1995 geology, mineralogy and magma evolution of gunung slamet volcano, java, i.pdf

7/18/2019 1995 Geology, mineralogy and magma evolution of Gunung Slamet Volcano, Java, I.pdf

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Pergamon

Journal of Southeast Asian Earth Sciences, Vol. I I, No. 2, pp. 135-164, 1995

Copyrig ht 0 1995 Elsevier Science Ltd

Printed in Great B ritain. All rights reserved

0743-9w7/95 9.50 + 0.00

Geology,

mineralogy and magma evolution of

Slamet Volcano Java Indonesia

Danilo Vukadinovic*t and Igan Sutawidjajat

Gunung

*Departm ent of Earth Sciences, Monas h University, Clayton, Victoria 3168 Australia, and

tVolcanologica1 Survey of Indonesia, Jl. Diponegoro 57, Bandung, Central Ja va, Indonesia

AhstracG-Gunung Slamet, Central Java, is a large stratovolcano within the Sunda magmatic arc of

Indonesia. The volcanic edifice includes products of two large overlapping Quaternary stratocones.

Basaltic andesites and andesites, with rare basalts, dom inate the western region of the comple% ,

known as Slamet Tua (old); and basalts and basaltic and esites c ompose the eastern c one, called

Slamet Muda (young).

On the basis of stratigraphy, trace-element content, Zr/Nb, Zr/K and *Sr/*Sr ratios, Slame t lavqs

can be broadly categorize d as relating to high abundance magm a (H AM ) and low abundance magm a

(LAM ) types. The Tua and Lebaksiu sequences generally com prise the LAM group, and are older,

more salic and have higher *Sr/%r ratios than those of HAM . L AM and esites contain some

amphibole, but HAM andesites do not.

Models involving randomized magm a replenishment, tapping and fractionation (RTF) wetie

developed to explain the geochem ical features o f both LAM and HAM rock groups. The salic lavas

of the LAM suite can be generated if fractionation was dominant relative to replenishment ati

tapping in LAM ma gma cham bers. Conversely, magm a chambers with a high proportion of

replenishment and tapping relative to fractionation can produce dominantly mafic lavas, such as

those of the HAM suite.

Concave-upward heavy-rare-earth element (HREE) patterns for LAM andesites are probably due

to significant amphibole fractionation; HAM andesites display flat HREE patterns and do not requite

amphibole fractionation from parental basalts. The high TiO, contents of HAM basalts and basaltic

andesites (relative to those of average arc rocks) are due to either suppressed crystallization-or

minor accum ulation-of Ti-magn etite, in conjunction with RTF processes.

Introduction

Gunung Slamet volcano, Central Java, lies about 190 km

above the northwa rd-dipping seismic Benioff zone

(Hamilton, 1979) and rests upon Neogene sediments of

domina ntly shallow marine regressive facies (Djuri,

1975) above a 20-25 km thick crust (Hamilton, 1979).

Compared with most other arc volcanoes, Gunung

Slam et has erupted significant a mou nts of mafic lava (cf.

Wh itford, 1975a ), allowing for detailed studies on the

origin of arc magm as (Vukadinovic, 1989; Vukadinovic

and Nicholls, 1989).

Neum ann van Padang (1951) presented the earliest

major-element analyses of Slamet lavas. The reconnais-

sance study of Wh itford (1975a) contains mode rn analy-

ses of Slamet rocks, i.e. basalts and basaltic andesites

(SiOZ c 56 wt%), some of which have TiOz in excess of

1.8 wt%. Whitford (1975a, b) classified Slamet as an

anomalous calcalkaline volcan-anomalous in the

sense that 87Sr/86Sr ratios are higher in Slamet lavas

relative to those of the majority of calcalkaline lavas

from Java. He also suggested that the large abundance

of high-field-strength (HFS) elem ents in Slam et lavas,

relative to typical arc rocks, may be due to incorporation

of a subduc ted oceanic island with its underlying litho-

sphere. Subsequent studies (Pardyanto, 1971; Aswin

et a l . , 1984; Sutawidjaja et al. , 1985; Vukaflinovic, 1989)

discussed older andesitic rocks associated 1with Gunung

Slam et. On the basis of stratigraphy and igeochem istry,

Vukadinovic (1989) and Vukadinovic and Nicholls

(1989) broadly categorized Slam et lavas #as relating to

high-abundance magm a (HAM) and low-abundance

magm a (LAM) types. Compared with HAM lavas,

LAM are older and more salic and have higher 87Sr/*6Sr.

