge and plateau (Downie & Wilkinson, 1972). The distinct Shira Ridge

(Figures 2b & 7 resulted from a caldera collapse (Wilcockson, 1956; Downie &

Wilkinson, 1972). The lavas on the western and southern slopes dip radially

outwards, from about 20° on the upper slopes to 2° to 3° on the lower slopes

and cover the lava units of Ol Molog in the north and Kibongoto in the south.

Reconstruction of the Shira volcano suggests it may have once reached a

height of 5400m (Downie & Wilkinson, 1972).

The geology of Shira (Figure 7) is described by Downie and Wilkinson (1972);

they conclude that the petrogenesis of its magmas reflect significant fractional

crystallisation of ferromagnesian minerals. Shira volcanic units are not dated,

however they are older than the Upper Rectangle Porphyry group of Kibo (Nvq2

– Figure 7), a unit that partially covers the degraded Shira crater.

Shira contains the most primitive and alkalic rocks of the Mt Kilimanjaro region.

The rocks are mainly mafic, silica-undersaturated lavas with considerable

amounts of pyroclastic material. Shira rocks include olivine basalt, trachybasalt,

trachyandesite, ankaramite, basanite, nephelinite, agglomerate and augite-

bearing tuff (Downie & Wilkinson, 1972). A 480m thick section, measured from

just below the Shira Ridge upwards identified 3 distinct groups. These are the

upper trachybasalt group (Nvd2 on Figures 6 & 7) ultramafite and

melanephelinite group (Nvu on Figures 6 & 7;inner face of the Shira Ridge) and

lower trachybasalt group (Nvd1 on Figures 6 & 7). The upper trachybasalt

groups (upper ridges and western escarpment) consists of trachybasalt with

large platy feldspar phenocrysts, the ultramafite and melanephelinite group

consists of large augite crystal tuffs, ankaramite, basanite and melanephelinite,

and the lower trachybasalt group (southern ridge and the upper slopes of

Shira) is comprised of trachybasalt with small platy feldspars.

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With a relief of approximately 240m, Platzkegel rises from the centre of the

Shira Plateau. Eruptive products from the Platzkegel vent consist of

agglomerates with analcime-basalt fragments in a tuffaceous matrix, and thin

basalt flows (Downie & Wilkinson, 1972). Platzkegel has been intruded by

various dolerite, analcime, syenite and essexite intrusions penetrating along

NNE-SSW fissures. Many other Shira dykes form a radial swarm focusing on

Platzkegel. These dykes are more or less vertical, ranging in thickness from

0.5 to 1m, and are generally of similar composition to the trachyandesite and

trachybasalt lavas with a few dykes representative of the basalts, ankaramites,

atlantites and melatrachybasalts. The density of dykes appears to decrease

with increasing distance from the crater. A number of inclined dykes dipping

outwards at about 45° intrude into the flanks. They are approximately 100m

apart, 1 to 1.5m thick, and are composed of the equivalents of the trachybasalt

and melatrachybasalt lavas (Downie & Wilkinson, 1972).

A nephelinite centre (Nvn – Figure 7) occurs approximately 3.5km northwest of

Platzkegel, and post dates the agglomerate and dykes.

METHODS

Field Investigations

Eleven samples were collected from Mt Kilimanjaro, near the East Shira Hill and

Shira Cathedral. Five samples were collected from a 20m vertical exposure

capped by pyroclastics. Thirty seven samples were obtained from Cambridge

University, and came from the 1953 and 1957 joint surveys of the Geological

Survey of Tanganyika (Tanzania) and Sheffield University. This sample set

includes 26 samples large enough for geochemical analysis, and 11 samples of

sufficient size for the creation of polished sections. Sample locations are shown

in Figures 6 and 7.

Sample Preparation

Petrography, Microprobe and Laser Ablation

Polished sections were made of all collected samples for petrographic,

microprobe and laser ablation ICP-MS analysis. Microprobe analyses were

undertaken at the QUT Analytical Electron Microscopy Facility using a JEOL

JXA-840A Scanning Electron Microprobe with an Energy-Dispersive

Spectrometry (EDS) detector. Operating conditions for the quantitative

grasso
grasso
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determination of mineral chemistry were: 15kV accelerating voltage, beam

current of ~3nA, count time of 100 seconds, 38mm working distance, 40° take

off angle for the EDS detector and a focused 5-10 µm diameter beam.

Calibration was performed using pure copper from the Astimex Scientific

MINM25-53 standard mineral mount, with a standards file based on albite for

Na, olivine for Mg and Si, plagioclase for Al, apatite for P, sanidine for K,

diopside for Ca, rutile for Ti, chromium for Cr, rhodonite for Mn, aluminium

garnet for Fe, nickel silicide for Ni and cobalt for Co. EDS spectra were

collected and interpreted through Moran Scientific quantitative EDS software.

13 samples were analysed, with phenocrysts probed between 2 and 12 times

from core to rim, dependant upon size and whether zonation was apparent in

backscattered images. Three groundmass analyses of each phase were also

determined. Table 1 shows comparisons between analysed and accepted

values for several relevant mineral standards. The maximum deviation from

accepted values is approximately five percent (relative) with most elements

being determined within two percent (relative).

Microprobe analyses are recalculated as a proportion of end member

compositions for olivine, clinopyroxene, feldspar, nepheline, and spinel. Olivine

microprobe analyses are calculated as a percentage of forsterite whereas

clinopyroxenes are calculated as a percentage of both enstatite (En) - ferrosilite

(Fe) - wollastonite (Wo) and Ti-Aliv-NaM2 (e.g. Kempton et al., 1987). Pyroxene

En-Fe-Wo calculations used PX-NOM, a pyroxene spreadsheet calculator

(Sturm, 2002), based on the classification schemes of the International

Mineralogical Association. Fe3+ values were determined using the methods of

Droop (1987). Plagioclase compositions are presented as percentage

anorthite-albite-orthoclase (An-Ab-Or), with nepheline cast as a percentage of

nepheline-kalsilite-silica (Ne-Ks-Q) (Deer, Howie & Zussman, 1992).

Mg/Mg+Fe2+ and Cr/Cr+Al for spinel analyses followed methods of Kempton et

al. (1987).

Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) analysis of

trace elements (Sc, V, Cr, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Nd, Sm, Eu,

Gd, Dy, Er, Yb, Lu, Hf, Ta, Pb, Th and U), Ca, Si and Al were performed at The

Australian National University (ANU) on an ArF (193nm) EXCIMER laser and a

Fisons PQ2 STE ICPMS. Full instrument details are outlined in Eggins and

Shelley (2003). A laser diameter of 71µm was used, with repetition rate of 5Hz.

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Counting time was set at 60 seconds, including 20 seconds of background data

collection. Instrument calibrations were performed after approximately 12

analyses on internal glass standard NIST 612 to account for drift. Analyses

were performed on 5 samples (KSH05, K2225, KSH01, KSH03 and K811), on

phenocryst points previously analysed through EDS in order to reduce the data

and gain quantitative results. Ca was used to reduce analyses of

clinopyroxene, Si for plagioclase and olivine, and Al for spinel.

Table 1. Comparison between EDS analysed and accepted compositions of

the Astimex Scientific MINM 25-53 mineral mount for a range of minerals

comparable to Shira samples.

Albite Albite Plagioclase Plagioclase Diopside Diopside Cr Diopside Cr Diopside Olivine Olivine Chromite Chromite

(analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted)SiO2 67.5 68.52 52.25 54.21 55.41 55.36 54.19 55.13 41.73 41.84 0.25Al2O3 19.46 19.54 28.64 28.53 0.32 0.09 0.27 0.08 12.61 13.79FeO 0.01 0.3 0.37 0.13 0.05 1.25 1.21 7.34 6.51 18.4 17.5MnO 0.03 0.05 0.03 0.12 0.34MgO 0.01 0.13 19.17 18.62 17.7 17.46 51.64 51.57 12.23 13.6CaO 0.06 0.13 12.37 11.8 26.93 25.73 25.48 25.55 0.01Na2O 11.02 11.59 4.43 4.35K2O 0.21 0.22 0.4 0.41 0.1 0.07 0.06TiO2 0.05 0.03 0.07 0.08 0.06 0.05 0.31 0.15P2O5 0.14 0.19Cr2O3 0.05 0.07 0.11 0.58 0.58 54.28 54.4NiO 0.09 0.03 0.11 0.35 0.14CoO 0.03 0.19 0 0.03 0.05 0.2 0.02Total 98.49 100 98.71 99.87 102.45 99.98 99.82 99.98 101.28 100.24 98.59 99.44

Geochemistry / Analytical Techniques

Samples were prepared for chemical analysis at the University of Queensland

(UQ) sample preparation laboratory. Samples were washed, crushed using a

hardened steel jaw crusher and dried overnight on a hot plate. Rock chips were

crushed for 15 minutes in an agate mill with approximately 70-120g of powder

produced.

Major elements (Si, Ti, Al, Fe, Mn, Ca, Na, K and P) and two trace elements

(Ba and Sr) were determined by Inductively Coupled Atomic Emission

Spectrometry (ICP-AES) at the QUT School of Natural Resource Science

Geochemical Analytical Facility using a Varian Liberty – 200 ICP-AES.

Solutions were prepared using the methods of Kwiecien (1993), involving

hydrofluoric acid digestion and dilution to 200ml. H2O+, CO2 and S were

determined as loss on ignition (LOI) for each sample by heating samples to

950ºC over a period of 5 hours, then maintaining this temperature for a period

of 15 minutes.

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Samples were run manually in batches of 10, including at least 1 blank and 4

calibration standards, along with USGS standards Nim-L and Nim-S for

comparison (Table 2). Calibration standards used were internal standards QUT

353, 446, 1552, 2769 and 161 which are referenced to USGS standards W1,

GSP1, AGV1, BCR1 and G2. Correlation coefficients were all extremely close

to 1, and the USGS standard values correlated extremely well with accepted

values for all elements.

Table 2. Comparison of ICP-AES analysed and accepted USGS standards

Nim-L and Nim-S. Sample Nim-L (analysed) Nim-L (accepted) Nim-S (analysed) Nim-S (accepted)SiO2 52.18 52.40 64.77 63.63Al2O3 12.98 13.64 16.78 17.34Fe2O3 (total) 9.93 9.96 1.46 1.40Fe2O3 8.74 1.07FeO 1.13 0.30MnO 0.76 0.77 0.01 0.01MgO 0.25 0.28 0.45 0.46CaO 2.57 3.22 0.72 0.68Na2O 7.97 8.37 0.43 0.43K2O 5.38 5.51 15.47 15.35TiO2 0.49 0.48 0.04 0.04P2O5 0.04 0.06 0.12 0.12LOI 3.72 2.48 0.31 0.31Total 96.26 97.17 100.56 99.77

Sr(ppm) 4396.00 4600.00 65.74 62.00Ba(ppm) 307.40 450.00 2550.00 2400.00

Trace element analyses were performed by Professor Richard Arculus (ANU).

Glass discs were prepared by fusing 0.5 grams of powdered sample with 1.5

grams of Li-borate flux for 15 minutes at 1190˚C. All trace element

concentrations (Sc, V, Cr, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Nd, Sm, Eu,

Gd, Dy, Er, Yb, Lu, Hf, Ta, Pb, Th and U) were determined on the glass discs

by Laser Ablation, Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-

MS) at the Research School of Earth Sciences, ANU. The LA-ICP-MS employs

an ArF (193nm) EXCIMER laser and a Fisons PQ2 STE ICPMS. Full instrument

details are outlined in Eggins and Shelley (2003). Analyses were performed on

Li-borate fusion discs using a spot size of 100µm and a repetition rate of 5Hz.

Counting time was set at 70 seconds. Instrument calibration was against NIST

612 glass and background analysis time was 30 seconds. 43Ca was employed

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17

as the internal standard isotope, based on CaO concentrations previously

measured by ICP-AES.

Sr and Ba results from LA-ICP-MS correlate well with ICP-AES results

indicating consistency between both techniques (Figure 8).

0 1000 20000

1000

2000

Ba

(ppm

)-

LA

-IC

P-M

S

Ba (ppm) - ICP-AES

0 1000 2000 3000 40000

1000

2000

3000

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(pp

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CP

-MS

Sr (ppm) - ICP-AES

Figure 8. Sr and Ba comparisons for ICP-AES and LA-ICP-MS analyses showing a close

correlation.

grad

ient

=1

gradient =

1

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18

RESULTS

Petrography

Petrographic examination of Kilimanjaro rocks show that they are relatively

fresh to slightly altered, microporphyritic to porphyritic rocks with varying

vesicularity. Phenocrysts of predominantly clinopyroxene, with lesser amounts

of olivine, plagioclase, nepheline and spinel, constitute up to 55 modal percent

in some samples. Shira samples are split into four groups based on

phenocryst assemblages (Table 3), with the presence of feldspar and/or

feldspathoid phenocrysts used to distinguish between Groups 1, 2 and 3;

Platzkegel samples (Group 4) are distinguished by their distinctive intergranular

textures. Results are discussed with respect to the following groups:

Group 1: Olivine and clinopyroxene phenocrysts ± microphenocrysts of

feldspar.

Group 2: Clinopyroxene and feldspar ± olivine phenocrysts.

Group 3: Feldspathoid, clinopyroxene and feldspar phenocrysts ±

microphenocrysts of olivine.

Group 4: Platzkegel samples.

Group 1 samples are similar to Group 2, which is distinguished by larger

feldspar phenocrysts and fewer olivine phenocrysts. Olivine phenocrysts in

Group 1 samples often contain magnesiochromite inclusions (absent in all other

groups), and lack ulvöspinel phenocrysts common to Group 2 samples.

Complex zonation patterns are apparent in many large clinopyroxene

phenocrysts in Groups 1, 2 and 3 with abundant melt, apatite, spinel and olivine

inclusions found in these phenocrysts. Smaller clinopyroxene phenocrysts and

olivines are generally normally zoned or unzoned and contain far fewer

inclusions (occasional magnetite speckles or feldspar) than the larger samples.

Some Group 2 clinopyroxenes are sector zoned and often occur in

glomerocrysts. Plagioclase phenocrysts occur predominantly in Group 2

samples and are extremely variable. In general the larger phenocrysts are

complexly zoned, whilst the smaller phenocrysts are normally zoned or

unzoned and occur as glomerocrysts or as individual crystals aligned in a

trachytic texture. Large nepheline and clinopyroxene phenocrysts are

prominent in Group 3 samples, with olivine appearing solely as

microphenocrysts. Group 4 samples are much more equigranular and contain

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19

distinctive intergranular textures. Groundmasses in Groups 1 to 3 are

predominantly cryptocrystalline to microcrystaline and are speckled with

titaniferous magnetite. Group 4 groundmasses are fine to medium grained and

composed of clinopyroxene, plagioclase, nepheline, titaniferous magnetite,

magnetite and in some samples, olivine, interstitial biotite and minor apatite.

Phase Chemistry

Samples for EDS microanalysis were taken from each group (Table 4).

Representative clinopyroxene and spinel results for EDS microanalysis are

presented in Table 5.

Table 3. Shira volcanic rocks and classification into four groups.

Group 1 Group 2 Group 3 Group 4KSH03 KSH01 K686 K361KSH05 KSH02 K689 K832aKSH07 KSH04 K811 K832bKSH08 KSH06 K813 K894KSH09 KSH10 K820 K897K693 KSH11 K825 K1043K2225 K679 K829

K695 K1039K696K802K803K804K821K822K895K891K1038

Table 4. Samples analysed by EDS microprobe and LA-ICP-MS

Samples for EDS Analysis Samples for LA-ICP-MS AnalysisSample Group Sample GroupKSH03 1 KSH05 1KSH05 1 K2225 1KSH08 1 KSH03 1KSH09 1 KSH01 2K2225 1 K811 3KSH01 2KSH04 2KSH06 2K802 2K811 3K820 3K361 4K894 4

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20

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a(m

2)

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g/(

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+F

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94.4

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e2O

3**

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g/(

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+F

e2

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r/(C

r+A

l)69.5

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om

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t1.8

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*=

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and

Fe

adju

ste

dsto

ichio

metr

ically

thro

ugh

the

meth

ods

of

Dro

op

(1987)

2+

3+

**=

Fe

an

dF

eadju

ste

dsto

ichio

metr

ically

2+

3+

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21

Olivine

Olivine phenocrysts occur in all groups and are compositionally homogenous or

normally zoned. Olivine phenocrysts from Group 1 samples have higher

forsterite contents than those from other groups; phenocryst cores (Fo85-92) are

compositionally homogenous, with thin rims that have dramatically lower

forsterite content (Fo77-80). Group 2 samples vary from Fo75 cores to ~ Fo40 rims.

Olivine phenocrysts from Group 3 and 4 samples are more homogenous (Fo65

to Fo60 and Fo55 to Fo48, respectively). Compositions of groundmass olivine

varies, but is always less than the least forsteritic phenocryst rim composition;

groundmass olivine in Group 3 is considerably lower than the rim compositions

of the phenocrysts. Olivine phenocryst compositions are less than

compositions calculated to be in equilibrium with the bulk rock (Figure 9), with

groundmass analyses considerably lower.

30 40 50 60 70 80

10

30

50

70

90

Fo

%

Mg number of rock

Figure 9. Comparison of forsterite values of phenocryst and groundmass olivines withMg number calculated from bulk rock analysis. The dashed line represents the olivinecomposition in equilibrium with the bulk rock using K = 0.3. (arrows show typical

Fo change from core to rim where applicable).D(Ol/Liq)

Fe/Mg

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass Group 3 Groundmass

Group 3 Phenocryst Group 4 Phenocryst

Group 4 Groundmass

rim

-co

re

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22

Clinopyroxene

Clinopyroxene phenocrysts commonly occur in all four petrographic groups.

Phenocrysts range from large subhedral crystals with resorption rims,

numerous melt, apatite, olivine and spinel inclusions and complex zonation

patterns to small, unzoned or normally zoned euhedral crystals with few

inclusions. Most clinopyroxenes are aluminium diopsides (Figure 10a).

Phenocryst cores contain significant amounts of Cr2O3 (eg. Table 5, KSH05

Cpx 4a (0.93 weight percent)); TiO2 contents reach 4.6 weight percent in some

rim / groundmass analyses. The majority of samples that plot in the “others”

quad (Figure 10b) are rim or groundmass analyses. According to the

boundaries defined by Aoki and Kushiro (1968) on an octahedral aluminium

(AlM1) versus tetrahedral aluminium (AlT) plot (Figure 10c), all clinopyroxene are

of low pressure origin.

