magmatic evolution of the shira volcanics, mt … · stephen john hayes b.app.sc. (qut) 2004...
TRANSCRIPT
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)
KEYWORDS
Kilimanjaro, East African Rift, alkalic magmatism, petrogenesis, magma
evolution, fractional crystallisation
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.
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
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
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
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:…………………………………..
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.
1
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).
2
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.
3
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).
4
5
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
6
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;).
7
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)
8
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°
4°
3°
2°
1°
0°
-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.
9
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).
10
11
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.
12
13
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
14
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.
15
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.
16
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
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
Sr
(pp
m)
-L
A-I
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
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
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
20
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81.7
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0.1
00.1
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70.2
10.2
40.1
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17.5
313.3
911.6
312.5
610.4
411.5
111.9
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50.0
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30.1
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tal
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a(m
2)
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94.4
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313.0
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r 2O
344.4
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3**
8.7
921.2
521.7
45.1
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21
7.3
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1F
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**16.9
149.0
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154.2
839.0
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643.9
352.5
6M
nO
0.4
50.7
30.5
10.5
80.5
10.8
31.9
10.3
8M
gO
11.5
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44.1
50.3
14.1
40.4
22.8
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0C
aO
0.0
00.3
60.0
60.5
60.0
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20.8
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2O
0.0
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10.0
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0.0
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2O
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tal
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Su
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ati
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s(4
O)
3.0
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g/(
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+F
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54.8
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r/(C
r+A
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t1.8
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*=
Fe
and
Fe
adju
ste
dsto
ichio
metr
ically
thro
ugh
the
meth
ods
of
Dro
op
(1987)
2+
3+
**=
Fe
an
dF
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ste
dsto
ichio
metr
ically
2+
3+
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
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.
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
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.
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.
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
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).
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
)
28
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).
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)
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
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
33
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.
34
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
35
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
36
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
37
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
38
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
39
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
40
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
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)
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)
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.
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.
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.
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
47
Oliv
ine
Ana
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s U
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in P
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odel
ling
Spi
n el A
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yses
Use
d in
Pet
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enet
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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
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1 O
l1a
K36
1 O
l1b
KS
H0 3
CrS
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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.
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
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).
50
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
51
52
53
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).
54
55
56
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
57
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 #).
58
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.
59
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.
60
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
61
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
62
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
63
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%
64
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
65
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.
66
67
.
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
68
69
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
70
71
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,
72
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
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
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.
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).
76
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
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).
78
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
79
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).
80
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
81
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.
82
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96
APPENDIX A
97
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
98
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).
99
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
100
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
101
APPENDIX B
102
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
103
APPENDIX C
104
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
105
APPENDIX D
106
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%
107
APPENDIX E
108
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%