A model w as developed showing that: (1) dompared with

parental HAM , parental LAM are generiCted by higher

degrees of melting w ithin the mantle wedhe; and (2) the

degree of melting is controlled by the adount of fluids

introduced by the dehydrating,

subducti g

?

lithosphere

(Vukadinovic, 1989; Vukadinovic and N cholls, 1989).

Gunung Slamet

Purwokerto, the largest town near Gunlung Slamet, is

located about 25 km south of the volcan4 summ it. The

highest villages are about 1500 m above s$a level; above

this height, only odd footpaths through d&se vegetation

exist. Below 1500 m, the density of rdads increases

rapidly w ith decreasing height, providing e)xcellent access

around the base of the volcano (where, in any case, the

exposure-in streams-is best).

t Present address: Geology Department, Brandon University,

The topographic maps (1: 50,000 scale; edition

Brandon, Manitoba, Canada R7A 6A9.

2-AM S) used during the field expeditions1 for this study

135


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136

D. Vukadinovic and I. Sutawidjaja

section is older than the eastern. Pardyanto (1971)

suggested that some of the numerous peaks in the

western region (e.g. Gunung Minggrik, Gunung

Sem bung) represent eruptive cen tres from this period of

activity. The large plateaux of the western sector may be

due to block fracturing and faulting (van Bemmelen,

1949). Sutawidjaja er al. 1985) stated that the western

half of Slam et is comp osed entirely of andesitic lava and

tephra (excluding the Semaya and Waka eruptive

centres) and called it Slamet Tua. This term is retained

in the present study; however, subseq uent field studies

(Vukad inovic, 1989) have found rare basaltic outcrops

within the Slamet Tua sequence. Future mapping will

undoubtedly uncover more new units.

The eastern part of the volcano is relatively y oung

and, as a consequence, topographically smooth due to

the relatively rece nt eruption of fluidal basaltic lavas;

Djuri (1975), Aswin et al. (1984), and Sutawidjaja et al.

(1985) termed it Slamet Mu da. The smooth topogra-

phy is disrupted on the northeastern slope by a field of

35 scoria cones studied by Sutawidjaja (1988). The cones

range in size from N 130 to 750 m in basal diameter and

from several to

-250 m in height. According to the

terminology of Porter (1972), H,, = 0.25 * WC, and

WC,= 0.4. WC, for Slamet scoria cones, where H, =

cone

height, W, = basal width and WC,= crater width. The

total volume of the scoria cones am ounts to a mere

0.357 km 3, but due to the vesicular nature of scoria, the

actual volume is even less. A K-Ar radiometric age date

on a sample of Slamet scoria gave 4 2 _+20 Ka (C. J.

Adam s, personal communication, 1988). Most cones

have single craters, although some may have as many a s

were prepared under the direction of CINC USA RPA C

by the U.S. Army Map Service, Far East, and the 29th

Engineer Battalion. With Java so densely populated, yet

overwhelm ingly rural in character, sma ll villages aboun d

throughout the island. As a result, the names of some

villages are occasionally mentione d in the text that are

not sh own on Fig. 1, due to a desire for clarity and

brevity. In these instances the reader is referred to the

CINCUSARPAC maps.

The geology of Gunung Slamet (Fig. 1) has been

referred to three sequences: Tua (Old), Lebaksiu and

Muda (Young). S ubdivisions w ithin each sequence have

been termed units. In most cases, the units are made up

of several lava flows. The relative ages of two of the

sequences, Tua and Muda , are well established (see

below). However, the stratigraphic position of the

Lebaksiu sequence is problematic. Also, the chemical

compo sition of Lebak siu lav as is transitional between

those of the Tua and Muda sequences (Vukadinovic,

1989).

Morphology

On Java, only Gunung Semeru (3676 m) exceeds

Gunung Slamet (3428 m) in height. Gunung Slamet can

be divided broadly into two parts: the rugged, dissected ,

western half of the volcano, consisting of deep valleys,

gullies, several plateaux and numerous peaks; and the

smoo th, gently sloping, eastern half (see contour lines on

Fig. 1). Previous workers (van Bemmelen, 1949;

Pardyanto, 1971; Djuri, 1975; Aswin et al., 1984;

Sutawidjaja

et al.,

1985) have shown that the western

Geologic Map of Gunung Slamet Lavas

1 Keruh ardldac .)

u Ce ndarm hb. emI.)

loo-es, : : ,

ICWO 1W.15 Top20

Fig. 1. Geologic map of Gunung Slamet lava and pyroclastic units. Modified from Sutawidjaja et al. (1985).