Group 1 samples vary from chromian aluminium augite to ferrian sub-silic

aluminium wollastonite. Groundmass analyses dominantly plot towards the

diopside/wollastonite end of this band. Group 3 and 4 samples show similar

trends however span much smaller compositional bands, whilst Group 2

phenocrysts and groundmass compositions overlap.

Feldspar

Feldspar phenocrysts are well developed only in Group 2 samples. These

phenocrysts are sub- to euhedral, coarse to very fine-grained with a variety of

zonation patterns. Most phenocrysts are unzoned or normally zoned; An

content varies from An70 to An45. Some larger Group 2 phenocrysts show

oscillatory or reverse zonation. Group 1 and 3 samples contain sub- to

euhedral micro phenocrysts that are normally zoned from An80 to An60 and An70

to An50, respectively. Groundmass plagioclase generally overlaps phenocryst

rim compositions and extends to lower An contents. Group 2 samples contain

sanidine in the groundmass.

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23

NaM2

AlT

CaTSJD

NaTi TiAlNaTiAl

Ti

enstatite ferrosilite

Hedenbergitediopside hedenbergite

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass Group 3 Groundmass

Group 3 Phenocryst Group 4 Phenocryst

Group 4 Groundmass

Figure 10. A) Microprobe analyses of phenocryst and groundmass clinopyroxenesfrom Shira samples presented in the Mg-Ca-Fe (enstatite-wollastonite-ferrosilite) triangle(arrows indicate general trend from core to rim, circles indicate groundmass compositionregions) and

.

Samples have been split into the four petrographic groups based on phenocryst assemblagesas discussed in the text. C) Plot of octahedral aluminium (Al ) versus tetrahedral aluminium(Al ) in clinopyroxenes, and the pressure fields of Aoki and Kushiro (1968).

M1

T

B) “others” quadrilateral (JD = jadeite, CaTS = Ca-Tschermaks, TiAl = Ti-Alaugite, NaTiAl = Na-Ti-Al augite, NaTi = Na-Ti augite (Ti end member is “fictive” CaTiAl O )2 6

0.0 0.1 0.2 0.3 0.4 0.50.0

0.1

0.2

0.3

0.4

AlM

1

AlT

Intermediate Pressure

High Pressure

Low Pressure

A)

B)

wollastonite

C)

core - rimtrend

groundmasssamples

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24

Ab An

Or

Albite Oligoclase Andesine Labradorite Bytownite Anorthite

Anorthocl

ase

Sanid

ine

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Figure 11. Microprobe analyses of phenocryst and groundmass feldspars from Shira samplesplotted as proportion anorthite-albite-orthoclase (An-Ab-Or), with arrow showing the general trendfrom core to rim, and circles showing the groundmass composition regions.

core

Spinel

Analyses of spinels are separated into inclusion, phenocryst and groundmass

phases. Inclusions occur dominantly in Group 1 olivine (one inclusion was

found in a Group 4 olivine). These spinel inclusions are dominantly

magnesiochromite spinels (Figure 12). Spinel phenocrysts and groundmass

phases in all groups are similar in composition, being dominantly titaniferous

magnetites.

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25

Figure 12. Microprobe analyses of phenocrysts (including inclusions) and groundmassspinels of Shira samples. Samples have been plotted using Mg numbers (Mg/(Mg + Fe ))and Cr numbers (Cr/(Cr+Al)). Groundmass samples plot very close to the lower left cornerof each diagram or not at all due to 0% Mg or Cr (magnetite / titaniferous magnetite).

2+

Cr/

Cr+

Al

Mg/Mg+Fe2+

00 10 20 30 40 50 60 700

20

40

60

80

10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

0 10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

0 10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

inclusions

inclusion

Group 1 Inclusion

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Feldspathoid

Nepheline phenocrysts occur in Group 3 and 4 samples; groundmass nepheline

is also present in these as well as in one Group 2 sample (K802). Nepheline

compositions of Group 3 samples have higher nepheline components (Ne70-80)

than Group 4 samples (Ne60-65) (Figure 13). Groundmass and phenocryst

nepheline compositions overlap.

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26

50 60 70 800

10

20

Ks

Ne

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Figure 13. Nepheline phenocryst and groundmass analyses from Shira samples plotted as

percent nepheline (Ne) versus kalsilite (Ks).

Laser Ablation Results

Samples for LA-ICP-MS (Table 4) were all analysed by EDS prior to LA-ICP-MS

analysis in order to reduce data and gain quantitative results. Results are

presented with respect to phenocryst type, and their respective sample names.

Olivine

Olivine shows consistent trace element concentrations for almost all core to rim

traverses (Figure 14), despite a significant decrease in forsterite content at the

phenocryst rim. KSH05 and K2225 (Group 1) olivine phenocrysts have similar

forsterite contents (Fo80-90) and very similar trace element concentrations (Ni ~

2000ppm, Cr ~ 250-400ppm, V ~ 5ppm and Mn ~ 1500ppm). KSH03 (Group 1)

olivine has slightly lower forsterite content (Fo75-82), and significantly lower Ni (~

1000ppm) and Cr (~ 50ppm), but higher V (~ 7-9ppm) and Mn (~ 2500ppm).

KSH01 (Group 2) olivine has the lowest forsterite contents (Fo72-55) and much

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27

lower Ni (178-379ppm) and Cr (1.8-20ppm), yet the highest V (10-33.16ppm)

and Mn (4500-7000ppm) concentrations of all groups.

Clinopyroxene

Traverses of clinopyroxene phenocrysts yield a range of zonation patterns from

relatively unzoned to complex and oscillatory zoned. Results are presented as

a series of chondrite-normalised (Sun & McDonough, 1989) REE diagrams,

with an inset diagram of Mg number (Mg #) and Sc variation from core to rim for

comparison (Figure 15). REE diagrams show smooth enriched curves,

increasing in degree of enrichment from La to Nd, then decreasing from Nd to

Lu. Many samples also show slight positive Gd anomalies. In general, Mg #

and Sc concentrations show opposing trends (increasing Mg # versus

decreasing Sc); low Mg #’s and high Sc concentrations correlate with greater

REE enrichment. Many samples show distinct steps in REE enrichment (i.e.

Figure 15, KSH05 cpx 2, 4 and 5) in which there is a drastic increase in REE

enrichment over a small increase in distance from the phenocryst core.

REE contents in individual clinopyroxene phenocrysts can vary by up to a factor

of 10, however in most cases, variation is restricted to a factor of ~3. The

overall degree of REE enrichment in clinopyroxene increases from Group 1

(samples KSH05, KSH03 and K2225) to Group 2 (KSH01) and Group 3 (K811).

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28

Sc V Cr Mn Ni Fo%

Figure 14. Results of LA-ICP-MS core to rim analyses of 3 olivine phenocrysts from

each sample KSH05, KSH03, K2225 and KSH01 showing the variation of Sc, V, Cr,

Mn and Ni, along with the EDS determined forsterite content.

1.00

10.00

100.00

1000.00

10000.00

KSH05 Olivine 1

Ol1a Ol1b Ol1c Ol1d Ol1e Ol2a Ol2b Ol2c Ol2d Ol2e Ol2f Ol3a Ol3b Ol3c Ol3d Ol3e

KSH03 Olivine 1

Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c

K2225 Olivine 1

Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c

KSH01 Olivine 1

Ol1a Ol1b Ol1c Ol4a Ol4b Ol4c Ol5a Ol5b

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

1.00

10.00

100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

1.00

10.00

100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

1.00

10.00

100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

KSH05 Olivine 2 KSH05 Olivine 3

KSH03 Olivine 2 KSH03 Olivine 4

K2225 Olivine 2 K2225 Olivine 4

KSH01 Olivine 4 KSH01 Olivine 5

Ab

un

dan

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)

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29

Feldspar

Three plagioclase phenocrysts from sample KSH01 were analysed, and are

presented in order of decreasing anorthite content (determined by EDS

microprobe analyses) (Figure 16a). Only minor changes in trace element

concentrations were noted between each analysis. Chondrite normalised (Sun

& McDonough, 1989) REE diagrams (Figure 16b) show decreases in degree of

enrichment from La (42 x chondrite) to Yb (0.14 x chondrite), with distinct

positive Eu anomalies (up to 22 x chondrite). Degree of enrichment increases

with decreasing An %.

Spinel

Due to the small size of spinel phenocrysts (samples KSH01 (Group 2) and

K811 (Group 3)) and inclusions (sample KSH05 (Group1)), traverses were

unable to be conducted. Analyses have instead been plotted against

decreasing Mg #’s as determined through EDS microprobe analyses in order to

show trace element variations (Figure 17). Distinct changes are observed with

the most notable being decreases in Cr and Ni, yet increases in Ti, Mn, V, Zr

and Nb. Zr/Nb is lowest in K811 analyses (0.56-0.61), and increases in KSH05

(0.93-1.33) and KSH01 (1.71).

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30

KSH01 Plagioclase LA-ICP-MS Results

1.00

10.00

100.00

1000.00

10000.00

Pl7aPl7bPl1a

Analyses

Lo

g(p

pm

)

40.00

60.00

80.00

An

ort

hit

e%

(ED

S)

Ti Mn Ga Sr Ce An%

Figure 16. A) LA-ICP-MS analyses of Ti, Mn, Ga, Sr, Ce and An% in plagioclase phenocrystsfrom sample KSH01. Samples have been plotted in order of decreasing anorthite content asdetermined through EDS microprobe analysis. B) Chondrite normalised (Sun & McDonough,1989) REE diagram of analysed plagioclase phenocrysts.

KSH01 Plagioclase LA-ICP-MS Results

0.10

1.00

10.00

100.00

1000.00

La Ce Nd Sm Eu Gd Dy Er Yb Lu

REE

Ch

on

dri

teN

orm

ali

sed

RE

E

(Su

n&

McD

on

ou

gh

,1989)

Plag 1a Plag 7b Plag 7a

A)

B)

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31

Figure 17. LA-ICP-MS analyses of spinel inclusions (KSH05 samples) and phenocrysts (K811and KSH01 samples). Samples have been plotted in order of decreasing Mg number asdetermined through EDS microprobe analysis.

Spinel LA-ICP-MS Results

1.00

10.00

100.00

1000.00

10000.00

100000.00

1000000.00

KS

H01-S

p3

K811-S

p2a

K811-S

p3a

K811-S

p4a

KS

H05-S

p3a

KS

H05-S

p4a

Analyses

Lo

g(p

pm

)

0.00

20.00

40.00

60.00

Mg

nu

mb

er

(ED

S)

Ti Mn V Cr Ni Zr Nb Mg#

Geochemical Results

All Shira samples were analysed for both major and trace elements. Results

are presented with respect to petrographic groups (Tables 6, 7, 8 and 9) and

graphically in Figures 18, 19 and 20. Mg #’s (Mg/Mg+Fe2+) were adjusted to a

FeO ratio of 0.85 (FeO/Fe2O3+FeO). CIPW normative mineralogy was

calculated using IGPET (Igpet32) petrologic software (Terra Softa Inc.).

Samples are classified using the total alkalis-silica (TAS) diagram (Le Bas et al.,

1986).

The Shira volcanic rocks are all strongly alkalic, ranging from nephelinite to

picro-basalt, basanite and trachybasalt (Figure 21) and are all nepheline

normative; Mg #’s vary from 77 to 36. The Shira samples have a limited range

in SiO2 content (40.46 wt % to 49.31wt %), a broad range in MgO content

(16.51wt % to 3.11wt %) and Al2O3 content (8.35wt % to 17.72wt %). CaO

abundances (15.76wt % to 7.09wt %) and CaO/Al2O3 (molecular proportions)

(0.73 to 3.71) have positive correlations with Mg # (Figure 22). Abundances of

Fe2O3, TiO2, K2O, P2O5, Na2O, Sr and Ba all show negative correlations with

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32

Mg# (Figure 18), however both Fe2O3 and TiO2 show inflections at

approximately Mg# 45.

Groups identified on the basis of petrographic character are easily discernible

on most major element and trace element graphs (Figures 18, 19 and 21) and

normative mineralogy (Figure 23). Group 1 samples (picrites, basanites and

alkali-olivine basalts) are easily separated due to their much higher Mg #’s and

CaO contents, and much lower Al2O3, Na2O, P2O5 and K2O abundances (Figure

18). Group 1 samples generally show low incompatible element concentrations

(Figures 19 & 20), relatively high normative plagioclase compositions (Figure

23b) and low normative albite contents (Figure 23c).

Group 3 (nephelinites and basanites) samples, although having similar Mg #’s

to Group 2 and 4 samples, are distinguished by their high P2O5 and MnO, and

low SiO2 content (Figure 18). Group 3 samples also have higher CaO contents

and CaO/Al2O3 ratios (Figure 22a) at comparable Mg #’s to Group 2 and 4

samples, as well as higher Sr, Ce, Yb, Zr, Nb and Ta abundances (Figure 19).

Group 3 samples have the highest normative nepheline contents (Figure 23a),

high normative plagioclase compositions (Figure 23b), and low albite contents

(Figure 23c) at comparable Mg #’s to Groups 2 and 4.

Group 2 and 4 samples (trachy-basalts and basanites) cover broad, but similar

chemical composition ranges (Figures 18, 19, 22 and 23). Group 2 samples

have lower CaO/Al2O3 (Figure 22a), than Group 3 and 4 samples of similar Mg

#. Group 2 samples generally contain slightly higher Sr, Ba, Rb, Ce, Yb, Zr, Hf,

Nb and Ta contents at comparable Mg #’s (Figure 19) than Group 4 samples,

whereas the majority of Group 4 samples contain higher normative plagioclase

compositions (Figure 23b) and lower normative albite content (Figure 23c) than

Group 2 samples of comparable Mg #’s.

Although broad geochemical trends are apparent over the entire range of Shira

samples (negative trends for incompatible elements (i.e. Sr, Ba, REE, Zr, Hf, Nb

& Ta) and positive trends for Cr, Sc and V), smaller intra-group trends are also

apparent, with some intra-group trends opposing the broader Shira trend.

Group 2 samples show positive trends for Nb and Ta, whilst Groups 1, 3 and 4

show negative trends. Similarly, Groups 1 and 3 show positive Rb trends,

whilst Groups 2 and 4 show negative trends. Hf shows a negative correlation

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for Groups 1 and 4, yet a positive correlation for Group 3 samples, and broad

scatter of Group 2 samples.

Chondrite-normalised REE patterns of Shira samples are light-REE enriched.

La concentrations range between 100 and 400 times chondritic levels, with Lu

concentrations approximately 10 to 20 times chondritic levels. Ce/Yb ratios

vary from 36 to 70. REE patterns shallow towards the heavy REE, with Ce/Sm

values between 9.9 and 16.3 and Sm/Yb values of between 3.17 and 4.41.

Chondrite-normalised REE patterns are smooth and near parallel (Figure 20),

with very minor Eu anomalies observed in only five Group 2 samples, three

Group 3 samples, and one Group 4 sample. The degree of REE enrichment

increases from Groups 1 to 3, with Group 4 covering a broader range. Groups

have distinct multi-element spider diagram trends when normalised against

primitive mantle values (Figure 20) (Sun & McDonough, 1989). All groups show

distinct K depletions, but uncharacteristically, Pb enrichments (not as

pronounced in Group 3 samples) (Figure 18). Group 1 and 2 samples have

similar characteristics, with Group 1 tending to be less enriched than Group 2.

Positive anomalies are shown for Pb, Nb, Nd, and Ti relative to neighbouring

elements, and negative anomalies are shown for P, K and Zr in Groups 1 and 2,

with larger anomalies in Group 1 than Group 2. Group 3 multi-element spider

diagrams are similar to Group 2, with larger negative K anomalies, but smaller

Pb anomalies. The multi-element diagram for Group 4 is very similar to that of

Group 1.