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Evolution of G. Slam et Volcano , In donesia

137

three. Many of the craters are breached towards the east

(downh ill). Breached scoria-cone craters at Slam et

usually have a lava flow associated with the cone.

The summ it of Gunung Slamet con sists of four nested

craters with a spatial arrangement indicating that the

volcanic vent has migrated slightly from northeast to

southwest (N 1 km) during Muda time. Neumann van

Padang (1936) examined the possible volcanic hazards

resulting from such shifts in activity and concluded that

the northeast foot of the volcano is now best shielded

from volcanic catastrophe.

Tua sequence geo l ogy: S i rum ia ng m ix ed andes i te

The Sirumiang mixed andesite, located on the western

side of the volcano at the foot of a fault sca rp, is a small

dome (-0.51 km in diameter) composed of andesitic

material containing abundant basaltic enclaves up to

N 15 cm in size. The leucocratic host rock contains

abundant xenocrysts of quartz and feldspar (up to 5 mm

in diame ter). All outcrops are densely overgrown, pro-

hibiting a precise estimate of the relative propo rtions

of host andesite and enclaves. The origin and geochem-

istry of the Sirumiang mixed an desite is complex-

involving processes such as magm a mixing, crustal

assim ilation, and liquid-liquid diffusion (Vukad inovic,

in preparation)--and will not be dealt with in this paper.

Tua sequence geo lo gy: M enda l a basa l t s

The Mend ala basalts are located in a confined area

west-northwest of the Slamet summ it. Neither the erup-

tion point for this rock unit nor contacts with other rock

units were found, restricting precise stratigraphic place-

ment of the Mendala basalts, which are believed to

belong to Slam et Tua activity (Vukad inovic, 1989). In

outcrop, Mendala basalts show crude columnar jointing.

Plagioclase , pyroxene and olivine are visible in hand

specimens. No mineral alignment is evident in these

nonvesicular rocks.

Tua sequence geology

:

Sumbaga andes i tes

Sutawidjaja et a l . (1985) subdivided Slamet Tua vol-

canic products into five units, wherea s Pardyanto (1971)

split Slamet Tua into seven units using air-photo

interpretation. The field investigations and subseq uent

chem ical da ta of Vuka dinovic (1989) are not in agree-

ment with either division. The Slamet Tua andesites are

probably composed of numerous domes of viscous lava

of limited area1 extent. Due to the poor outcrops,

determina tion of stratigraphic/tem poral relationships

between these domes is difficult. In this study, the

andesitic rocks of this area are collectively nam ed

Sumbaga. In general, the Sumba ga andesites are

nonvesicular, phenocryst-rich

(N40-50%) two-

pyroxene andesites with varying amounts of amphibole.

Note, however, that the Gunung Cendana, Kalipagu

and Keruh units have been distinguished on morpho-

logical and chemical grounds from Slamet Tua material

(Vukadinovic, 1989) and are discussed below.

Lebaks iu sequence geo logy

The term Lebaksiu was given by Aswin et a l . (1984)

and Sutawidjaja

et a l .

(1985) to the products of flank

eruptions on the lower southwestern slopes of Slam et.

They identified two separate eruptive centres near the

village of Semaya and Gunung W aka (Fig. 1). Both

centres have highly vesicular, thin (N 30 cm), basa ltic

flows separated by thin lenses of agglutina ted spatter

mate rial. The high degree of vesicularity and low disper-

sal of tephra indicate a mild ma gma tic style of eruption

for the Lebak siu flank eruptions. The term Lebaksiu

has been extended to include lavas with chemical com po-

sition similar to those of the Semaya and Waka basalts

(Vukadinovic, 1989).

Near the village of Siwarak on the eastern flank of the

volcano, extensive lava caves exist within Lebaksiu-type

basalts called Sirawak basalts (Vukadinovic, 1989).

The cave walls display tide marks indicating the rise and

fall of the outpouring lava, due to chang ing effusion

rates from the vent.

Basalts similar to those of Waka and Semaya occur on

the eastern slope above the site of the lava caves (Fig. 1).

Aswin et a l . (1984) and Sutawidjaja et a l . (1985) assumed

that the source of these flows is Gunung Malang, a

vent located approximately 600 m east of the summit.

The basalts contain olivine and pyroxene phenocrysts

and are relatively vesicular. Aswin et a l . (1984) and

Sutawidjaja et a l . (1985) called this unit Lawa Ganung

Malang and distinguished it from the Lebaksiu se-

quence. The Gunung Malang unit is here incorporated

in the Lebaksiu sequence solely on the basis of similar

chemical composition (Vukadinovic, 1989).

A unit of massive basalt between the village of Batu-

raden and the Cendana andesites is also tentatively

placed within Lebaksiu sequence on the basis of chemi-

cal parameters (Fig. 1).

M uda sequence geo l ogy

The youngest Slamet Muda volcanic products-

excluding the vent area-occur on the northeast slopes

of the volcano, and the oldest on the southern

(Pardyanto, 1971; Aswin et a l . , 1984; Sutawidjaja et a l . ,

1985). In this study, the stratigraphy of Sutaw idjaja et a l .

(1985) has been adopted with slight changes.

M uda sequence geo logy : Ba t u raden basa l t un i t

The Baturaden basalt unit occurs on the

south-southe astern slopes of Slam et. This unit is the

Lava Slamet 2 of Sutawidjaja et a l . (1985)b who found

that Lava Slamet 2 and L ava Slam et 3 the (Banyumudal

unit of this study) sandwich a widespread scoriaceous

airfall deposit. For this reason the distinction between

the two lava units is retained.

Columnar-jointed flows that are 4-5 m thick crop up

at the village of Baturad en. Other o utcrops are thick and

massive or thin and fluidal with variable vesicularity.

The source for the Baturaden unit is one of the summ it

vents (Sutawidjaja

et al . ,

1985).

M uda sequence geo logy : Bany umuda l basa l t un i t

The Banyumudal basalts occur predominantly in the

northeast and east sectors of the study area. The lava

flows are generally < 3 m thick; thicker flows exist where

the underlying topography allowed the lava to accum u-

late. The chemical co mposition of these rocks, as with

the scoria cones, is similar to that of Baturad en


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138


unit rocks. The scoria cones formed during the

hiatus between Baturaden and Banyumudal activity

(Sutawidjaja et a l . , 1985).

Muda sequence geol ogy : Legokm ene basa l t i c an s i t e un i t

In the northwest of the study a rea, a series of pre-

viously unm apped outcrops of basaltic-an desite lava lies

within Slamet Tua andesites. The outcrops extend north-

ward from the eruptive centre, the Angg rung scoria cone

situated east of the town of Legokm ene (Fig. 1). Geo-

chemically, these basaltic andesites show affinities with

Slamet M uda volcanic rocks and, therefore, are classed

with them. The Legokmene lavas are generally massive

to slightly vesicular, whereas the Anggrung scoria cone

is built up of both airfall ma terial, in which cla sts range

from lapilli to bomb size, and associated surge deposits

with lapilli-sized tephra.

M uda sequence geo logy : Kaw ah basa l t un i t

In 1973, magm atic activity at Slamet consisted of

relatively mild lava fountaining and the emplac emen t of

a ring of lava within the lowermost of Slamets summ it

craters. Aswin

et a l .

(1984) and Sutawidjaja

et a l .

(1985)

suggested that the lava ring resulted from the explosive

disintegration of a lava dome , requiring the dome-

forming mag ma to be relatively viscous (> 10 poise;

Sutawidjaja, 1988). Indeed, in a magm a undergoing

significant de gassing (e.g. via fire fountaining), enough

undercooling will occur to promote rapid crystal growth,

increasing the viscosity and yield strength of the magma

(Sparks and Pinkerton, 1978). However, it is also poss-

ible that the ring was formed by a sma ll volume of

basaltic lava rising to a height slightly above the top of

the vent. The outermost part of the lava may then have

chilled and solidified against the vent walls, leaving a

ring of basalt after the central part of the plug drained

back into the bowels of the volcano. This mechanism

avoids assuming viscosities for the Kawah basalt that are

characteristic of dacites and rhyolites. The name as-

signed previously to the Kawah unit was Lawa K ubah

(i.e. lava dome; Aswin et al., 1984; Sutawidjaja et al.,

1985). Since the mode of emplacement is debatable, the

name has been changed here to Kawah (crater), in

order to avoid genetic connotations.

In the Guci graben, northwest of the summ it, an

unwe lded scoria-flow dep osit, chemically similar to that

of Kawah basal@ overlies Baturaden basaltic lava flows.