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Table 6. Geochemical results of Group 1 samples (BSN=basanite,

PBAS=picrobasalt, AOB=alkali olivine basalt). ICP-AES Major Element Results (values in weight percent except where stated)Sample KSH03 KSH05 KSH07 KSH08 KSH09 K2225Rock Type BSN AOB PBAS AOB AOB BSNSiO2 44.46 46.37 44.37 45.18 45.15 43.33Al2O3 13.28 7.96 9.33 7.52 8.35 12.91Fe2O3 (total) 12.93 10.62 11.49 10.23 11.10 13.33MnO 0.21 0.16 0.18 0.16 0.17 0.21MgO 10.13 16.17 13.06 16.51 14.53 8.46CaO 11.55 14.30 15.68 15.34 15.76 13.13Na2O 2.02 1.16 1.43 1.04 1.14 3.33K2O 0.96 0.45 0.36 0.37 0.34 0.56TiO2 1.97 1.50 1.75 1.49 1.65 2.21P2O5 0.44 0.23 0.29 0.24 0.24 0.49LOI 1.65 1.09 1.06 0.98 1.14 2.47Total 99.59 100.01 99.00 99.07 99.58 100.42

Sr (ppm) 622.10 388.10 355.50 291.90 338.10 590.60Ba (ppm) 371.80 123.80 152.70 82.31 109.10 271.70

LA-ICP-MS Trace Element Results (values in ppm)Sc 34.23 47.94 59.23 52.29 59.51 35.03V 276.81 237.98 305.80 254.00 289.19 325.96Cr 316.44 1417.44 764.80 1571.65 886.98 193.40Ga 18.48 11.07 13.59 11.83 12.44 18.79Rb 22.35 85.98 44.29 11.09 46.04 9.57Sr 671.84 395.26 378.01 329.97 369.07 595.74Y 22.64 14.04 17.55 14.82 16.38 24.42Zr 194.15 100.10 120.73 105.55 107.93 193.67Nb 63.50 32.25 37.88 33.64 33.30 71.26Cs 0.24 0.98 0.55 0.11 0.44 0.28Ba 455.77 197.76 208.96 188.02 207.47 366.92La 46.78 24.50 26.09 24.83 24.71 48.82Ce 85.65 43.67 49.92 45.89 45.82 88.91Nd 39.39 21.35 25.09 22.11 22.76 39.90Sm 7.15 4.30 5.03 4.32 4.62 7.31Eu 2.13 1.23 1.53 1.29 1.38 2.16Gd 5.99 3.61 4.46 3.85 4.29 6.53Dy 4.47 2.70 3.28 2.97 3.29 4.79Er 2.10 1.28 1.69 1.40 1.57 2.39Yb 1.68 1.05 1.38 1.14 1.27 1.91Lu 0.26 0.16 0.21 0.16 0.19 0.28Hf 4.48 2.57 3.36 2.58 2.99 4.61Ta 3.69 1.89 2.28 2.03 2.13 4.25Pb 6.50 4.34 4.68 4.67 4.61 6.00Th 4.89 2.68 2.88 2.80 2.57 5.96U 0.88 0.57 0.63 0.58 0.49 1.34

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.16 1.77 1.92 1.71 1.85 2.22FeO 10.99 9.03 9.77 8.70 9.44 11.33

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)

Mg/(Mg+Fe2+) 62.2 76.2 70.5 77.2 73.3 57.1CaO/Al2O3 1.6 3.3 3.1 3.7 3.4 1.8Fo% (0.3) 84.6 91.4 88.8 91.9 90.1 81.6%AN 66.9 68.1 91.0 88.5 87.0 75.5CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 5.78 2.68 2.17 2.23 2.04 3.37ab 12.28 7.19 1.81 1.95 2.52 6.16an 24.79 15.32 18.32 15.02 16.90 18.97ne 2.78 1.47 5.70 3.79 3.93 12.21di 24.97 43.84 47.57 48.64 48.84 36.15ol 21.37 23.49 17.52 22.40 19.31 14.43mt 3.19 2.59 2.84 2.52 2.72 3.28il 3.81 2.87 3.39 2.88 3.18 4.28ap 1.04 0.54 0.68 0.57 0.56 1.16Total 100.00 100.00 100.00 100.00 100.00 100.00

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Table 7. Geochemical results of Group 2 samples (BSN=basanite,

TBAS=trachybasalt). ICP-AES Major Element Results (values in weight percent except where stated)Sample KSH01 KSH02 KSH04 KSH06 KSH10 KSH11 K679 K802 K803 K804 K895Rock Type TBAS TBAS BSN TBAS BSN TBAS TBAS TBAS BSN TBAS TBASSiO2 45.73 49.31 42.79 49.01 43.38 46.61 46.62 47.93 44.52 48.57 47.25Al2O3 16.08 17.03 16.27 17.72 15.74 16.77 15.71 17.47 16.44 16.91 15.81Fe2O3 (total) 13.66 12.05 14.20 11.62 15.17 12.42 12.42 12.57 13.02 11.56 12.57MnO 0.22 0.20 0.21 0.19 0.22 0.19 0.21 0.23 0.24 0.18 0.20MgO 5.57 4.41 4.49 3.94 4.74 4.85 6.31 3.70 5.66 4.26 6.20CaO 9.15 7.36 9.74 7.09 11.95 8.47 9.50 7.93 10.22 7.88 9.06Na2O 4.12 4.42 3.37 3.74 2.12 3.30 3.62 4.64 4.34 3.37 3.86K2O 1.27 2.25 1.64 2.48 1.13 1.95 2.05 1.89 1.68 2.40 2.15TiO2 2.40 2.06 2.60 2.08 2.74 2.36 2.26 2.56 2.49 2.17 2.24P2O5 0.58 0.73 0.61 0.68 0.58 0.60 0.73 0.70 0.75 0.59 0.71LOI 0.51 0.65 4.82 1.87 2.88 2.20 1.26 0.83 0.79 1.17 0.69Total 99.28 100.46 100.73 100.41 100.66 99.71 100.68 100.43 100.14 99.05 100.73

Sr (ppm) 848.30 959.20 741.60 869.80 728.40 827.10 848.60 877.80 941.10 1289.00 875.00Ba (ppm) 645.90 787.00 424.40 758.10 526.90 631.80 617.70 680.80 635.60 589.50 586.20

LA-ICP-MS Trace Element Results (values in ppm)Sc 19.50 13.97 15.55 11.72 20.79 14.43 19.77 9.38 17.32 15.04 18.49V 263.11 180.06 241.78 170.59 338.87 246.89 235.06 145.12 235.50 210.71 224.18Cr 36.65 35.39 29.08 30.48 18.63 - 99.07 - - 69.24 151.10Ga 23.62 23.20 23.52 23.10 23.72 24.99 22.32 22.95 22.91 27.93 22.40Rb 66.38 62.75 53.66 92.84 125.47 89.15 54.02 53.94 59.30 116.09 58.71Sr 822.05 945.01 771.66 872.76 778.23 881.56 877.47 1000.05 954.54 1316.21 892.83Y 27.84 29.74 27.85 29.67 28.69 30.29 28.34 29.68 29.17 39.01 27.47Zr 258.20 279.91 226.23 268.65 221.08 270.48 296.65 262.01 328.48 325.54 287.77Nb 90.25 97.42 82.56 99.83 75.50 92.91 107.72 88.29 107.97 94.29 107.37Cs 0.36 0.56 0.37 0.57 1.31 0.71 0.54 0.76 0.49 0.38 0.85Ba 634.96 781.96 487.50 769.35 587.53 695.26 690.43 710.36 703.35 704.94 672.67La 67.88 75.85 53.14 75.95 51.50 72.09 71.85 69.41 74.34 78.98 72.38Ce 121.85 137.00 97.43 137.06 95.52 129.50 131.61 124.89 136.24 143.20 129.80Nd 52.98 58.10 43.03 57.58 44.38 53.39 55.55 53.15 58.48 60.55 53.98Sm 9.44 9.68 7.88 9.85 8.31 9.75 9.85 9.46 10.11 10.80 9.00Eu 2.84 2.64 2.46 2.57 2.51 2.71 2.72 2.63 2.98 2.91 2.68Gd 7.53 7.60 6.72 7.56 7.24 7.66 7.53 7.41 8.02 8.78 7.32Dy 5.37 5.65 5.35 5.72 5.68 5.88 5.52 5.64 5.80 7.24 5.37Er 2.53 2.79 2.63 2.85 2.73 2.89 2.67 2.80 2.71 3.83 2.60Yb 2.16 2.51 2.18 2.55 2.23 2.48 2.23 2.48 2.37 3.40 2.28Lu 0.31 0.37 0.34 0.41 0.32 0.37 0.34 0.38 0.32 0.51 0.34Hf 5.61 5.97 4.84 6.21 4.93 6.04 6.42 5.66 6.82 7.24 6.16Ta 5.14 6.14 4.92 6.09 4.52 5.08 6.41 4.99 6.48 5.18 6.49Pb 5.45 9.57 7.24 10.04 6.08 9.56 8.66 8.48 7.49 8.78 8.49Th 7.50 8.83 6.84 9.59 5.90 9.13 9.38 8.99 7.68 10.52 9.42U 1.40 1.65 1.43 2.99 1.39 1.02 1.12 1.89 1.55 2.60 1.15

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.28 2.01 2.37 1.94 2.53 2.07 2.07 2.10 2.17 1.93 2.10FeO 11.61 10.24 12.07 9.88 12.89 10.56 10.56 10.68 11.07 9.83 10.68

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)Mg/(Mg+Fe2+) 46.1 43.4 39.9 41.6 39.6 45.0 51.6 38.2 47.7 43.6 50.9CaO/Al2O3 1.0 0.8 1.1 0.7 1.4 0.9 1.1 0.8 1.1 0.8 1.0Fo% (0.3) 74.0 71.9 68.9 70.3 68.6 73.2 78.0 67.3 75.2 72.0 77.5%AN 51.6 41.0 65.4 44.8 69.0 51.5 54.6 44.4 65.0 46.4 51.4CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 7.58 13.29 10.08 14.84 6.81 11.79 12.16 11.19 9.97 14.46 12.67ab 20.49 28.77 13.43 30.25 13.82 24.23 17.18 26.62 11.02 28.18 18.37an 21.85 19.98 25.40 24.55 30.70 25.77 20.64 21.30 20.50 24.39 19.41ne 7.97 4.67 8.79 0.97 2.43 2.35 7.35 6.89 14.01 0.48 7.69di 16.77 9.81 17.35 5.50 21.73 10.99 18.05 11.36 21.13 9.61 17.17ol 16.04 14.96 14.77 15.45 14.09 15.79 15.62 13.11 13.72 14.43 15.76mt 3.34 2.91 3.57 2.85 3.74 3.07 3.01 3.05 3.16 2.85 3.04il 4.60 3.91 5.14 4.00 5.31 4.59 4.31 4.87 4.75 4.20 4.24ap 1.36 1.69 1.47 1.60 1.37 1.42 1.70 1.62 1.74 1.39 1.64Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

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Table 8. Geochemical results of Group 3 samples (BSN=basanite,

NEPH=nephelinite/foidite). ICP-AES Major Element Results (values in weight percent except where stated)Sample K686 K811 K813 K820 K825Rock Type BSN BSN NEPH NEPH NEPHSiO2 41.32 42.72 40.67 40.46 41.48Al2O3 14.49 14.89 14.12 15.29 16.89Fe2O3 (total) 16.07 14.27 14.64 15.86 14.71MnO 0.27 0.24 0.29 0.31 0.28MgO 5.50 5.52 6.22 5.20 4.22CaO 11.94 11.67 13.99 11.37 10.03Na2O 3.71 3.89 3.03 4.46 5.76K2O 0.78 1.83 0.76 1.10 1.47TiO2 2.68 2.41 2.63 2.39 2.27P2O5 0.85 0.75 0.75 0.88 0.92LOI 3.38 2.00 3.14 1.86 1.82Total 100.99 100.19 100.25 99.17 99.84

Sr (ppm) 935.10 977.20 807.50 1138.00 1156.00Ba (ppm) 545.30 669.40 393.70 545.70 719.30

LA-ICP-MS Trace Element Results (values in ppm)Sc 9.30 16.88 13.25 9.37 6.67V 300.48 270.26 335.77 278.34 253.13Cr - 41.65 19.27 13.06 4.12Ga 23.91 21.80 24.44 23.56 23.38Rb 74.02 59.61 126.66 63.60 55.26Sr 980.81 916.55 912.24 1182.32 1151.69Y 34.62 30.99 36.28 39.22 37.90Zr 351.34 297.07 385.83 360.25 353.74Nb 134.44 108.10 121.95 138.41 148.53Cs 0.58 0.44 0.66 0.57 0.61Ba 520.52 574.16 532.86 616.73 762.33La 85.81 70.69 81.92 90.80 92.75Ce 154.99 128.87 150.89 162.13 165.72Nd 65.64 56.95 67.70 68.37 67.35Sm 11.71 10.16 12.09 11.87 11.46Eu 3.36 2.98 3.62 3.32 3.32Gd 9.31 8.17 9.98 9.61 9.38Dy 6.76 6.02 7.26 7.42 7.17Er 3.30 2.95 3.43 3.68 3.63Yb 2.73 2.40 2.90 3.15 3.16Lu 0.40 0.34 0.39 0.48 0.43Hf 6.48 5.75 7.57 6.52 6.02Ta 9.30 7.22 8.51 10.03 10.62Pb 5.92 5.24 5.75 5.65 5.44Th 9.18 7.38 8.50 9.98 10.62U 1.65 1.15 1.85 2.60 2.78

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.68 2.38 2.44 2.64 2.45FeO 13.66 12.13 12.44 13.48 12.50

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)

Mg/(Mg+Fe2+) 41.8 44.8 47.1 40.7 37.6CaO/Al2O3 1.5 1.4 1.8 1.4 1.1Fo% (0.3) 70.5 73.0 74.8 69.6 66.7%AN 75.2 87.6 99.7 89.9 81.8CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 4.71 10.99 4.61 6.66 8.84ab 6.94 2.55 0.06 2.12 3.60an 21.03 18.05 23.30 18.91 16.17ne 13.61 16.73 14.24 19.80 24.92di 28.29 29.82 35.66 27.67 23.87ol 14.24 11.94 11.57 14.17 12.43mt 3.97 3.51 3.63 3.92 3.61il 5.20 4.65 5.13 4.65 4.39ap 2.01 1.77 1.79 2.09 2.17Total 100.00 100.00 100.00 100.00 100.00

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Table 9. Geochemical results of Group 4 samples (BSN=basanite,

TBAS=trachybasalt). ICP-AES Major Element Results (values in weight percent except where stated)Sample K361 K832A K897 K832B K894Rock Type BSN BSN BSN TBAS BSNSiO2 44.27 45.07 43.92 45.58 43.09Al2O3 16.22 14.20 15.78 15.37 17.51Fe2O3 (total) 14.00 14.47 15.18 14.50 11.83MnO 0.20 0.22 0.23 0.21 0.23MgO 6.10 6.37 5.63 4.31 3.11CaO 11.41 12.51 10.29 9.12 8.07Na2O 3.55 3.31 4.17 2.96 4.83K2O 0.64 0.41 1.44 2.14 2.42TiO2 2.19 2.28 2.46 2.63 2.00P2O5 0.39 0.54 0.63 0.46 0.91LOI 1.36 0.92 0.91 2.13 6.05Total 100.32 100.30 100.63 99.39 100.04

Sr (ppm) 769.80 558.60 702.00 617.10 1198.00Ba (ppm) 384.10 366.50 441.60 597.00 811.80

LA-ICP-MS Trace Element Results (values in ppm)Sc 20.56 28.79 13.98 20.51 4.25V 317.62 290.64 268.64 301.86 133.06Cr - 60.88 24.13 76.76 -Ga 21.58 20.32 24.12 22.97 23.52Rb 13.81 20.97 27.59 48.70 78.61Sr 813.04 574.22 734.79 625.53 1232.35Y 20.56 26.56 27.23 28.47 29.89Zr 165.97 192.93 228.59 260.35 274.90Nb 52.07 62.27 83.86 83.27 152.97Cs 0.29 0.44 0.44 0.18 0.75Ba 439.70 443.48 546.68 687.21 904.48La 38.08 46.88 60.24 52.96 95.73Ce 70.29 86.61 108.49 94.56 161.86Nd 32.13 41.26 46.76 42.85 61.69Sm 6.17 7.61 8.36 7.96 9.95Eu 1.99 2.29 2.49 2.47 2.82Gd 5.18 6.72 6.98 6.86 7.77Dy 4.07 5.17 5.24 5.49 5.68Er 1.93 2.51 2.58 2.73 2.90Yb 1.73 2.14 2.09 2.33 2.31Lu 0.24 0.30 0.32 0.34 0.33Hf 3.94 4.46 4.98 5.66 4.65Ta 3.11 3.73 5.00 5.06 9.17Pb 6.11 7.58 7.53 12.46 10.39Th 4.49 5.12 7.53 5.95 12.71U 1.07 1.17 1.74 1.23 2.89

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.33 2.41 2.53 2.42 1.97FeO 11.90 12.30 12.90 12.33 10.06

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)Mg/(Mg+Fe2+) 47.7 48.0 43.8 38.4 35.6CaO/Al2O3 1.3 1.6 1.2 1.1 0.8Fo% (0.3) 75.3 75.5 72.2 67.5 64.8%AN 66.1 59.8 66.4 54.5 59.7CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 3.81 2.43 8.51 12.97 15.18ab 13.69 15.33 10.18 19.11 13.60an 26.65 22.77 20.09 22.90 20.12ne 8.99 6.93 13.61 3.56 16.13di 23.11 29.87 22.40 17.14 13.56ol 15.24 13.57 15.41 14.51 12.10mt 3.41 3.51 3.67 3.60 3.03il 4.19 4.35 4.67 5.12 4.03ap 0.91 1.26 1.46 1.09 2.24Total 100.00 100.00 100.00 100.00 100.00

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40 50 60 7040

42

44

46

48

SiO2

Mg/(Mg+Fe2+)

40 50 60 705

10

15

Al2O3

Mg/(Mg+Fe2+)

40 50 60 705

10

15

Fe O

(total)2 3

Mg/(Mg+Fe2+)

40 50 60 701

2TiO2

Mg/(Mg+Fe2+)

40 50 60 70

1

2

3

4

5

Na2O

Mg/(Mg+Fe2+)

40 50 60 700.1

0.2

0.3

MnO

Mg/(Mg+Fe2+)

40 50 60 700

1

2

K2O

Mg/(Mg+Fe2+)

0

40 50 60 70 805

10

15

CaO

Mg/(Mg+Fe )2+

40 50 60 70 800

P2O5

Mg/(Mg+Fe )2+

0.8

0.6

0.4

0.2

40 50 60 70 80

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

Figure 18. ICP-AES major element resultsfor Shira samples in weight percent plottedagainst Mg numbers.

Group 1

Group 2

Group 3

Group 4

Shira Samples

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40 50 60 70

200

600

1000

1400

Sr

Mg/(Mg+Fe2+)

40 50 60 70

200

400

600

800

1000

Ba

Mg/(Mg+Fe2+)

40 50 60 700.0

0.5

1.0

1.5

Cs

Mg/(Mg+Fe2+)

40 50 60 700

50

100

Rb

Mg/(Mg+Fe2+)

40 50 60 700

40

80

120

160

Ce

Mg/(Mg+Fe2+)

40 50 60 701

2

3

Yb

Mg/(Mg+Fe2+)

40 50 60 700

100

200

300

400

Zr

Mg/(Mg+Fe2+)

40 50 60 700

2

4

6

8

Hf

Mg/(Mg+Fe2+)

40 50 60 70 800

50

100

150

Nb

Mg/(Mg+Fe )2+

40 50 60 70 80

2

6

10

Ta

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

Figure 19. LA-ICP-MS trace element results (Sr, Ba, Cs, Rb, Ce, Yb, Zr, Hf, Nb & Ta) for Shirasamples in parts per million (ppm) plotted against Mg numbers.

Group 1

Group 2

Group 3

Group 4

Shira Samples

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1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

1000

CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Rock/Primitive Mantle

1

10

100

1000

CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Rock/Primitive Mantle

1

10

100

1000

CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Rock/Primitive Mantle

1

10

100

1000

CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Rock/Primitive Mantle

Figure 20. REE and multi element spider diagrams for all Shira samples (separated into

groups and normalised to Sun & McDonough, 1989 chondrite and primitive mantle respectively)

Group 1

Group 2

Group 3

Group 4

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41

Figure 21. A)Total alkalis silica (TAS) diagram of all Kilimanjaro samples showing the

classification scheme of Le Bas , (1986) with inset showing sample numbers.