The scoria-flow deposit is

-4-5 m thick and contains

normally graded, dense, scoria c lasts (up to 30 cm in

diameter) and abundant charcoal in a matrix of black

lapilli and ash. Where the scoria flow overlies unconsol-

idated sediments, large fragments from the latter are

incorporated into the lower portion of the former. The

charcoal and rounded nature of the scoria clasts suggests

that the driving force of transport wa s mag matic , poss-

ibly caused by the collapse of a sma ll eruption column

(J. V. Wright, personal communication).

O the r un i t s

The following section describes units from Gunung

Slamet that contain rocks with chemical characteristics

that are either distinct from or transitional between

those of the Tua and Muda sequences.

Othe r un i t s : Ke ruh dac i t e un i t

The Keruh ignimb rite occurs in the valleys of the

Keruh River system o n the western slopes of the vol-

cano. A small quarry near the village of Pengasinan

provides the best exposure. The deposit conta ins an

undetermined number of ignimbrite sheets, each

N 3-7 m in thickness, with associated basal ground-surge

and co-ignimbrite ash-fall deposits. In the ignimb rites

proper, pumice clasts range from 1 to 15 cm in size and

are evenly distributed throughout a matrix of ash and

lapilli. The associated surge deposits, com posed of

ash- and lapilli-sized particles, exhibit low-angle cross-

stratification. Unidirection al sedimentary bedforms in

surges are evidence for pyroclastic transport by a

ground-hugging, expanded, turbulent, gas-solid dis-

persion; th is contrasts with the ignimb rite units, which

were probably transported as lamin ar, high-density-par-

ticle-to-gas concentrations (e.g. Cas and Wright, 1987).

O the r un i t s : Cendana am ph ibo l e andesi t es

Located 2 km west of Baturaden village, the Cendana

amp hibole andesite s form several steep hills of

-200-300 m relief. The morphology and limited area1

extent (~0.5 km diameter) of the individua l hills

suggests that the amphibole andesites were extruded as

thick, viscou s dome s. The dom es are located in a circular

depression, which Aswin et a l . (1984) interpreted as an

old crater. Poor e xposure prevented a clear assess men t

of the relative age of the Cendana andesites. Compared

with the Tua sequence, the Cendana andesites have

similar trace-elemen t ratios and contents, but lower

*Sr/*?Sr values (V ukadinov ic, 1989; see below).

O the r un i t s : Ka l i pagu basa l t i c andesi t es

The Kalipagu basaltic andesites occur on the

south-sou thwest slope of the volcano an d extend from

the summit down to the Cendana crater (Sutawidjaja

et

al. ,

1985). Kalipagu rocks are grey, massive, and

generally phenocryst-rich and occasionally show flow

foliation. T he Kalipa gu unit also contains andes ites with

chemical similarities to both the Muda and Lebaksiu

sequences (Vukadinovic, 1989). Determination of the

temporal and chemical relationship between the

Kalipa gu and other units requires further field studies.

Petrography

Slam et T ua sequence: M enda l a basa l t s

Mendala basalts are strikingly porphyritic (h 50%

phenocrysts) comp ared with other basa lts of the Slam et

volcanic complex. Indeed, plagioclase phenoc rysts are so

strongly zoned and abundant that the rocks can be

mistaken for andesites. Most of the plagioclase phe-

nocrysts have a concentric arrangem ent of internal gla ss

and other cryptocrystalline inclusions, which docum ents

the growth history of the mineral. Large (3.5 mm )

euhedral clinopyroxene phenocrysts are strongly zoned,

more than in any other samples thus far discussed.

Inclusions of plagioclase, opaque granules and minor

olivine occur in the pyroxene phenocrysts.


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Evolution of G. Slamet Volcano, Indonesia 139

Very rare orthopyroxene phenocrysts exist in some

thin sections. Common subhedral olivine phenocrysts,

with relatively extensive iddingsitization and ma rginal

resorption, reach a maximum size of 1.5 mm . R are

magnetite is always associated with other ferromagne-

sian minera ls. The hyalopilitic ground mas s consists of

plagioclase , clinopyroxene, opaque , glass and olivine.

Slamet Tua sequence: Sumbaga an s i t es and basa l t ic

andes i tes

The highly porphyritic Tua andes ites contain phe-

nocrysts (w 50 ~01% ) of abundant plagioclase; common

orthopyroxene, clinopyroxene and mag netite; and rare

amphibole and apatite. Groundmass textures are either

hyalopilitic or pilotaxitic/felty. The ground ma ss usually

contains plagioclase, opaque granules, clinopyroxene,

cryptocrystalline comp onents and minor orthopyroxene

and clear glass. Zoning in plagioclase phenocrysts, which

is more noticeable in andesites than in basalts, consists

of well-defined c oncentric patterns with slight convolu-

tions in many rings . Plagioclase phenocryst cores are

typically resorbed and encircled by a man tle of zoned

plagioclase containing inclusions of glass and rare

opaque m aterial and pyroxene. Subhedral bladed plagio-

clase is typically 3 mm in length and, on average ,

comprises 3040 ~01% of the rock.

Subhe dral prisma tic pyroxene varies in size, attaining

maximum lengths of 4 mm, and combines with plagio-

clase and Ti-magn etite phenocrysts to form crystal

aggregates. Occasionally, augite jackets the prism faces

of hypersthene crystals.

Subhedral equant Ti-magnetite microphenocrysts

(~0.5 mm dia.) are rarely embayed and generally

inclusion free (except for rare apatite) and constitute

-2 ~01% of the rock. M agnetite tends to occur in close

association with other ferromagn esian mine rals, particu-

larly a s inclusions and a s members of crystal aggregates.

Anhedral-subhedral pargasitic amphibole occurs in

many of the andesite samples, som etimes as large

mega crysts that are uniformly rimme d by a very fine

aggrega te of Fe-Ti oxides and clinopyroxenes. These

crystals usually contain plagiocla se an d clinopyroxene

inclusions and display abundant disequilibrium textures

such as embayments and reaction coronas.

Most Slamet basaltic andesites and andesites contain

sma ll apatite crystals with distinctive optical properties

that include moderate pleochroism (with absorption

strongest in the direction of the promine nt cleavage) a nd

interference figures that vary from biaxial (2 V x 40)

negative to uniaxial negative within the same thin sec-

tion. The andesites contain the greatest am ount of

apatite (< 1% modal), occurring as phenocrysts sur-

rounded by mesostasis, grains within multi-phase crystal

aggregates, and inclusions in all phenocryst phases.

Very rare olivine is present in some ande sites. Olivine

is highly resorbed, alters to iddingsite, and sometim es

displays a corona of clinopyroxene and plagioclase.

Lebaks i u sequence: Gu nung W ak a and Sema ya basa l ts

Sema ya rock s have a coarse intersertal texture

represented by plagioclase microlaths (ave. length =

0.25 mm) with interstices occupied by abundant clinopy-

roxene, opa que granules , brow n gla ss and lesser olivine.

Phenocrysts, particularly plagioclase , tend to be larger

than in other b asalts, reaching 5-6 mm in length. Olivine

and clinopyroxene phenocrysts are present in all sam ples

and have features similar to those of Muda basalts.

Nearer to source, the Gunung Wak a ba@ ts have a

felty ground ma ss of opaque oxides, plagioclase an d

clinopyroxene; approximately 5 km from the $ource, the

textures-besides being coarser grained-vary from

intersertal to intergranula r to suboph itic within single thin

sections. Subhedral equant ohvine phenoc

#

sts (max.

size e 2 mm ) in the Gunung Wa ka basalts ,how more

extensive alteration, represented by opaqu es rims and

internal iddingsitization, than either the Sema$ or Mud a

basalts. Gunung Waka plagioclase, on the other hand, is

relatively free of internal melt inclusions and resorbed

margins. Subhedral equant clinopyroxene

1

henocrysts

attain widths of 2.5 mm and contain com mo inclusions

of plagioclase . Ti-magnetite microphe nocrysts~ are absen t

from both flow units of the Lebaksiu sequence..

Lebaksiu sequence : Gunung M a l a ng ba sal t s

All samples collected from the Gunung Malang

basa lts are rich in glomeroporphyritic, seriate olivine

and plagioclase. Plagioclase laths have a maximum

length of 3 mm , and olivine grains are usually 2 mm or

less in diame ter. Clinopyroxene phenocrysts hnd crystal

aggregates are rare.

Zonation, both conc@ric and

sector, is comm on within the pyroxene rains. The

groundmass is intergranular, with the plagio

1

ase micro-

lite framework filled by opaque , clinopyroxene and

olivine granules.

M uda sequence

:

l a v a l o w s

The Slamet Muda basalts are rich in porphyritic,

seriate plagioclase and contain les ser am ounts of olivine,

clinopyroxene and Ti-magn etite. These mine rals

comm only form glomerophe nocrysts andyor crystal

aggregates.