B) Plot of normative plagioclase composition (AN=(anx100)/(ab+an)) versus normative

nepheline and hypersthene for all Shira samples. The centre line represents the plane

of silica undersaturation, whereas the shaded band represents the approximate trace of

the plane of silica saturation (e.g. Best & Brimhall, 1974), showing that all Shia samples

are alkalic.

et al.

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Picro-

basalt

Basalt Basaltic

andesite Andesite Dacite

Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Basaltictrachy-andesite

Trachy-basalt

Tephrite

Basanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

Kibo samples

Parasitic vent samples

Other Localities

Group 1

Group 2

Group 3

Group 4

Shira Samples

Na

O+

KO

%2

2

SiO %2

2

4

6

40 45 50

Picro-basalt

Basalt

Trachy-basalt

Tephrite

Basanite

Foid

ite

KSH01

KSH02

KSH03

KSH04

KSH05

KSH06

KSH07

KSH08KSH09

KSH10

KSH11

K361

K679

K686

K802

K803

K804K811

K813

K820

K825

K832A

K832B

K894

K895

K897

K2225

10

20

30

ne

%AN

0

10

20

30

hy

50 70 90

Group 1

Group 2

Group 3

Group 4

Shira Samples

approximate plane of silica saturation

A)

B)

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42

Figure 23. Plots of normative nepheline (A), anorthite percent (B) (%AN = an/ (ab+an)),

and albite (C) versus Mg number in order to show normative differences

between groups.

Figure 22. A) Plot of Mg number versus CaO/Al O for Shira samples showing the broad

negative trend (decrasing with decreasing Mg number) and differing

values for groups 1 to 4. B) Plot of K O versus P O for Shira samples showing the regions

in which groups 1 to 4 plot.

2 3

2 2 5

CaO/Al O CaO/Al O2 3 2 3

40 50 60 70 800

1

2

3

4

Ca

O/A

lO

23

Mg/(Mg+Fe2+)

40 50 60 700

10

20

30

ne

Mg/(Mg+Fe2+)

40 50 60 7030

50

70

90

%AN

Mg/(Mg+Fe2+)

0 1 2 30.0

0.5

1.0

K O2

PO

25

Group 1

Group 2

Group 3

Group 4

Shira SamplesShira Sample

A) B)

A) B)

40 50 60 70 800

10

20

30

ab

Mg/(Mg+Fe2+)

Group 1

Group 2

Group 3

Group 4

Shira SamplesC)

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43

DISCUSSION

Four distinct groups based upon petrographic characteristics are identified as

composing the Shira Volcanics of Mt Kilimanjaro. In general, these groups also

have distinctive petrographic, geochemical and geographic characteristics.

Group 1 (East Shira Hill and Shira Cathedral areas) are high-Mg strongly phyric

basanites, picrobasalts and alkali olivine basalts. Group 2 samples (from the

same area) are trachybasalts and basanites, Group 3 samples (nephelinites)

are from the main Shira Ridge near Klute and Kente peaks, and Group 4

samples (basanites and trachybasalts) are from Platzkegel.

Understanding the relationship between these different groups may help to

understand the petrogenesis of the Shira volcanic rocks and magma chamber

dynamics beneath Mt Kilimanjaro. The petrogenetic relationship of each group

is developed through modelling of magmatic processes such as fractional

crystallisation, mixing and assimilation, and ultimately, speculating on their

source composition and mineralogy, and melting processes.

Fractional Crystallisation Models

Major and trace element trends indicate that the diversity of the Shira suite can

be explained by the process of fractional crystallisation (Figure 24) commencing

from a suite of slightly different primary magmas.

Quantitative models of fractional crystallisation for the Shira lavas are based on

major element mass balance equations using inputs of major element

geochemistry and microprobe data. These models provide a basis for trace

element calculations using the Rayleigh fractional crystallisation formula

CL/CO=F(D-1) (Allegre et al., 1977; Allegre & Minister, 1978) and compilation of

relevant partition coefficients (Table 10). Modelling has been performed through

both IGPET petrologic software and the creation of Microsoft Excel

spreadsheets for comparing calculated daughter trace element concentrations

with observed parent and daughter concentrations.

Several criteria are used for selecting suitable solutions to major element mass

balance calculations. The first criterion is that the sum of the residuals squared

be less than 0.6 (using a weighting of 0.4 for Si, 0.5 for Al and Mn and 1 for Ti,

Comment [ADG1]: You will find that I have not changed anything in the discussion yet. This is not to say that it is perfect. It does need some changes some in presentation and some in science), but you certainly have the bulk of it there. Why I haven’t written anything is that after 2 readings, I am still mulling it all over in my mind. I am not quite sure what the issues are, but I have this slightly unsettled feeling that there is another angle that needs to be thought about. I will do this thinking over the weekend and get some comment back to you on the discssion by Monday.

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44

Mg, Ca, Fe, Na, K and P). The second criterion is the plausibility of the

fractionating assemblage (whether the relative abundances of fractionated

phases from the model is suitable for the observed mineralogy of the samples)

whilst the third criterion is the match between calculated daughter trace element

concentrations and observed daughter concentrations. When these criteria fail

to discriminate between models, the model that uses the least number of

fractionating phases is preferred. Comprehensive major and trace element

results are presented in Appendix A, and simplified model results are shown in

Table 12.

Potential fractionating phases are limited to observed phenocrysts

(clinopyroxene, olivine, plagioclase, nepheline and spinel). In some cases, the

compositions of the selected phase vary significantly between core and rim.

The phase chemistries used in the fractional crystallisation models for each

group are presented in Table 11. The vectors produced by fractionation of the

wide range of phenocryst compositions are shown in Figure 25 on a CaO

versus Mg # diagram. The major observations that can be made from this

graphical presentation are that fractionation of olivine drives liquid compositions

to lower Mg #’s and higher CaO, whereas fractionation of clinopyroxene drives

liquid compositions to lower Mg #’s and lower CaO. In general it takes

approximately twice the amount of clinopyroxene fractionation to decrease the

Mg # the same amount that olivine fractionation would decrease it. Variation in

olivine composition and clinopyroxene compositions create a range of liquid

compositions. Plagioclase fractionation produces minimal change, whereas

spinel fractionation drives liquids to higher Mg ‘s. The essential conclusion is

that the Shira trends can be explained by dominantly clinopyroxene

fractionation, or a combination of clinopyroxene and olivine fractionation.

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45

40 50 60 70 80

8

10

12

14

16

CaO

Mg/(Mg+Fe )2+

KSH01

KSH02

KSH03

KSH04

KSH05

KSH06

KSH07KSH08

KSH09

KSH10

KSH11

K361

K679

K686

K802

K803

K804

K811

K813

K820

K825

K832A

K832B

K894

K895

K897

K2225

Figure 24. Mg number versus CaO plot showing the fractional crystallisation paths

modelled and microprobed samples used in modeling.

Group 1 and 2 fractionation paths ( , )

Group 3 fractionation path ( )

Group 4 fractionation path ( )

Samples with phase chemistrydetermined

PathA

Path

B

Path C

40 50 60 70 80

8

10

12

14

16

CaO

Mg/(Mg+Fe )2+

Fractionation of the rangeof cpx phenocrysts found in Shira rocks

5%

10%

15%

20%

25%

30%

Fractionation of olivine phenocrystsFo89 Fo67

15%

10%

5%

Fractionation direction of the rangeof spinels found in Shira rocks

Fractionation of plagioclase (An76)(arrow tip = 30% fractionation)

Fractionation of plagioclase (An59)(arrow tip = 30% fractionation)

Figure 25. Mg number versus CaO plot showing the bulk rock chemistry change (starting from

sample K2225 black star) produced by the fractionation of a wide range of Shira olivine, spinel,

plagioclase and clinopyroxene compositions as determined by microprobe analyses.

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46

Table 10. Partition coefficients used for petrogenetic modelling (CPX =

clinopyroxene, PLAG = plagioclase, SP = spinel, OL = olivine, NEPH =

nepheline, OPX = orthopyroxene, GT = garnet, AMPH = amphibole and

PHLOG = phlogopite)

Mineral CPX PLAG SP OL NEPH OPX GT AMPH PHLOGMn 0.75 2 0 2 0 2 1.45 2 0.044 6 1.4 2

Sc 2.8 4 0.02 4 1.59 4 0.15 4 0.056 6 1.2 2 8.5 9 2.9 1

Ti 0.4 2 0.04 2 7.5 2 0.02 2 0.08 6 0.1 2 0.3 2 1.4 1 0.9 2

V 1.35 2 0 2 26 2 0.06 2 0.6 2

Cr 5.3 7 0.08 7 4.2 7 2.8 7 10 2 1.345 9

Rb 0.03 4 0.39 4 0.32 4 0.02 4 1.5 6 0.022 2 0.042 2 0.45 1 5.8 1

Sr 0.16 7 2.7 7 0.68 7 0.02 7 0.04 2 0.012 2 0.376 1 0.081 2

Y 0.9 2 0.03 2 0.2 2 0.01 2 0.18 2 9 2 0.333 1 0.03 2

Zr 0.3 4 0.09 4 3.6 4 0.07 4 0.18 2 0.65 2 0.8 1 0.6 2

Nb 0.005 2 0.01 2 0.4 2 0.01 2 0.15 2 0.02 2 0.2 1 0.14 1

Cs 0.04 7 0.13 7 0.08 7 0.05 7 0.8 6

Ba 0.04 7 0.56 7 0.4 7 0.03 7 0.42 6 0.013 2 0.023 9 0.5 1 2.9 1

La 0.13 4 0.12 4 0.19 4 0.01 4 0.041 6 0.003 3 0.001 9 0.039 1

Ce 0.092 5 0.111 5 2.15 8 0.006 5 0.047 6 0.02 10 0.007 9 0.067 1 0.034 2

Nd 0.23 5 0.09 5 2 8 0.0059 5 0.03 10 0.026 9 0.142 1 0.032 2

Sm 0.445 5 0.072 5 1.65 8 0.007 5 0.059 6 0.05 10 0.102 9 0.188 1 0.031 2

Eu 0.59 4 0.21 4 0.22 4 0.02 4 0.061 6 0.05 10 0.243 9 0.27 1 0.03 2

Gd 0.556 5 0.071 5 0 8 0.01 5 0.09 10 0.68 9 0.03 2

Dy 0.582 5 0.063 5 0 8 0.013 5 0.15 10 1.94 9 0.36 1 0.03 2

Er 0.583 5 0.057 5 0 8 0.0256 5 0.23 10 4.7 9 0.034 2

Yb 0.542 5 0.056 5 1.35 8 0.0491 5 0.086 6 0.34 10 6.167 9 0.61 1 0.042 2

Lu 0.506 5 0.053 5 0.0454 5 0.066 6 0.42 10 6.95 9 0.046 2

Hf 0.5 4 0.05 7 0.37 4 0.03 4 0.11 6 0.02 3 0.14 9 0.5 4

Ta 0.08 4 0.03 4 0.56 4 0.02 4 0.06 2 0.19 4

Th 0.06 4 0.05 4 0.19 4 0.02 4 0.04 6 0.06 3 0.001 3 0.05 1

U 0.07 4 0.05 4 0.26 4 0.03 4 0.043 6 0.08 4

1 Spath et al. , 2001 (comp of McKenzie & O'Nions, 1991; Frey et al. , 1978; 4 Lemarchand et al ., 1987

Dalpe' & Baker, 1994; Chazot et al., 1996; Adam et al., 1993; 5 Fujimaku, 1984

Le Roex et al ., 1990 and references therein; Lemarchand et al ., 1987 6 Onuma et al ., 19812 Rollinson, 1993 (comp of Arth 1976; Pearce & Norry, 1979; Green et al., 1989; 7 Villemant et al ., 1981

Schock, 1979; Fujimaku, 1984; Dostal et al ., 1983; Henderson, 1982; 8 Schock, 1979

Leeman & Lindstrom, 1978; Lindstrom & Weill, 1978; Green & Pearson, 1987) 9 Irving & Frey, 19783 Kempton et al ., 1987 10 Arth, 1976

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47

Oliv

ine

Ana

lys e

s U

sed

in P

etro

gene

tic M

odel

ling

Spi

n el A

nal

yses

Use

d in

Pet

rog

enet

ic M

ode

lling

Gro

up 1

Gro

up 1

Gro

up 1

Gro

up 4

Gro

up 4

Gro

up 1

Gro

u p 3

Gro

up 3

Gro

up 4

Gro

up 4

K

SH

05 o

l 8c

KS

H05

ol 5

c K

SH

03 O

l3b

K36

1 O

l1a

K36

1 O

l1b

KS

H0 3

CrS

pK

811 M

gTiS

pK

820M

gTiS

p K

361M

gTiC

r Sp

K89

4TiS

p av

g rim

av

g c

o re

r imS

iO2

39.4

339

.33

3 8.7

234

.97

34.0

6S

iO2

0.8

0.48

0.45

0.26

0 .75

TiO

20

00

0.05

0.02

TiO

24.

4617

.12

15.2

515

.81

20.4

3A

l 2O3

00.

150

00.

22A

l 2O3

1 9.3

87.

747.

975.

166.

98Fe

O14

.02

16.5

119

.88

36.9

940

.13

FeO

43.7

666

.39

64.6

863

.45

58.8

MnO

0.1

0.39

0.29

0.72

0.99

MnO

0.06

0.61

0.51

0.32

2.5

MgO

44.7

842

.89

40.4

526

.01

22.4

MgO

9.09

3.93

4.14

1.7

0.4

CaO

0.55

0.49

0.38

0.57

0.4 2

CaO

0.02

0.03

00.

030.

12N

a 2O

00

00

0N

a 2O

00

00

0.11

K2O

00.

10.

10

0.06

K2O

0.01

0.01

0.01

00

P2O

50

00

0.14

0.18

P2O

50.

120

0.01

00

Cr 2

O3

00

00.

010

Cr 2

O3

17.1

60.

10 .

045.

380.

11T O

TAL

98.8

899

.86

99.8

299

.46

98.4

8TO

TAL

94.8

696

.41

93.0

692

.11

90.2

# of

Cat

ions

3.00

3.00

3.00

3 .01

3 .00

# of

Cat

ions

3.00

3.00

3.00

3.00

3.0 1

Fo85

82.2

78.3

55.6

49.8

Mg#

39.5

14.3

15.9

6.7

1.5

Clin

opy

roxe

ne A

naly

ses

Use

d in

Pet

roge

netic

Mo d

ellin

gP

lag

iocl

ase

Ana

lyse

s U

sed

in P

etro

g ene

tic M

odel

ling

Gro

up 1

Gro

up 1

Gro

up 3

Gro

up 3

Gro

up 4

Gro

up 4

Gro

up 4

Gro

up 4

KS

H05

cpx

8c

KS

H03

Cpx

4b

K81

1 C

p x4c

K

820

Cpx

1cK

361

Cpx

1a

K89

4 C

px2c

K36

1 G

M P

lag1

K89

4 G

M P

lag1

avg

avg

r im ri

mco

re ri

m a

vg

avg

SiO

249

.66

48.3

243

.26

46.5

646

.39

42.1

9S

iO2

52.2

252

.9Ti

O2

0.77

1.5

2.82

1.62

1.75

3.41

TiO

20.

1 40.

18A

l 2O3

4.08

6.17

9.49

6 .44

5 .63

1 0.6

5A

l 2O3

26.3

927

.5Fe

O5 .

326.

369.

818.

677.

318.

81Fe

O0.

040.

15M

nO0.

10.

010.

210.

250.

140.

07M

nO0

0M

gO15

.21

13.9

79.

3111

.74

12.7

9.31

MgO

0 .02

0C

aO23

.38

22.6

122

.47

23.3

722

.26

2 2.2

CaO

9.77

10.1

1N

a 2O

0.1

00 .

310.

240.

480.

29N

a 2O

5.17

4.75

K2O

0.12

0.11

0.17

0.1

0.1

0.04

K2O

0.5

0.49

P2O

50

0.19

00

0.18

0.41

P2O

50

0C

r 2O

31.

20.

280.

070.

020.

160.

03C

r 2O

30

0.06

TOTA

L99

.94

99.5

297

.92

99.0

197

.197

.41

TOTA

L94

.25

96.1

4#

of C

atio

ns4.

034.

0 14 .

034.

044.

044.

01#

o f C

atio

ns20

.01

19.9

0E

n43

.39

4 1.3

429

.95

3538

.63

30.7

8A

b47

.44

44.5

6W

o4 7

.93

48.0

951

.96

50.0

848

.66

52.7

5A

n49

.54

52.4

1Fe

8.68

10.5

718

.09

14.9

212

.71

16.4

7O

r3.

0 23.

03

avg

= av

e rag

e co

mpo

sitio

n fo

r pha

se in

sa m

ple,

rim

= a

vera

ge r

im c

ompo

sitio

n fo

r pha

se in

sam

ple,

cor

e =

aver

age

core

com

posi

tion

for p

hase

in s

ampl

eN

umb e

r of c

atio

ns (

# of

Cat

ions

) cal

cula

ted

on th

e ba

sis

of 4

oxy

gens

for o

livin

e an

d s p

inel

, 6 o

xyge

n s fo

r clin

opyr

oxe n

e an

d 32

oxy

g ens

for p

lagi

ocla

se

Tabl

e 11

. R

epre

sent

ativ

e m

icro

prob

e an

alys

es u

sed

in f r

actio

nal c

ryst

allis

atio

n m

odel

s.

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48

Groups 1 and 2

Although different with respect to phenocryst mineralogy, samples from Group 1

and 2 define a broadly coherent trend on variation diagrams that suggest they

are probably genetically related. Three plausible fractionation paths that

include samples from both groups are considered. Path A (samples KSH08-

KSH03-K679-KSH02), path B (samples K2225-K803) and path C (samples

KSH01-K802) (Figure 24) model variation from basanite (Mg # 78) to

trachybasalt (Mg # 43). Several other samples from these two groups are

difficult to reconcile with a fractional crystallisation origin and suggest magma

mixing.