Subhedral bladed plagioclase (max. size range from 6

to 0.5 mm) is the most abundant phenocryst phase,

averaging 20-30 vol%, and displays a flow alignment in

some rocks. Generally, plagioclase cores contain abun-

dant m elt inclusions and are surrounded by clear rims

showing m arginal resorption, which is less pronounced

in rocks without clinopyroxene phenocrysts. Minor

olivine inclusions are present in the feldspar phenocrysts

of some samples. Norm al concentric zoning is not as

common as in the andesites.

Olivine is a comm on phenocryst phase , occurring in

varying amounts in the groundmass, and has an average

diameter of 0.5 mm , yet can be as large as 4.0 mm . The

crystals are subhedral, with some larger grains display-

ing embayments. A thin coating of Ti-magnetite

occasionally rims some grains. Olivine phenocrysts lack

zoning and are largely devoid of inclusions-although

rare plagioclase, mag netite, spine1 and clihopyroxene

(very rare) occur.

Subh edral prisma tic clinopyroxene phenocrysts

(N 5 ~01%) have diameters ranging between 0.5 and

4 mm and are pale green; some rocks contain crystals

that display very faint pleochroism (pale ~ reen-faint

pink), indicating higher than normal Ti contents. Most

clinopyroxene phenocrysts have some concentric a nd

sector zoning and contain com mon inclusions of olivine,

plagioclase and occasional glass.


6/30

140


Although ubiquitous in the groundmass, equant

Ti-magnetite ( m 0.25 mm) grains are variably present as

microphenocrysts (< 1 vol%), ranging in shape from

euhedral cubes to anhedral blobs. On progression from

basa lt to more evolved com positions, the morpho logies

of mag netite phenocrysts change from skeletal or den-

dritic forms-indicating a significant degree of ma gma

undercooling (e.g. quench extensions )-to squat,

equant, well-formed crystals. This implies that magnetite

had not reached saturation in many of the more primi-

tive rock compositions, suggesting that delayed m ag-

netite precipitation is the cause of relatively high Ti02

contents in some basalts and basaltic andesites from the

Mud a sequence. Significantly, the high-TiOz basalts of

Slam et are the only magne tite-free volcanic roc ks on

Java (Whitford, 1975a).

The ground mas s textures are predomina ntly inter-

granula r with Ti-magn etite, clinopyroxene and olivine

granule s (in decreasing order of abund ance) filling the

interstices between m icrohtes of plagioclase (0.05 mm in

length); some rocks possess an intersertal texture w ith

the addition of brown glass. Orientation of the microlites

varies from strongly aligned to randomly arranged .

M uda sequence: scor i a cones

Seven scoria cones were samp led, prim arily on the

basis of availability of fresh scoria. Texturally, the rocks

are hyalo-ophitic with pervasive da rk glass enclosing

microlites of plagioclase , granule s of olivine and rare

mag netite. Porphyritic phas es include plagioclase ,

olivine, and clinopyroxene-all with the sam e character-

istics and relationships as in Mud a sequence rocks.

Occasion ally, olivine forms skeletal crystals.

M uda sequence: Kaw ah basa l t

The porphyritic Kawah basalt samples contain

phenocrysts of olivine, clinopyroxene and plagioclase .

Although the abundance of plagioclase phenocrysts is

relatively low, groundmass plagioclase is more abundant

and coarser than in other basa lts. Interstices b etween

plagioclase laths hold murky brown glass, olivine,

mag netite and clinopyroxene granules .

Othe r un i t s : Ke ruh dac i t e

Pum ice from the unwe lded Keruh pyroclastic flow is

extremely vesicular. The vitrophyric texture results from

subhe dral ph enocrysts of plagioclase , orthopyroxene,

clinopyroxene, amphibole and opaques set in clear,

vesicular glass. Minor apatite occurs as inclusions w ithin

amp hibole and orthopyroxene. Plagioclas e is usually

riddled with brown glass, although some crystals may

have anhedral inclusion-free cores or are completely

devoid of inclusions. Oth er phenocryst phases-exc lud-

ing magnetite-may also have brown glass inclusions,

particularly the pyroxenes.