Path A (Samples KSH08-KSH03-K679-KSH02)

Examination of variation diagrams for a number of trace elements indicate that

Sr, Ba, Rb, LREE, Y, and Ga are incompatible throughout the entire

fractionation path (decreasing Mg #), and Cr and Sc behave compatibly. This

pattern of trace element variation suggests fractionation of clinopyroxene ±

olivine. Inflections in V, Ti and Fe at KSH03 may indicate the onset of spinel

fractionation. Zr, Hf and Th behave incompatibly from KSH08 to K679, yet

compatibly from K679 to KSH02, possibly due to minor accessory phase

fractionation. Nb and Ta also show inflections at this point attributable to a

change in spinel fractionation. Quantitative fractional crystallisation models that

incorporate these suggestions are presented as Table 12.

The most primitive sample, a basanite (Mg# 78 (KSH08) probably contains

cumulate olivine and clinopyroxene, as evidenced by the high modal

abundance of these phases and high forsterite content in some of the olivines.

A fractional crystallisation model that uses this sample as the parental magma

requires significant fractionation of clinopyroxene and olivine. Fractional

crystallisation is dominated by clinopyroxene and olivine with the addition of

minor Cr spinel for the parent / daughter pairs KSH03 / K679 and for K679 /

KSH02.

Trace element calculations for Rayleigh fractional crystallisation based on each

model of Path A show a high degree of consistency between calculated and

observed values (Figure 26). Slight discrepancies between observed versus

calculated REE for the model that links the most evolved magmas

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49

(trachybasalts; K679-KSH02) may reflect minor accessory phase fractionation

that was not included in the calculation.

Path B (Samples K2225-K803) and Path C (Samples KSH01-K802)

The two samples that constitute Path B are distinct from those that constitute

Path A in that they are richer in CaO than Path A samples with similar Mg #.

For the parent / daughter pair (K2225/K803) the general increase in the

abundance of a wide variety of trace elements (except for the transition

elements) suggests fractionation controlled by clinopyroxene ± minor olivine

and spinel. A quantitative fractional crystallisation model for this pair (Table 12)

has a sum of squares of 0.55, and a good agreement between calculated and

observed trace element abundances (Figure 27).

Path C is represented by samples KSH01 (Mg # 46) and K802 (Mg # 38). A

fractional crystallisation model that relates these two significantly evolved

magmas requires clinopyroxene, olivine and spinel fractionation (Table 12).

This suggestion is supported by the overall incompatible behaviour of Sr, Ba,

Al, Th, Pb, Na, Er, U and K, and the compatible behaviour of Sc and V

(indicating clinopyroxene, spinel ± olivine as fractionating phases). Trace

element calculations overlap between calculated and observed values for

almost all elements except Rb and Cs (Figure 28).

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Table 12. Fractional crystallisation models and trace element calculations for

Group 1 and 2 (Paths A, B and C), Group 3 and Group 4.

Fractional Crystallisation Models

Group 1 and 2PATH A

KSH08-KSH03 KSH03-K679 K679-KSH02Proportion of Phases FractionatedCpx 77 KSH05 Cpx8c avg 69.7 KSH03 Cpx4b avg 78.7 KSH03 Cpx4b avgOl 23 KSH05 Ol5c rim Fo82.2 16.0 KSH03 Ol3b avg Fo78.3 1.2 KSH03 Ol3b avg Fo78.3Sp 14.3 KSH03 Cr Sp 20.1 KSH03 Cr Sp

F 0.562 0.335 0.203R2 0.26 0.419 0.200

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 45.39 46.06 46.77 46.89 45.39 45.14 49.4 46.89 47.26MgO 10.34 16.83 16.8 6.35 10.34 10.35 4.42 6.35 6.25CaO 11.79 15.64 15.47 9.55 11.79 11.88 7.37 9.55 9.5

PATH B PATH CK2225-K803 KSH01-K802

Proportion of Phases Fractionated Proportion of Phases FractionatedCpx 66.8 K2225 Cpx3b avg Cpx 66.6 KSH01 Cpx1b avgOl 24.8 K2225 Ol3a avg Fo78.4 Ol 23.7 KSH01 Ol5b avg Fo64.9Sp 8.4 KSH03 MgTiSp Sp 9.7 KSH01 MgTiSp

F 0.307 F 0.153R2 0.552 R2 0.428

Major Element Calculations Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 44.81 44.23 44.52 SiO2 48.11 46.29 46.87MgO 5.7 8.64 8.6 MgO 3.71 5.64 5.6CaO 10.29 13.4 13.34 CaO 7.96 9.26 9.13

Group 3K813-K820 K820-K825

Proportion of Phases FractionatedCpx 96.8 K811 Cpx4c rim 78.6 K820 Cpx1c rimSp 3.2 K811 MgTiSp 21.4 K820 MgTiSp

F 0.27 0.176R2 0.181 0.239

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 41.58 41.89 41.81 42.32 41.58 41.45MgO 5.34 6.41 6.4 4.31 5.34 5.37CaO 11.68 14.41 14.47 10.23 11.68 11.73

Group 4K361-K897 K897-K894

Proportion of Phases FractionatedCpx 29.8 K361 Cpx1a core 45.4 K894 Cpx3a corePlag 50.8 K894 Plag2 An56.9 20.9 K894 Plag2 An56.9Ol 12.1 K361 Ol1a core Fo55 22.1 K361 Ol1b rim Fo49.8Sp 7.3 K361 MgTi Sp 11.6 K894 Ti Sp

F 0.381 0.324R2 0.298 0.355

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 44.04 44.73 44.98 45.84 44.04 44.23MgO 5.65 6.16 6.29 3.31 5.65 5.73CaO 10.32 11.53 11.39 8.58 10.32 10.21

F = amount of fractionation requiredR2 = sum of residuals squared

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Group 3 (K813-K820-K825)

The trend of Group 3 samples, from the Shira Ridge, range in composition from

Mg # 47 to 37. With respect to CaO versus Mg # (Figure 24), this group

displays a slightly steeper trend that is richer in CaO than Group 1 and 2, and

Group 4 samples of similar Mg #. This difference may reflect either a different

composition of, or an increased proportion of clinopyroxene fractionation in the

fractionating assemblage.

Sr abundances do not decrease with differentiation (decreasing Mg #) of Group

3 samples, indicating feldspar fractionation is negligible. Decreases in Cr and

Sc abundances with decreasing Mg # support clinopyroxene (± olivine) as the

fractionating assemblage with minor decreases in V and Ti indicating spinel

fractionation.

Fractional crystallisation models for the parent/daughter pairs K813/K820 and

K820/825 demonstrate that fractionation of clinopyroxene with minor spinel can

account for the major element changes observed (Table 12). No olivine or

plagioclase is required to create better quantitative models. Trace element

abundances calculated for these models (Table 12) agree with observed values

(except for Rb and Cs).

Group 4 (K361-K897-K894)

The highly evolved (Mg # 48 to 35) basanites from Platzkegel (Group 4) plot

between Group 3 and Group 1 and 2 with respect to CaO versus Mg # (Figure

24). The overall trend on this diagram is similar to that of Group 1 and 2

samples suggesting a similar scenario of fractional crystallisation. Sr content

decreases with decreasing Mg # in Group 4 samples suggesting feldspar

fractionation, with similar decreases in V, Ti, Cr and Sc abundances supporting

clinopyroxene, spinel (± olivine) fractionation.

Quantitative models of fractional crystallisation for the parent/daughter pairs

K361/K897 and K897/K894 support fractionation of clinopyroxene, plagioclase,

olivine and spinel (Table 12). Calculated abundances of trace elements for

these models agree well with observed values (Figure 30).

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Summary

The lithologic and chemical diversity observed in the Shira suite reflects low

pressure fractional crystallisation of dominantly ferromagnesian phases from

multiple, geochemically distinct ‘primary’ magmas. Clinopyroxene is the major

fractionating phase, with olivine and spinel included in most fractionating

assemblages. Plagioclase fractionation is negligible except in the evolution of

Group 4 (Platzkegel).

The shallow fractionation of ferromagnesian minerals has been identified as the

most important process during magma evolution in several East African Rift

volcanic provinces as well as many intraplate volcanic provinces globally.

Kabeto (2001) identified fractionation of olivine, clinopyroxene, apatite and Fe-

Ti oxides as responsible for the evolution of mafic Samburu Hills rocks, with the

addition of plagioclase to the fractionation assemblage in more evolved

samples. Similarly, the evolution of nephelinitic magmas from East Africa

nephelinitic volcanoes has been shown to incorporate significant fractionation of

olivine, clinopyroxene, nepheline and Fe-Ti oxides, with clinopyroxene

remaining on the liquidus over a wide range of melt temperatures (Peterson,

1989a;1989b). Shaw et al., (2003) and Ho et al., (2003) identified the

fractionation of clinopyroxene ± olivine, spinel and plagioclase as the most

important magmatic evolution processes within the intraplate volcanic provinces

of Jordan and China respectively, whilst Al c et al., (2002) identified

fractionation of dominantly olivine and clinopyroxene as responsible for major

element trends in extension related alkalic volcanism in Turkey.

Crustal Contamination / Magma Mixing Models

The absence of xenocrysts or xenoliths of crustal materials in both field and

petrographic observations, along with the absence of textural heterogeneities

such as cognate inclusions, banded lavas and sieve textures or corroded cores

in plagioclase phenocrysts (Gourgaud & Vincent, 2004) suggests that crustal

contamination is not significant in the evolution of Shira samples. Ratios of

specific trace elements also argue against crustal contamination in the Shira

volcanic rocks; Zr/Hf ratios between 35 and 60 (Figure 31a) are consistent with

mantle-derived intraplate basaltic rocks (Zr/Hf between 35 and 80) (Dupuy et

al., 1992), that are not contaminated with crustal material. In the Shira volcanic

suite, the highest Zr/Hf are observed in the most silica-undersaturated

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nephelinites (Group 3) (~50 to 60) with the other groups having lower ratios

(~35 to 50) but still considered as dominantly mantle-derived. Nb/Ta ratios are

lower in Group 3 samples, than Group 1, 2 and 4 samples, however all values

are between 14 and 18 (Figure 31a), and remain fairly constant within each

group, for all derivative magmas (Figure 31b) (decreasing Mg #). Nb/Ta ratios

for mantle–derived magmas range from 12.5 to 20 and are substantially

different from Nb/Ta ratios of the crust (8 to 12.5) (Green, 1995). The Nb/Ta

values of the Shira volcanic suite argue against crustal contamination.

30 40 50 60 70 80

13

15

17

19

Nb

/Ta

Mg/(Mg+Fe )2+

A)

30 40 50 60 70 8030

40

50

60

Zr/

Hf

Mg/(Mg+Fe2+)

B)

Man

tle-D

eri

vie

dN

b/T

aR

ati

os

Crustal Nb/Ta Ratios (8-12.5)

Figure 31. A) Plot of Zr/Hf versus Mg # showing ratios between 35 and 80,

consistent with mantle derivation. B) Plot of Mg # versus Nb/Ta showing that

Groups 1 and 2, Group 3 and Group 4 Nb/Ta ratios remain constant between

14 and 18, with evolution (decreasing Mg #).

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A linear trend apparent on the Mg# versus CaO diagram that suggests magma

mixing (Figure 32) (K804-KSH11-K2225). Petrographic, microprobe and LA-

ICP-MS observations identify complex major and trace element zonation

patterns in clinopyroxene (Figures 33 & 15) and plagioclase phenocrysts that

imply complex crystallisation paths. Such complexity is compatible with magma

chamber convection or magma mixing (Simonetti et al., 1996).

A successful magma mixing model was created that has a sum of squares

value of 0.75 (using the same weightings as fractional crystallisation models)

having slightly higher K2O, CaO, Na2O, SiO2 and MgO, and slightly lower Al2O3,

P2O5, FeO and TiO2 than the observed sample (Figure 34) (full results in

Appendix B). This model suggests that it is possible to create a composition

similar to KSH11 through the mixing of 16.3% of K2225 with 83.7% of K804

(Table 13). Trace element calculations performed using simple linear

calculations broadly support mixing of K2225 and K804 (Figure 35), with minor

discrepancies in the heavy REE and Sr results.

The complex zonation patterns and textures observed in clinopyroxene, as well

as the reverse and oscillatory zonation of plagioclase phenocrysts in the same

sample, can be attributed to decompression and complex crystallisation paths

(Gençalio lu Ku cu & Floyd, 2001). Together with the magma mixing model

suggested by the geochemical data, this evidence implies that some Shira

samples reflect complex patterns of magma convection or magma mixing in

sub-volcanic magma chambers.

40 50 60 70 80

8

10

12

14

16

CaO

Mg/(Mg+Fe )2+

KSH01

KSH02

KSH03

KSH04

KSH05

KSH06

KSH07KSH08

KSH09

KSH10

KSH11

K361

K679

K686

K802

K803

K804

K811

K813

K820

K825

K832A

K832B

K894

K895

K897

K2225

Figure 32. CaO versus Mg number plot showing the possible magma mixing

path K804-KSH11-K2225.

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13 X SEM Backscattered image - 15KVWorking Distance = 38mm

KSH05Clinopyroxene 1

1mm

KSH05 Clinopyroxene 1a REE Plot

1.00

10.00

100.00

La* Ce* Pr* Nd* Sm* Eu* Gd* Tb* Dy* Ho* Er* Tm* Yb* Lu*

RE

E/C

ho

nd

rite

(Su

n&

McD

on

ou

gh

1989)

KSH05-Cpx1a

KSH05-Cpx1b

KSH05-Cpx1c

KSH05-Cpx1d

KSH05-Cpx1e

KSH05-Cpx1f

KSH05-Cpx1g

KSH05-Cpx1h

KSH05-Cpx1i

KSH05-Cpx1j

KSH05-Cpx1k

a

j

b

c

h,ki

f,g

e

d

progressive enrichment

change from

Core Rim

a-b-c-d-e-f-g-h-i-j-k

KSH05 Clinopyroxene 1 Sc and Ce Variation

0

20

40

60

80

100

120

140

a b c d e f g h i j k

Core to Rim

Sc

pp

m

0

5

10

15

20

25

30

35

Sc Ce

Ce

pp

m

KSH05 clinopyroxene 1

A) B)

C)

Figure 33. A) Back scanned image of clinopyroxene 1 in sample KSH05,

showing analysed points and major element zonation. B) Sc and Ce variation

from core to rim. C) LA-ICP-MS reduced quantitative data showing the change

in degree of chondrite normalised (Sun & McDonough, 1989) REE enrichment

from core to rim.

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Table 13. Magma mixing models for mixing path K804-KSH11-K2225.

Groups 1 and 2 Magma Mixing ModelsK804-KSH11-K2225

Evolved Magma Observed Magma Calculated Magma Primitive MagmaPercent Mixing K804 KSH11 K2225K2225 0.00% 16.30% 100%K804 100.00% 83.70% 0%

R2 0.753

Major Element Calculations (weight %)K804 KSH11 K2225

observed observed calculated observedSiO2 49.61 47.79 48.75 44.23MgO 4.35 4.97 5.05 8.64CaO 8.05 8.69 8.93 13.4

R2 = sum of residuals squared

Major Element

Magma Mixing Model

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

K2O CaO Na2O SiO2 MgO MnO Al2O3 P2O5 FeO TiO2

KS

H1

1C

alc

ula

ted

/K

SH

11

Ob

se

rve

d

calculated sample normalised observed KSH11 normalised

Trace Element

Magma Mixing Model

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

U Sr

Yb

Sc

Lu Er Y

Dy

Hf

Zr

Rb

Gd

Th

Nd

Sm Ce

Eu

La

Ta

Nb

Ba V

Pb

Cs

KS

H11

Calc

ula

ted

/K

SH

11

Ob

serv

ed

calculated sample normalised observed KSH11 normalised

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Figure 34. Comparison of major and trace element magma mixing models

results normalised to KSH11 (the observed mixed magma)

1

10

100

1000

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Co

nc

en

tra

tio

n/C

ho

nd

rite

(Su

n&

Mc

Do

no

ug

h1

98

9)

K804-observed KSH11-observed KSH11-calculated K2225-observed

1

10

100

1000

Cs Rb Ba Th U Nb K La Ce Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu

Co

ncen

trati

on

/Pri

mit

ive

Man

tle

(Su

n&

McD

on

ou

gh

1989)

K804-observed KSH11-observed KSH11-calculated K2225-observed

Figure 35. A) Chondrite normalised (Sun & McDonough, 1989) REE diagram

and B) Primitive mantle normalised (Sun & McDonough, 1989) multi element

spider diagram for the magma mixing model K804-KSH11-K2225.

A

B

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Primitive Magmas, Melting and Sources

The lack of primitive and primary samples (aphanitic, Mg # 68-72, high Ni, Cr,

Co and Sc) (Frey et al., 1978) in the Shira suite limits the determination of

partial melting conditions and consequently, source characteristics. To

overcome this limitation, two approaches are followed to calculate primary

magma compositions. The first approach corrects non-primary magmas to

‘primary’ compositions by the addition of olivine in one percent increments until

the magma has an Mg # of 71 (Frey et al., 1978). This approach requires 16%

olivine addition for KSH03, 23% for K2225, 38% for K813 and 36% for K832a

(Figure 36) (these samples are chosen as they are the most primitive of groups

1, 3 and 4). These ‘primary’ samples have low CaO abundances (8-10%), with

two of the ‘primary’ basanites having higher CaO abundances than the ‘primary’

nephelinite. Trace element abundances are corrected using the Rayleigh

fractional crystallisation rule (Figure 37) rather than by simple dilution.

The addition of such quantities of olivine is simplistic as the fractional

crystallisation models calculated for the more evolved magmas indicate the

importance of clinopyroxene and/or spinel fractionation. Thus a second method

of fractionation correction uses phases theoretically determined to be on the

liquidus of the chosen samples. Modelling of crystallisation histories for

selected Shira samples involved the use of MELTS (Java applet version 1.1.0),

a program written by Mark S. Ghiorso (Ghiorso & Sack, 1995; Asimow &

Ghiorso, 1998; Ghiorso et al., 2002). Models were created for samples KSH03,

K2225, K813 and K832a using 0.2%, 1% and 2% H2O, 1 and 5 kb pressures,

oxygen fugacity (fO2) of quartz-fayalite-magnetite (QFM) over temperatures

from liquidus to 100°C below the liquidus in 5°C intervals. These models show

that clinopyroxene is either on, or very close to the liquidus for all four modelled

samples.

MELTS results thus indicate that both olivine and clinopyroxene probably

fractionated from the primary magma to produce the analysed samples. The

amount of clinopyroxene that fractionated is difficult to determine, and thus a

database of primary and primitive melilitites, nephelinites, basanites and alkali-

olivine basalts was compiled to establish the maximum and minimum CaO

contents for a range of primitive magmas at Mg # 71 (Figure 38). These CaO

values are used to determine the maximum and minimum amounts of

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clinopyroxene fractionation (in addition to olivine) required to reach these limits.

The compilation demonstrates that primary nephelinites and melilitites generally

contain higher CaO than basanites and alkali-olivine basalts at Mg # 71. The

Shira samples that are modelled are nephelinites and basanites; corresponding

CaO limits at Mg # 71 are set at 11.75%-13% and 9.5%-11.75% respectively.

The general direction of the olivine vector was known from the first approach.

When combined with the minimum and maximum CaO end points determined

from the database of primary and primitive samples, a point mid range is

portrayed that has to be reached by clinopyroxene fractionation. The

appropriate quantity of clinopyroxene (using appropriate phenocryst core

compositions) was added in 1% increments until the olivine addition vectors

graphically corresponded with the CaO limits set by the primary and primitive

sample database (Figure 39). Although addition of clinopyroxene and olivine

has been modelled as two separate stages, both were most likely fractionating

concurrently. The resultant vector produced through the combination of the two

separate stages reflects the more complex fractionation path.

Modelling of clinopyroxene and equilibrium olivine addition in order to reach the

compositional limits of the ‘primary’ nephelinites and basanites show olivine is

the dominant fractionating phase. Addition of between 12% and 18%

clinopyroxene and 26% and 21% olivine is required for K813 to reach the upper

and lower limits of the ‘primary’ nephelinite field. Sample KSH03 requires 0% to

10% clinopyroxene addition and 16% to 10% olivine addition to reach the limits

of the ‘primary’ basanites field. Sample K2225 (basanite) requires 0% to 6%

clinopyroxene addition, and 23% to 19% olivine addition, and sample K832a

(basanite) requires 4 to 16% clinopyroxene addition and 32 to 21% olivine

addition.

Trace element concentrations, corrected for both minimum and maximum

clinopyroxene addition using Rayleigh fractional crystallisation, are nearly

identical (possibly due to the low amount of clinopyroxene added relative to

olivine) (Figure 40) (full results in Appendix C).

The fractionation model was then ‘checked’ by additional MELTS models to

determine which model best represents ‘primary’ magmas for samples KSH03,

K2225, K813 and K832a (Table 14). Crystallisation sequence modelling was

performed utilising variables of 1, 5, 10 and 20 kb pressures, 0.2%, 1% and 2%

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H2O, oxygen fugacity (fO2) of quartz-fayalite-magnetite (QFM) over

temperatures from liquidus to solidus in 20°C intervals (Figure 41). Models

were then qualitatively assessed through comparison of determined

crystallisation paths and proportions of phases added to account for

fractionation correction.

Low pressure models showed that olivine was generally the first phase

fractionated, but was followed very closely by clinopyroxene. Increasing

pressure did not significantly alter the olivine liquidus, however it did increase

the temperature of the clinopyroxene liquidus such that the crystallisation

sequence begins with clinopyroxene in models where the pressure was greater

than 20kb. Increasing the H2O content had the opposite effect, reducing the

temperature of the clinopyroxene liquidus far more than the olivine liquidus.

Four models are selected (one for each ‘primary’ magma) based on this method

(Figure 41), which qualitatively represents the crystallisation sequence,

proportion of phases added, and mineralogy of the original samples to which

olivine and/or clinopyroxene has been added. The models which crystallised

equivalent quantities of the phases added (clinopyroxene and olivine) before

reaching compositions similar to the ‘uncorrected’ samples were selected.

MELTS models show that sole olivine addition is too simplistic for samples

K813 and K832a, as at the clinopyroxene liquidus only 20 to 25% olivine has

crystallised, however 36% and 38% olivine has been added to reach the

‘primary’ compositions respectively. Models involving maximum clinopyroxene

addition were also disregarded for samples K813 and K832a, as at the point

where 18% and 16% clinopyroxene has crystallised (the quantity of

clinopyroxene added respectively) the amount of olivine crystallised is 12 to

15%, whereas 21% olivine was added to both samples. The models involving

minimum quantities of clinopyroxene addition were accepted for samples K813

and K832a, as the crystallisation sequences and proportion of phases

crystallised were broadly comparable with the added phases. Both models for

samples K2225 and KSH03 appear equally plausible, with crystallisation

sequences and proportion of phases crystallised comparable to the amount of

olivine, and olivine/clinopyroxene added. Due to the low volume of

clinopyroxene added in these samples, and both models appearing equally

plausible, the simplest method involving addition of olivine only has been

accepted. In summation, the models which correlated closest with determined

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crystallisation sequences were K2225+23% olivine, KSH03+16% olivine,

K813+12% clinopyroxene + 26% olivine and K832a+4% clinopyroxene + 32%

olivine, and will hence be used in partial melting models.

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.

Primitive melilitites

Primitive nephelinites

Primitive basanites

Primitive alkali olivine basalts

CaO

40 50 60 70 806

8

10

12

14

16

Mg/(Mg+Fe )2+

Melil

itite

Nephelin

ite

Basanite

Alk

ali-

oliv

ine

basalt

Figure 38. Compilation of primitive melilitites, nephelinites, basanites and alkali olivine

basalts showing their CaO ranges at Mg number 71 (sourced from Brey, 1978; Frey

., 1978; Phelps ., 1983; Kempton ., 1987; Jung, 1998; and Spath ., 2001).

et

al et al et al et al

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Table 14. Corrected “primitive” magma major element compositions for both

correction methods used in MELTS crystallisation sequence modelling

(minimum clinopyroxene addition models for K2225 and KSH03 required no

clinopyroxene and were identical to olivine addition model). Sample K2225 K2225 KSH03 KSH03 K813 K813 K813 K832a K832a K832aOlivine added 23% 19% 16% 10% 38% ol 26% 21% 36% ol 32% 21%Clinopyroxene added 6% 10% 12% 18% 4% 16%

SiO2 42.68 43.38 43.85 45.04 40.42 41.87 42.68 43.49 43.95 45.44Al2O3 10.25 10.03 11.31 10.81 9.64 9.57 9.38 10.08 9.88 9.66TiO2 1.75 1.72 1.68 1.60 1.80 1.78 1.75 1.59 1.59 1.55Fe2O3 2.23 2.13 2.14 1.99 2.47 2.23 2.12 2.44 2.37 2.14FeO 11.35 10.84 10.94 10.16 12.58 11.40 10.80 12.46 12.06 10.89MnO 0.17 0.16 0.18 0.17 0.20 0.20 0.19 0.15 0.15 0.15MgO 16.28 15.61 15.63 14.47 18.53 16.49 15.66 18.11 17.45 15.54CaO 10.42 11.35 9.83 11.51 9.55 11.64 12.69 8.71 9.38 11.53Na2O 2.64 2.59 1.72 1.64 2.07 2.05 2.01 2.31 2.30 2.25K2O 0.44 0.43 0.82 0.78 0.52 0.52 0.50 0.29 0.29 0.28P2O5 0.39 0.38 0.37 0.36 0.51 0.51 0.50 0.38 0.38 0.37Totals 98.56 98.59 98.44 98.52 98.18 98.20 98.24 99.92 99.74 99.75

Mg # 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71

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Partial Melting Models

Compositional differences of primary/primitive alkalic rocks are traditionally

attributed to differences in the degree of partial melting, source composition and

crustal contamination (Frey et al., 1978). In a now classic study, Frey et al.,

(1978) concluded that a single enriched source composition could yield olivine

melilitite (4-6% melting), olivine nephelinite to basanite (5-7% melting), alkali-

olivine basalt (11-15% melting) and olivine basalt to olivine tholeiite (20-25%

melting). Although contents of H2O and CO2 are critical factors for low degree

melts and are important in the genesis of strongly silica-undersaturated olivine

melilitites and nephelinites (Green, 1969; Brey & Green, 1976; 1977; Brey,

1978; Eggler, 1978; Frey et al., 1978; Wyllie, 1987), their precise effect is not

quantified. The contribution of these volatile phases to the petrogenesis of the

Shira volcanic rocks may therefore be important, but difficult to assess.

The trace element patterns of corrected ‘primitive’ samples show near parallel

trends (Figures 37 & 40), differing only in degree of enrichment. Degree of

enrichment increases from K832a (Group 4) to K2225 and KSH03 (Group 1), to

K813 (Group 3), with abundances increasing from 128 to 219 times chondrite

La, and 8 to 10.5 times chondrite Lu. These differences may be due to different

degrees of partial melting, with nephelinite being produced by lower degrees of

melting than basanite, resulting in greater incompatible element enrichment.

Negative K anomalies are observed for all samples including fractionation-

corrected samples (Figures 20, 37 & 40). Potassium depletion relative to

elements of similar incompatibility (e.g. Th, Nb and La) is common in intraplate

volcanics, and is explained most easily by either fractionation of a K-bearing

phase or retention of K in the source during partial melting. As no significant K-

bearing phases are identified in Shira rocks, retention of a K-bearing phase in

the source during partial melting is the most plausible explanation. Zhang and

O’Reilly (1996) identified phlogopite retention as the most likely reason for

similar characteristics in eastern Australia samples. Similarly, either residual

amphibole or phlogopite is identified as the most likely cause of K depletions in

Grand Comore magmas (Indian Ocean) (Deniel, 1998) and South African

melilitites (Rogers et al., 1992). Amphibole and phlogopite retention in mantle

source regions of alkalic basalts is postulated for many regions globally (e.g.

Mertes & Schmincke, 1985; Wilson & Downes, 1991; Hoernle & Schmincke,

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1993; Class et al., 1994; Spath et al., 1996; Class & Goldstein, 1997) and is

supported by both experimental evidence, and their occurrence in mantle

derived xenoliths and as mantle xenocrysts in alkalic rocks (e.g. Best, 1974;

Wilshire et al., 1980; Erlank et al., 1987; Gamble & Kyle, 1987; Harte et al.,

1987).

Amphibole and phlogopite are identified in numerous mantle xenoliths from

Kenya and northern Tanzania (Dawson et al., 1970; Dawson & Smith, 1973;

Dawson & Smith, 1988; Henjes-Kunst & Altherr, 1992; Johnson et al., 1997).

Primitive mantle normalised multi-element spider diagrams of Shira volcanic

rocks are very similar to the neighbouring Chyulu Hills Volcanic Province (Spath

et al., 2001). Spath et al., (2001) attributes the K depletion in the Chyulu Hills

volcanic rocks to residual amphibole during non-modal equilibrium batch partial

melting of an amphibole-bearing spinel lherzolite (53% olivine, 22%

orthopyroxene, 18% clinopyroxene, 2% spinel, 5% amphibole melting in mode

of 5%, 10%, 33%, 2% and 50% respectively). Forward modelling indicates that

the relative K depletion and REE trends can not be accounted for by partial

melting of a four phase spinel lherzolite (Spath et al., 2001).

As the negative K anomalies (relative to elements of similar incompatibility)

observed in ‘corrected’ primary Shira samples cannot be accounted for through

partial melting of a four phase spinel lherzolite, sources require residual

phlogopite or amphibole. K, Rb and Ba concentrations are consistent with

derivation from a kaersutitic or pargasitic amphibole rather than a phlogopite-

bearing source (Beswick, 1976; Spath et al., 2001). The REE trends of

‘corrected’ primary Shira magmas are very similar, differing only in degree of

enrichment. Chondrite normalised REE enrichment decreases from La to Er,

then remains rather constant at 10 times chondritic LREE (Er to Lu)

concentrations (Figures 37a & 40a). This style of REE enrichment is indicative

of derivation form a spinel, rather than garnet lherzolite source, and when

combined with K anomalies, suggest that the Shira lavas, like the nearvy

Chyulu Hills lavas (Spath et al., 2001) are most likely sourced from an

amphibole-bearing spinel lherzolite.

Fractional and/or continuous/dynamic melting (Langmuir et al., 1977; McKenzie,

1985; Albarede, 1995; Zou & Zindler, 1996; Shaw, 2000) of garnet-bearing and

garnet-free sources with low residual source porosity produce REE trends

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73

significantly different to those observed in the Shira volcanic rocks. To produce

compositions comparable to the Chyulu rocks, melting models that involve the

pooling and accumulation of melt fractions (i.e. aggregated fractional melting

and continuous melting with high residual source porosity) are required. These

variations in melting models produce very similar results to simple batch melts

(Spath et al., 2001; Maaløe & Pedersen, 2003). Therefore, although open to

criticism on the grounds that the actual process may be too simplistic and

therefore unreal, the minimal difference between results from simple batch

partial melting calculations, and more complex models (Spath et al., 2001;

Maaløe & Pedersen, 2003), coupled with the uncertainty inherent in using

‘corrected’ primary magmas justifies the use of simplistic batch melting models

to speculate on the petrogenesis of the Shira rocks.

Reverse partial melting models of both modal and non-modal equilibrium batch

melting (CL/CO=1/[DRS+F(1-DRS)]) of an amphibole-bearing spinel lherzolite were

created (Figures 42 & 43). These models utilise the modal composition of

Spath et al., (2001) (53% olivine, 22% orthopyroxene, 18% clinopyroxene, 2%

spinel, 5% amphibole) and partition coefficients presented in Table 10. The

models show that the production of fractionation-corrected ‘primary’ Shira

samples can best be accounted for through modal equilibrium batch melting

(Figure 42). Degrees of melting required to produce nephelinites and basanites

from an enriched source are very similar to values calculated by Frey et al.,

(1978) for the alkalic rocks of south eastern Australia. Sample K813

(nephelinite) requires 4% partial melting, KSH03 and K2225 (basanites) require

7% partial melting, whereas K832a (basanite) requires 10% partial melting.

The same degrees of melting also produce the closest correlation with non-

modal equilibrium batch melting (Figure 43). Using these degrees of partial

melting in non-modal batch melting calculations, produce source patterns

similar in appearance to modal melting, but with a greater degree of variation in

source concentrations (full results in Appendix D).

Forward modelling was used to confirm if the observed negative K anomalies

could be created by retaining amphibole during partial melting. Sample K2225

was modelled as it is the least phyric Shira sample analysed. The calculated

trace element source concentrations for corrected sample K2225 (7% modal

equilibrium batch melting) and an assumed source K concentration of 0.2%

were used (correlating with the degree of enrichment of elements with similar

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74

incompatibility (Nb, Th and La)). Modal equilibrium batch melting of between

1% and 10% was used with partition coefficient values adjusted such that the

negative K anomaly observed could be reproduced (Figure 44). By using

DKamphibole partition coefficients of between 3 and 15, (such that K preferentially

remains in the solid during melting) similar sized anomalies were produced (full

results in Appendix E).

These models support modal equilibrium batch melting of between 4 and 10%

(calculated values for corrected samples) of an enriched amphibole-bearing

spinel lherzolite, in which amphibole is residual to produce ‘primary’ Shira

magmas.

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75

1

10

100

1000

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Co

ncen

trati

on

/Ch

on

dri

te(S

un

&M

cD

on

ou

gh

1989)

PM OIB Av Sp LherzK813 + 12%cpx+26%ol source K813-4% melt KSH03 + 16%olsource KSH03-7% melt K2225 + 23%ol source K2225-10% meltK832a + 4%cpx+32%ol source K832a-10% melt

1

10

100

1000

Cs Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu

Co

ncen

trati

on

/Pri

mit

ive

Man

tle

(Su

n&

McD

on

ou

gh

1989)

OIB K813 + 12%cpx+26%ol source K813-4% melt

KSH03 + 16%ol source KSH03-7% melt K2225 + 23%ol

source K2225-7% melt K832a + 4%cpx+32%ol source K832a-10% melt

1

10

100

1000

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Co

ncen

trati

on

/Ch

on

dri

te(S

un

&M

cD

on

ou

gh

1989)

PM OIB Av Sp LherzK813 + 12%cpx+26%ol source K813-4% melt KSH03 + 16%olsource KSH03-7% melt K2225 + 23%ol source K2225-10% meltK832a + 4%cpx+32%ol source K832a-10% melt

1

10

100

1000

Cs Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu

Co

ncen

trati

on

/Pri

mit

ive

Man

tle

(Su

n&

McD

on

ou

gh

1989)

OIB K813 + 12%cpx+26%ol source K813-4% melt

KSH03 + 16%ol source KSH03-7% melt K2225 + 23%ol

source K2225-7% melt K832a + 4%cpx+32%ol source K832a-10% melt

Figure 42. Reverse modal equilibrium batch melting

models of corrected samples K813 (nephelinite), K2225,

KSH03 and K832a (basanites). Models calculated source

trace element abundances by using variables of between

0.001%, 0.01%. 0.1% and 1% to 20% (in 1% increments).

Models chosen are those in which the source trace

element abundances were as similar as possible.

Models indicate that through modal batch melting of a

relatively homogenous source with LREE enrichment

~ 20 x chondrite, and HREE enrichment ~ 3 to 4 x

chondrite it is possible to reproduce the ‘corrected’

primary magma trace element concentrations through

4% melting for nephelinite (K813), 7% melting for Group

1 basanites (K2225 & KSH03) and 10% melting for Group

4 basanite (K832a). A) Chondrite normalised

(Sun & McDonough, 1989) REE diagram, B) Primitive

mantle normalised (Sun & McDonough, 1989) multi-

element spider diagram (Mode = 53% olivine, 22%

orthopyroxene, 18% clinopyroxene, 2% spinel and 5%

amphibole).

A

B

A

B

Figure 43. Reverse non-modal equilibrium batch melting

models of corrected samples K813 (nephelinite), K2225,

KSH03 and K832a (basanites). Models calculated source

trace element abundances by using variables of 0.001%,

0.01%. 0.1% and 1% to 20% (in 1% increments).

Models chosen are those in which the source trace

element abundances were as similar as possible.

Models indicate that through non-modal batch melting of a

relatively homogenous source with LREE enrichment

~ 20 to 30 x chondrite, and HREE enrichment ~ 3 to 4 x

chondrite (with positive Eu, Dy and Yb anomalies) it is

possible to reproduce the ‘corrected’ primary magma

trace element concentrations through 4% melting for

nephelinite (K813), 7% melting for Group 1 basanites

(K2225 & KSH03) and 10% melting for Group 4 basanite

(K832a). A) Chondrite normalised (Sun & McDonough,

1989) REE diagram, B) Primitive mantle normalised

(Sun & McDonough, 1989) multi-element spider diagram

(Mode = 53% olivine, 22% orthopyroxene, 18%

clinopyroxene, 2% spinel and 5% amphibole melting in

mode of 5%, 10%, 33%, 2% and 50% respectively).

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1

10

100

1000

Cs Rb Ba Th U Nb K La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu

Co

ncen

trati

on

/Pri

mit

ive

Man

tle

(Su

n&

McD

on

ou

gh

1989)

1 3 5

7 10 K2225+23%ol

K2225_Source_7%melt

Figure 44. Forward modal equilibrium batch melting of calculated source for corrected

sample K2225 (assuming 7% partial melting required to produce this sample) showing the size

of the K anomaly produced through between 1 and 10% partial melting in which K remains

residual in amphibole (D = 6). (source K value was assumed at 0.2% in order to create a

smooth trend with elements of similar incompatibility Nb, Th, and La) shown on a primitive

mantle normalised (Sun & McDonough, 1989) multi-element spider diagram.

K

amphibole

Source Characteristics and Formation

Seismic, tomographic and petrologic studies of southeastern Kenya indicate

average crustal thicknesses between 40km and 43km, with total lithospheric

thicknesses of approximately 105km to 115km (Henjes-Kunst & Altherr, 1992;

Novak et al.,1997a; 1997b; Ritter & Kaspar, 1997). Velocity perturbations

observed between 40km and 70km are interpreted as partial melts within the

upper mantle (Novak et al., 1997b).

As the spinel-garnet transition occurs between 21kb (1100°C) and 24kb

(1300°C) (Green & Ringwood, 1967b, 1970), representing depths of

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77

approximately 65-80km, and the plagioclase-spinel transition occurs at

approximately 10kb (30km) Shira sources are constrained to between 30 and

90km depth. Pargasitic or kaersutitic amphibole upper stability limits range

from 21kbar to 30kbar (e.g. Green & Ringwood, 1970; Boettcher et al., 1975;

Olafsson & Eggler, 1983; Mengel & Green, 1986) representing depths of

approximately 70 to 90km. This stability range is consistent with the derivation

of the Shira magmas from a spinel lherzolite source that is part of the sub

continental lithosphere (to approximately 115km depth). Thus the Shira

volcanic rocks reflect melting of an enriched sub continental lithosphere rather

than an asthenospheric source.

Isotopic studies of numerous volcanic centres in northern Tanzania (Paslick et

al., 1995) suggest ancient (>1Ga) underplating and metasomatism of the

continental lithosphere by OIB melts is responsible for enrichment of the sub

continental lithosphere. Megacryst vein studies identify metasomatism by

alkaline silicate and possibly carbonatite melts beneath northern Tanzania

(Johnson et al., 1997). No constraint is available for the timing of this

metasomatic event. Asthenospheric-sourced carbonatite melts are identified as

responsible for metasomatism of peridotite xenoliths from Olmani cinder cone,

northern Tanzania (Rudnick et al., 1993) with numerous other authors

postulating metasomatism of the lithospheric mantle in other East African

regions (e.g. Vollmer & Norry, 1983a, 1983b; Cohen et al., 1984; Rogers et al.,

1992; Furman, 1995). Fractionation-corrected Shira samples have trace

element patterns similar to OIB-melts, although slightly less enriched and with K

and Rb anomalies (Figure 45). An age of >1Ga for OIB-like continental

lithosphere enrichment (Paslick et al., 1995) implies that source enrichment

occurred long before the onset of rifting (Figure 46a). Although additional

recent enrichment by plume-derived OIB melts may have occurred (i.e. Spath et

al., 2001), the source of the Shira magmas probably acquired its enrichment

signature from ancient (>1Ga) OIB underplating and metasomatism.

It is argued that ancient metasomatism created a sub continental lithosphere

that was capable of producing highly-enriched silica under-saturated magmas

by low degrees of partial melting. The ultimate cause of this melting event

however, remains unclear. The EAR is the result of ‘active’ rifting processes in

which a plume of asthenospheric mantle ascends, and undergoes

decompression melting (Kampunzu & Mohr, 1991; Spath et al., 2001).

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Kilimanjaro is located well away from the rift axis, thus it is expected that the

impact of the asthenosperic plume may be less. The major effect however,

would be the provision of heat (Turner et al., 1996), and probably volatiles to

the sub continental lithosphere to induce partial melting. In particular, the

introduction of volatiles such as H2O and CO2 would both lower the solidus

(Brey, 1969; Brey & Green, 1976) of the sub continental lithosphere and

stabilise amphibole, as well as providing additional incompatible element

enrichment. Thus while the Shira magmas are sourced from the sub

continental lithosphere, the ultimate reason for their generation lies in the

interaction between the upwelling asthenospheric mantle and the

subcontinental lithosphere

.1

1

10

CsRbBaTh U Nb K LaCe Sr P Nd ZrSmEu Dy Y YbLu

Rock/OIB

Figure 45. Fractionation corrected “primitive” Shira samples normalised to Sun & McDonough

(1989) OIB values.

Upon segregation from their source, melts follow structural weaknesses in the

lithosphere (Figure 46b) before ponding at shallow depths (1-5kb) in small

volume magma chambers, being subject to significant ferromagnesian

fractionation (Figure 46c). Fractionation paths result from separate,

geochemically distinct ‘primary’ magmas, in which the fractional crystallisation

of basanites (samples K2225, KSH03 and K832a) is initially dominated by

olivine, becoming clinopyroxene dominated between Mg # 45 and 60.

Nephelinite fractional crystallisation is initially subject to both olivine and

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clinopyroxene fractionation, becoming increasingly dominated by clinopyroxene

fractionation with evolution. Magma mixing by subsequent partial melts or

convective currents within magma chambers is thought to occur episodically,

resulting in oscillatory zoned phenocrysts.

The Shira volcanic centre has since collapsed, forming the present caldera, and

been intruded by late stage dykes, sills and vent infill (Platzkegel) before

cessation (Figure 46d).

Individual nephelinite cones in intraplate volcanic provinces (i.e. southeastern

Australia, various regions in East Africa, New-Mexico etc.) may simply

represent the extrusion of short-lived, single melts. Large mixed-association

continental rift stratovolcanoes however may represent longer-lived polygenetic

volcanoes, formed through the extrusion of many successive melts.

Polygenetic volcanoes may form due to their positions relative to magmatic

conduits (i.e. major faults in the case of Mt Kilimanjaro), raised geothermal

gradients or source region differences (i.e. extent of metasomatism,

mineralogy).

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Dehydration melting and structural weakening forming the initialdepression and magmatic conduits to the crust

(i.e. Kilimanjaro depression).

Crust

B

Rising asthenospheric plume

0km

40km

105km

SCLM

Metasomatised SCLMVariable partial melts (4-10%)

Crust

Introduction of volatiles to the sub continentallithospheric mantle (SCLM), enriched by ancient

OIB underplating and melts

A

Rising asthenosphericplume

0km

40km

105km

The collapse of Shira, followed by the eruption of Platzkegellavas and intrusion of various dykes/sills. This is in turn followed

by the cessation of the Shira and Mawenzi volcanic centresresulting in Kilimanjaro’s present morphology

Crust

SCLM

ShiraKibo

Mawenzi

D

Platzkegel

Small volume magma chambers ponding and undergoingextensive fractional crystallisation in the crust. Differring degrees ofpartial melting are responsible for nephelinites and basanites, with

cumulate samples erupted upon either emptying of magma chamber oras a result of magma mixing. Numerous partial melts (most likely of short

lifespans) create a polygenetic volcano, resulting in the formation ofKilimanjaro and its three main volcanic centres (Shira, Kibo and Mawenzi).

Crust

SCLM

ShiraKibo

Mawenzi

Magma chambers(shallow fractional

crystallisation)

Figure 46. Proposed model for the genesis and evolution of Mt Kilimanjaro starting withA) Introduction of volatile phases to a previously enriched subcontinental lithospheric mantle resultingfrom upwelling asthenosphere. B) Lithospheric faulting and low degree partial melts of the SCLMfollowing structural weaknesses, with larger polygenetic volcanoes occurring along major faults, wheremultiple partial melts may easily ascend and smaller monogenetic volcanoes occurring along minorfaults or structural weaknesses. C) Fractional crystallisation of small volume melts occurringpredominantly in small, shallow magma chamber, erupting to form Shira, Kibo and Mawenz.D) Formation of Shira caldera and Platzkegel followed by the cessation of Shira and Mawenziresulting in the present morphology of Mt Kilimanjaro (not to scale).

C

CONCLUSION

The Shira volcanic suite consists of silica-undersaturated nephelinites,

basanites, picro-basalts and hawaiites (Mg #’s ranging from 35.5 to 77.2).

Groups identified on the basis of phenocryst assemblages and textures

correlate with geographic location. Samples (East Shira Hill) contain olivine

and clinopyroxene phenocrysts + microphenocrysts of plagioclase (Group 1), or

plagioclase and clinopyroxene phenocrysts + microphenocrysts of olivine

(Group 2). Samples with high Mg #’s contain abundant cumulate clinopyroxene

and olivine (Fo92-Fo85). Group 3 samples (Shira Ridge) contain nepheline

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phenocrysts and Group 4 samples (Platzkegel) have distinct intergranular

textures.

Trends on many geochemical diagrams identify fractional crystallisation paths

reflecting fractionation of clinopyroxene ± olivine and spinel. Complex major

and trace element zonation in clinopyroxene and feldspar phenocrysts suggest

magma mixing.

Primitive samples corrected to ‘primary’ compositions indicate low degrees of

partial melting of between 4% and 10%. Trace element abundances similar to

OIB-melts, with negative K anomalies require retention of amphibole in the

source during partial melting. REE trends are compatible with an amphibole-

bearing spinel lherzolite source. When combined with local geophysical and

mineralogical studies, this conclusion implies a source in the sub continental

lithospheric mantle. The enrichment of this source reflects ancient (>1Ga) OIB

under-plating and metasomatism. Rising asthenosphere provides a source of

heat as well as volatiles, necessary in inducing melting and stabilising hydrous

K-bearing phases.

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APPENDIX A

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APPENDIX A – FRACTIONAL CRYSTALLISATION MODELS

Group 1 – Path A. Fractional crystallisation models and trace element

calculations for Path A (KSH08-KSH03-K679-KSH02).

Groups 1 and 2 Fractional Crystallisation Models

KSH08-KSH03 KSH03-K679 K679-KSH02Proportion of Phases FractionatedCpx 77 KSH05 Cpx8c avg 69.7 KSH03 Cpx4b avg 78.7 KSH03 Cpx4b avgOl 23 KSH05 Ol5c rim Fo82.2 16.0 KSH03 Ol3b avg Fo78.3 1.2 KSH03 Ol3b avg Fo78.3Sp 14.3 KSH03 Cr Sp 20.1 KSH03 Cr Sp

F 0.562 0.335 0.203S2 0.26 0.419 0.200

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 45.39 46.06 46.77 46.89 45.39 45.14 49.4 46.89 47.26TiO2 2.01 1.52 1.22 2.27 2.01 2.11 2.06 2.27 2.08Al2O3 13.56 7.67 7.74 15.8 13.56 13.02 17.06 15.8 15.43FeO 13.2 10.44 10.27 12.49 13.2 13.25 12.07 12.49 12.57MnO 0.21 0.16 0.19 0.21 0.21 0.16 0.2 0.21 0.16MgO 10.34 16.83 16.8 6.35 10.34 10.35 4.42 6.35 6.25CaO 11.79 15.64 15.47 9.55 11.79 11.88 7.37 9.55 9.5Na2O 2.06 1.06 0.95 3.64 2.06 2.42 4.43 3.64 3.53K2O 0.98 0.38 0.49 2.06 0.98 1.4 2.25 2.06 1.82P2O5 0.45 0.24 0.2 0.73 0.45 0.54 0.73 0.73 0.62

Trace Element CalculationsParent Daughter Calc. Daughter Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter

Sc 52.29 34.23 19.57 34.23 19.77 20.95 19.77 13.97 13.99V 254.00 276.81 579.91 276.81 235.06 416.26 235.06 180.06 294.93Cr 1571.65 316.44 72.59 316.44 99.07 68.73 99.07 35.39 39.53Rb 11.09 22.35 24.75 22.35 54.02 32.66 54.02 62.75 66.44Sr 329.97 671.84 677.92 671.84 877.47 926.59 877.47 945.01 1037.23Y 14.82 22.64 19.06 22.64 28.34 26.04 28.34 29.74 30.00Zr 105.55 194.15 196.51 194.15 296.65 216.31 296.65 279.91 299.32Nb 33.64 63.50 76.41 63.50 107.72 93.09 107.72 97.42 132.59Cs 0.11 0.24 0.24 0.24 0.54 0.35 0.54 0.56 0.67Ba 188.02 455.77 416.12 455.77 690.43 660.69 690.43 781.96 844.50La 24.83 46.78 52.09 46.78 71.85 67.00 71.85 75.85 87.32Ce 45.89 85.65 98.71 85.65 131.61 110.64 131.61 137.00 147.27Nd 22.11 39.39 43.56 39.39 55.55 49.35 55.55 58.10 61.06Sm 4.32 7.15 7.42 7.15 9.85 8.60 9.85 9.68 10.59Eu 1.29 2.13 2.02 2.13 2.72 2.67 2.72 2.64 3.04Gd 3.85 5.99 6.16 5.99 7.53 7.69 7.53 7.60 8.55Dy 2.97 4.47 4.67 4.47 5.52 5.69 5.52 5.65 6.24Er 1.4 2.1 2.20 2.10 2.67 2.67 2.67 2.79 3.02Yb 1.14 1.68 1.83 1.68 2.23 2.00 2.23 2.51 2.39Lu 0.16 0.26 0.26 0.26 0.34 0.34 0.34 0.37 0.39Hf 2.58 4.48 4.26 4.48 6.42 5.71 6.42 5.97 7.24Ta 2.03 3.69 4.39 3.69 6.41 5.24 6.41 6.14 7.73Pb 4.67 6.5 10.66 6.50 8.66 9.77 8.66 9.57 10.87Th 2.8 4.89 6.13 4.89 9.38 7.14 9.38 8.83 11.54U 0.58 0.88 1.26 0.88 1.12 1.28 1.12 1.65 1.37

F = amount of fractionation requiredS2 = sum of residuals squared

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Groups 1 and 2 Fractional Crystallisation Models Groups 1 and 2 Fractional Crystallisation Models

K2225-K803 KSH01-K802Proportion of Phases Fractionated Proportion of Phases FractionatedCpx 66.8 K2225 Cpx3b avg Cpx 66.6 KSH01 Cpx1b avgOl 24.8 K2225 Ol3a avg Fo78.4 Ol 23.7 KSH01 Ol5b avg Fo64.9Sp 8.4 KSH03 MgTiSp Sp 9.7 KSH01 MgTiSp

F 30.7 F 0.153S2 0.552 S2 0.428

Major Element Calculations Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 44.81 44.23 44.52 SiO2 48.11 46.29 46.87TiO2 2.51 2.26 2.41 TiO2 2.57 2.43 2.74Al2O3 16.55 13.18 12.99 Al2O3 17.54 16.28 15.63FeO 13.11 13.61 13.59 FeO 12.62 13.83 13.76MnO 0.24 0.21 0.2 MnO 0.23 0.22 0.24MgO 5.7 8.64 8.6 MgO 3.71 5.64 5.6CaO 10.29 13.4 13.34 CaO 7.96 9.26 9.13Na2O 4.37 3.4 3.03 Na2O 4.66 4.17 3.96K2O 1.69 0.57 1.17 K2O 1.9 1.29 1.61P2O5 0.75 0.5 0.54 P2O5 0.7 0.59 0.6

Trace Element Calculations Trace Element CalculationsParent Daughter Calc. Daughter Parent Daughter Calc. Daughter

Sc 35.03 17.32 20.22 Sc 19.50 9.38 16.37V 325.96 235.50 470.36 V 263.11 145.12 310.63Cr 193.40 44.29 Cr 36.65 20.16Rb 9.57 59.30 13.51 Rb 66.38 53.94 77.64Sr 595.74 954.54 797.64 Sr 822.05 1000.05 942.40Y 24.42 29.17 26.59 Y 27.84 29.68 29.65Zr 193.67 328.48 222.27 Zr 258.20 262.01 277.51Nb 71.26 107.97 101.10 Nb 90.25 88.29 105.77Cs 0.28 0.49 0.40 Cs 0.36 0.76 0.43Ba 366.92 703.35 514.84 Ba 634.96 710.36 740.68La 48.82 74.34 67.21 La 67.88 69.41 78.73Ce 88.91 136.24 115.00 Ce 121.85 124.89 137.53Nd 39.90 58.48 49.77 Nd 52.98 53.15 59.03Sm 7.31 10.11 8.66 Sm 9.44 9.46 10.33Eu 2.16 2.98 2.58 Eu 2.84 2.63 3.12Gd 6.53 8.02 7.95 Gd 7.53 7.41 8.36Dy 4.79 5.80 5.78 Dy 5.37 5.64 5.94Er 2.39 2.71 2.88 Er 2.53 2.80 2.80Yb 1.91 2.37 2.22 Yb 2.16 2.48 2.34Lu 0.28 0.32 0.34 Lu 0.31 0.38 0.35Hf 4.61 6.82 5.63 Hf 5.61 5.66 6.22Ta 4.25 6.48 5.85 Ta 5.14 4.99 5.96Pb 6.00 7.49 8.65 Pb 5.45 8.48 6.43Th 5.96 7.68 8.38 Th 7.50 8.99 8.77U 1.34 1.55 1.88 U 1.40 1.89 1.63

F = amount of fractionation required F = amount of fractionation requiredS2 = sum of residuals squared S2 = sum of residuals squared

Group 1 – Path B. Fractional

crystallisation models and trace

element calculations for Path B

(K2225-K803).

Group 1 – Path C. Fractional

crystallisation models and trace

element calculations for Path C

(KSH01-K802).

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Group 3. Fractional crystallisation models and trace element calculations for

the Group 3 fractionation path (K813-K820-K825).

Group 3 Fractional Crystallisation Models

K813-K820 K820-K825Proportion of Phases FractionatedCpx 96.8 K811 Cpx4c rim 78.6 K820 Cpx1c rimSp 3.2 K811 MgTiSp 21.4 K820 MgTiSp

F 0.27 0.176S2 0.181 0.239

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 41.58 41.89 41.81 42.32 41.58 41.45TiO2 2.46 2.71 2.69 2.32 2.46 2.76Al2O3 15.71 14.54 14.06 17.23 15.71 15.43FeO 16.29 15.07 15.09 15 16.29 16.23MnO 0.32 0.3 0.29 0.29 0.32 0.29MgO 5.34 6.41 6.4 4.31 5.34 5.37CaO 11.68 14.41 14.47 10.23 11.68 11.73Na2O 4.58 3.12 3.43 5.88 4.58 4.87K2O 1.13 0.78 0.87 1.5 1.13 1.25P2O5 0.9 0.77 0.66 0.94 0.9 0.77

Trace Element CalculationsParent Daughter Calc. Daughter Parent Daughter Calc. Daughter

Sc 13.25 9.37 7.61 9.37 6.67 6.95V 335.77 278.34 459.96 278.34 253.13 337.79Cr 19.27 13.06 5.03 13.06 4.12 5.95Rb 126.66 63.60 171.38 63.60 55.26 75.82Sr 912.24 1182.32 1182.07 1182.32 1151.69 1361.45Y 36.28 39.22 37.70 39.22 37.90 41.16Zr 385.83 360.25 465.20 360.25 353.74 359.82Nb 121.95 138.41 166.13 138.41 148.53 165.09Cs 0.66 0.57 0.89 0.57 0.61 0.69Ba 532.86 616.73 718.21 616.73 762.33 731.69La 81.92 90.80 107.66 90.80 92.75 107.19Ce 150.89 162.13 196.68 162.13 165.72 177.49Nd 67.70 68.37 84.74 68.37 67.35 73.75Sm 12.09 11.87 14.22 11.87 11.46 12.57Eu 3.62 3.32 4.13 3.32 3.32 3.65Gd 9.98 9.61 11.54 9.61 9.38 10.72Dy 7.26 7.42 8.33 7.42 7.17 8.24Er 3.43 3.68 3.93 3.68 3.63 4.09Yb 2.90 3.15 3.32 3.15 3.16 3.33Lu 0.39 0.48 0.46 0.48 0.43 0.54Hf 7.57 6.52 8.87 6.52 6.02 7.22Ta 8.51 10.03 11.31 10.03 10.62 11.75Pb 5.75 5.65 7.88 5.65 5.44 6.86Th 8.50 9.98 11.41 9.98 10.62 11.91U 1.85 2.60 2.47 2.60 2.78 3.09

F = amount of fractionation requiredS2 = sum of residuals squared

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Group 4. Fractional crystallisation models and trace element calculations for

the Group 4 fractionation path (K361-K897-K894).

Group 4 Fractional Crystallisation Models

K361-K897 K897-K894Proportion of Phases FractionatedCpx 29.8 K361 Cpx1a core 45.4 K894 Cpx3a corePlag 50.8 K894 Plag2 An56.9 20.9 K894 Plag2 An56.9Ol 12.1 K361 Ol1a core Fo55 22.1 K361 Ol1b rim Fo49.8Sp 7.3 K361 MgTi Sp 11.6 K894 Ti Sp

F 0.381 0.324S2 0.298 0.355

Major Element CalculationsDaughter Parent Calc. Parent Daughter Parent Calc. Parent

SiO2 44.04 44.73 44.98 45.84 44.04 44.23TiO2 2.47 2.21 2.53 2.13 2.47 2.66Al2O3 15.82 16.39 16.31 18.63 15.82 15.99FeO 15.22 14.14 14.02 12.59 15.22 15.14MnO 0.23 0.2 0.22 0.24 0.23 0.37MgO 5.65 6.16 6.29 3.31 5.65 5.73CaO 10.32 11.53 11.39 8.58 10.32 10.21Na2O 4.18 3.59 3.55 5.14 4.18 3.8K2O 1.44 0.65 1.01 2.57 1.44 1.8P2O5 0.63 0.39 0.42 0.97 0.63 0.7

Trace Element CalculationsParent Daughter Calc. Daughter Parent Daughter Calc. Daughter

Sc 20.56 13.98 20.77 13.98 4.25 11.53V 317.62 268.64 513.12 268.64 133.06 397.40Cr 0.00 24.13 0.00 24.13 8.96Rb 13.81 27.59 19.95 27.59 78.61 38.69Sr 813.04 734.79 648.53 734.79 1232.35 819.86Y 20.56 27.23 28.77 27.23 29.89 33.90Zr 165.97 228.59 220.64 228.59 274.90 268.60Nb 52.07 83.86 82.64 83.86 152.97 121.51Cs 0.29 0.44 0.45 0.44 0.75 0.63Ba 439.70 546.68 606.57 546.68 904.48 751.24La 38.08 60.24 58.22 60.24 95.73 85.41Ce 70.29 108.49 101.13 108.49 161.86 141.83Nd 32.13 46.76 45.80 46.76 61.69 60.16Sm 6.17 8.36 8.67 8.36 9.95 10.53Eu 1.99 2.49 2.78 2.49 2.82 3.22Gd 5.18 6.98 7.59 6.98 7.77 9.29Dy 4.07 5.24 5.95 5.24 5.68 6.95Er 1.93 2.58 2.82 2.58 2.90 3.42Yb 1.73 2.09 2.43 2.09 2.31 2.62Lu 0.24 0.32 0.36 0.32 0.33 0.43Hf 3.94 4.98 5.77 4.98 4.65 6.58Ta 3.11 5.00 4.83 5.00 9.17 7.08Pb 6.11 7.53 9.87 7.53 10.39 11.14Th 4.49 7.53 7.05 7.53 12.71 10.86U 1.07 1.74 1.67 1.74 2.89 2.50

F = amount of fractionation requiredS2 = sum of residuals squared

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APPENDIX B

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APPENDIX B – MAGMA MIXING MODEL

Magma mixing models for mixing path K804-KSH11-K2225.

K804-KSH11-K2225

Evolved Magma Primitive MagmaPercent Mixing K804 KSH11 K2225K2225 0.00% 16.30% 100%K804 100.00% 83.70% 0%

S2 0.753

Major Element Calculationsobserved observed calculated observed

SiO2 49.61 47.79 48.75 44.23TiO2 2.22 2.42 2.22 2.26Al2O3 17.27 17.2 16.61 13.18FeO 11.82 12.74 12.11 13.61MnO 0.18 0.19 0.19 0.21MgO 4.35 4.97 5.05 8.64CaO 8.05 8.69 8.93 13.4Na2O 3.44 3.38 3.44 3.4K2O 2.45 2 2.15 0.57P2O5 0.6 0.62 0.59 0.5

Trace Element Calculations observed observed calculated observedSc 15.04 14.43 18.30 35.03V 210.71 246.89 229.50 325.96Cr 69.24 89.48 193.40Rb 116.09 89.15 98.73 9.57Sr 1316.21 881.56 1198.77 595.74Y 39.01 30.29 36.64 24.42Zr 325.54 270.48 304.04 193.67Nb 94.29 92.91 90.54 71.26Cs 0.38 0.71 0.36 0.28Ba 704.94 695.26 649.84 366.92La 78.98 72.09 74.06 48.82Ce 143.20 129.50 134.35 88.91Nd 60.55 53.39 57.18 39.90Sm 10.80 9.75 10.23 7.31Eu 2.91 2.71 2.79 2.16Gd 8.78 7.66 8.41 6.53Dy 7.24 5.88 6.84 4.79Er 3.83 2.89 3.59 2.39Yb 3.40 2.48 3.16 1.91Lu 0.51 0.37 0.47 0.28Hf 7.24 6.04 6.81 4.61Ta 5.18 5.08 5.02 4.25Pb 8.78 9.56 8.32 6.00Th 10.52 9.13 9.78 5.96U 2.60 1.02 2.40 1.34

S2 = sum of residuals squared

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APPENDIX C

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APPENDIX C – PRIMARY MAGMA COMPOSITIONS

Major and trace element compositions of ‘fractionation corrected’

primary/primitive magmas. Models include olivine only addition, clinopyroxene

and olivine addition required to reach minimum CaO content as determined

from primary/primitive sample database, and clinopyroxene and olivine addition

required to reach maximum CaO content determined from database.

Sample K2225 K2225 KSH03 KSH03 K813 K813 K813 K832a K832a K832aOl Added 23% 19% 16% 10% 38% 26% 21% 36% 32% 21%Cpx Added 6% 10% 12% 18% 4% 16%Major elements (weight percent)SiO2 42.68 43.38 43.85 45.04 40.42 41.87 42.68 43.49 43.95 45.44Al2O3 10.25 10.03 11.31 10.81 9.64 9.57 9.38 10.08 9.88 9.66TiO2 1.75 1.72 1.68 1.60 1.80 1.78 1.75 1.59 1.59 1.55Fe2O3 2.23 2.13 2.14 1.99 2.47 2.23 2.12 2.44 2.37 2.14FeO 11.35 10.84 10.94 10.16 12.58 11.40 10.80 12.46 12.06 10.89MnO 0.17 0.16 0.18 0.17 0.20 0.20 0.19 0.15 0.15 0.15MgO 16.28 15.61 15.63 14.47 18.53 16.49 15.66 18.11 17.45 15.54CaO 10.42 11.35 9.83 11.51 9.55 11.64 12.69 8.71 9.38 11.53Na2O 2.64 2.59 1.72 1.64 2.07 2.05 2.01 2.31 2.30 2.25K2O 0.44 0.43 0.82 0.78 0.52 0.52 0.50 0.29 0.29 0.28P2O5 0.39 0.38 0.37 0.36 0.51 0.51 0.50 0.38 0.38 0.37Totals 98.56 98.59 98.44 98.52 98.18 98.20 98.24 99.92 99.74 99.75Mg # 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71Trace elements (ppm)Sc 28.05 32.93 29.52 38.06 8.89 13.17 15.93 19.70 22.47 33.01V 250.99 244.47 232.52 221.45 209.86 208.18 204.82 186.01 186.01 183.10Cr 309.59 385.76 433.10 624.98 44.91 66.45 82.98 135.94 153.88 230.45Rb 7.41 7.22 18.84 17.98 79.91 79.40 78.21 13.54 13.55 13.36Sr 461.12 453.75 566.32 548.38 575.53 583.22 580.23 370.80 373.38 375.47Y 18.85 19.53 19.05 20.05 22.78 25.85 27.25 17.07 17.84 20.08Zr 151.88 150.58 165.09 161.87 249.21 256.09 256.77 127.39 128.85 131.44Nb 55.01 53.58 53.43 50.89 76.58 75.91 74.67 40.03 40.02 39.37Cs 0.22 0.21 0.20 0.19 0.42 0.42 0.41 0.29 0.29 0.28Ba 284.75 277.77 384.85 367.47 337.77 335.65 330.65 287.65 287.80 283.86La 37.69 37.02 39.36 38.01 51.44 51.97 51.61 30.14 30.32 30.39Ce 68.56 67.19 72.02 69.27 94.57 95.05 94.15 55.58 55.82 55.66Nd 30.77 30.45 33.12 32.35 42.43 43.54 43.59 26.48 26.77 27.26Sm 5.64 5.66 6.01 6.02 7.58 8.04 8.18 4.89 4.99 5.25Eu 1.67 1.69 1.80 1.82 2.28 2.47 2.54 1.48 1.52 1.63Gd 5.04 5.10 5.04 5.10 6.27 6.75 6.93 4.32 4.44 4.74Dy 3.70 3.75 3.76 3.82 4.57 4.94 5.07 3.33 3.42 3.67Er 1.85 1.87 1.77 1.80 2.17 2.34 2.41 1.62 1.67 1.79Yb 1.49 1.50 1.42 1.44 1.85 1.98 2.03 1.40 1.43 1.52Lu 0.22 0.22 0.22 0.22 0.25 0.26 0.27 0.20 0.20 0.21Hf 3.58 3.60 3.78 3.80 4.80 5.11 5.22 2.89 2.96 3.13Ta 3.29 3.22 3.11 2.99 5.37 5.38 5.31 2.41 2.42 2.40Th 4.61 4.51 4.12 3.95 5.36 5.35 5.28 3.31 3.31 3.28U 1.04 1.02 0.74 0.71 1.17 1.17 1.16 0.76 0.76 0.75

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APPENDIX D

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APPENDIX D – REVERSE PARTIAL MELTING MODELS

Source trace element concentrations calculated by reverse partial melting

models of modal and non-modal equilibrium batch partial melts. Calculations

are performed on ‘fractionation’ corrected samples which best represent

primary/primitive magmas for samples K832a, K2225, KSH03 and K813. The

degree of melting required to produce these primary/primitive samples is

shown, corresponding with trace element abundances for the source required to

produce them. Source Concentrations

Modal Non-ModalSample K832a K2225 KSH03 K813 K832a K2225 KSH03 K813Olivine Added 32 23 16 26 32 23 16 26Clinopyroxene Added 4 12 4 12

Degree of Batch Meltingto Produce Fractionation 10% 7% 7% 4% 10% 7% 7% 4%Corrected Magmas

Trace Elements (ppm)Sc 22.96 28.68 30.18 13.47 53.47 68.04 71.60 32.55V 173.76 233.90 216.69 193.55 190.78 257.64 238.68 213.87Cr 669.35 1381.21 1932.26 303.88 427.13 877.65 1227.79 192.31Rb 1.96 0.86 2.19 6.97 4.34 2.20 5.60 21.81Sr 64.42 66.84 82.09 68.46 124.51 143.52 176.26 168.56Y 5.44 5.31 5.37 6.68 9.59 9.84 9.94 13.10Zr 41.03 44.91 48.82 69.91 81.60 94.32 102.52 155.91Nb 6.06 6.78 6.58 7.21 8.51 10.25 9.96 12.16Cs 0.04 0.02 0.02 0.03 0.03 0.02 0.02 0.02Ba 44.05 35.55 48.04 32.42 99.75 92.49 125.01 101.72La 3.99 3.87 4.04 3.83 4.86 4.99 5.21 5.42Ce 9.12 9.29 9.76 10.23 11.07 11.76 12.35 13.76Nd 5.04 4.97 5.34 5.85 7.26 7.60 8.18 9.69Sm 1.12 1.11 1.19 1.38 1.75 1.86 1.98 2.48Eu 0.35 0.34 0.37 0.44 0.62 0.64 0.69 0.91Gd 0.94 0.94 0.94 1.08 1.21 1.26 1.26 1.52Dy 0.84 0.82 0.83 0.97 1.54 1.59 1.62 2.03Er 0.42 0.42 0.40 0.47 0.49 0.50 0.48 0.58Yb 0.47 0.46 0.44 0.57 0.85 0.86 0.82 1.12Lu 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06Hf 0.68 0.73 0.77 0.90 1.43 1.67 1.76 2.29Ta 0.34 0.37 0.35 0.45 0.53 0.64 0.60 0.90Th 0.45 0.50 0.45 0.42 0.50 0.56 0.50 0.50U 0.10 0.11 0.08 0.09 0.12 0.14 0.10 0.13

Mode Non-Modal Melting Proportions 53% Olivine 5%

22% Orthopyroxene 10%18% Clinopyroxene 33%

2% Spinel 2%5% Amphibole 50%

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APPENDIX E

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APPENDIX E – FORWARD PARTIAL MELTING MODELS

Trace element results for forward modal batch melting models from the

calculated source for 'Fractionation Corrected' Sample K2225.

Forward Modelling Trace Element Concentrations

Source Degree of Melting0% 0.01% 0.10% 1% 2% 3% 4% 5% 7% 10%

Sc 28.68 28.00 28.00 28.01 28.02 28.02 28.03 28.04 28.05 28.07V 233.90 252.37 252.36 252.18 251.98 251.78 251.58 251.38 250.99 250.40Cr 1381.21 292.53 292.74 294.83 297.19 299.59 302.03 304.51 309.59 317.53Rb 0.86 17.27 16.98 14.53 12.52 11.00 9.81 8.85 7.41 5.95Sr 66.84 828.38 819.97 744.41 675.27 617.88 569.48 528.12 461.12 387.41Y 5.31 23.33 23.25 22.57 21.85 21.18 20.54 19.95 18.85 17.42Zr 44.91 185.00 184.48 179.46 174.19 169.22 164.52 160.08 151.88 141.05Nb 6.78 118.28 116.56 101.71 89.11 79.28 71.40 64.95 55.01 44.74Cs 0.02 0.64 0.62 0.50 0.41 0.35 0.30 0.27 0.22 0.17Ba 35.55 601.94 593.42 519.91 457.01 407.69 367.97 335.31 284.75 232.23La 3.87 109.89 107.24 86.43 71.11 60.39 52.49 46.41 37.69 29.40Ce 9.29 131.68 130.14 116.49 104.34 94.48 86.32 79.46 68.56 56.87Nd 4.97 50.50 50.09 46.30 42.71 39.63 36.97 34.65 30.77 26.35Sm 1.11 8.12 8.07 7.64 7.22 6.83 6.49 6.18 5.64 4.99Eu 0.34 2.35 2.34 2.22 2.11 2.00 1.91 1.82 1.67 1.48Gd 0.94 7.50 7.45 7.01 6.58 6.20 5.87 5.56 5.04 4.42Dy 0.82 5.03 5.00 4.78 4.56 4.36 4.17 4.00 3.70 3.32Er 0.42 2.49 2.47 2.37 2.26 2.17 2.08 2.00 1.85 1.67Yb 0.46 1.79 1.79 1.74 1.69 1.65 1.61 1.57 1.49 1.39Lu 0.06 0.27 0.27 0.26 0.25 0.25 0.24 0.23 0.22 0.20Hf 0.73 5.08 5.05 4.80 4.54 4.31 4.10 3.91 3.58 3.18Ta 0.37 8.08 7.93 6.70 5.71 4.98 4.41 3.96 3.29 2.62Th 0.50 12.16 11.91 9.87 8.30 7.15 6.29 5.61 4.61 3.64U 0.11 2.90 2.83 2.31 1.92 1.64 1.44 1.28 1.04 0.82K 0.20 0.67 0.67 0.65 0.64 0.62 0.61 0.60 0.57 0.54

Modal MeltingSource Compositions

CPX 18%SP 2%OL 53%OPX 22%AMPH 5%