O the r un i t s : Cendana amph i bo le andes i t es

These rocks are distinctive in having pa rgasitic horn-

blende as their only ferromagn esian phenocryst phase

(< 1 cm). Zoning within amp hiboles is optically visible;

and inclusions of plagioclase , apatite and minor pyrox-

ene (both clinopyroxene and orthopyroxene) define the

crystal growth history by forming conce ntric patterns

about an inclusion-free core in larger crystals. The

euhedral prismatic am phibole grains show minimal evi-

dence of reaction with the groundmass. Subhedral tabu-

lar plagioclase phenocrysts are (N 2 mm in diameter)

very strongly zone d and contain melt inclusions. Equa nt

magnetite (up to 0.25 mm) is rare to common in abun-

dance. The groundmass is characterized by abundant

equant feldspar laths, and the interstices are occupied

by orthopyroxene, minor Ti-magn etite, minor clinopy-

roxene, ab unda nt clear glass, and cryptocrystalline

components.

b

Fig. 2. Analyses of plagioclase phenocryst interiors plotted in terms of An-A& Or (mol ). Sym bols: filled

circles =

Muda (including Keruh); open circles

= Tua (including Cendan a); filled squares = Lebaksiu; open

triangles = Sirumiang.


7/30

Evolution of G. Slam et Volcano , Indonesia

141

Mineralogy

A table of minera l analyses and analytical procedures

and methods may be obtained from the main author.

P lag ioc lase

Plagiocla se core compositions are plotted in terms of

anorthite (An), albite (Ab), and orthoclase (Or) end-

mem bers (Fig. 2). Maximum An core values within the

bulk-rock MgO ranges of Fig. 2 (i.e. >6,4-6, 6 wt% have the largest range fin orthopy-

roxene com positions, with mg# as high as 80. These

mg # s are com patible with orthopyroxene ~ orming in

equilibrium with the coexisting C a-rich clinopyroxenes.

The lower-mg # (60-7 0) orthopyroxenes are anhed ral

and rimm ed by clinopyroxenes with hi her mg#s

(80-85) indicating that the

$

rthopyroxe es may be

xenocrystic. Amongst rocks with MgO < 4 wt%,

orthopyroxenes from Slam et Mu da andesites are richer

in Ca relative to those from Slam et Tua ande sites,

possibly due to higher temperature s of form tion for the

former. In some basaltic a ndesites, pigeo

,

te rims on

orthopyroxene cores are comm on. O the r s co ponents-

as defined by Papike et a l . (1974)---in Sla

ntn

t orthopy-

roxenes are relatively low (N 5 mol% on average), with

Al(IV) typically m ore abund ant than Ti, $Ia or Fe+.

In Ca-Mg-Fe space, Slamet Ca-rich clinopyroxenes

concentrate near the point where diopside,l endiopside,

salite and augite fields meet (cf. Deer ei

a l . ,

1966).

With decreasing bulk-rock MgO contents, the maxi-

mum amount of Ca (relative to Mg and Fe*+) in

clinopyroxene cores decrease s, and the comp ositions

generally shift from diopside to salite-augitie (Fig. 3): a

comm on phenom enon in pyroxenes from arc volcanic

rocks. Slam et clinopyroxenes contain approxim ately

15-20 mol% Othe r s comp onents. As in orthopyroxenes,

Al(IV) makes up most of the Othe r s component in

clinopyroxenes. Mu da clinopyroxenes generally have

(a) MgO> 6 wt%

(b) 6>MgOw t%.4 (c) MgO < 4 wt.96

Fo

._ ~_ _~* -- ~__~__---_--_ ~_ _

c

fli fTr -- - ~----~--

- _- Fa -b

Fig. 3. Compositional variation of pyroxenes, olivines (symbols plot below the pyroxene qu adrants) and

amphiboles (symbols plot in the centre of the pyroxene quadrants) in terms of Ca, Mg and Fe (at ic

proportions). Hash marks at 10% increments. Rocks are divided according to MgO wt% [(a) MgO 2 6; (b)

6 > MgO 2 4; (c) MgO < 41. Symbols as in Fig. 2.


8/30

142


100

T

35 40

45 50 55 60 65

Mg (host rock)

Fig. 4. mg # of pyroxenes vs 100 * Mg/(Mg + Fe*+ ) of rock samples

(Fe

*+ = 0.85*XFe). Lines represent range

of phenocryst compositions: thick, solid = Muda; horizontally dashed = Lebaksiu; and diagonally

dashed = Tua (including Sirumiang). Closed symbols and x s represent clinopyroxene and orthopyroxene

inclusion c ompositions, respectively. Num bered curves (0.23 and 0.27) denote Kd values (Grove et al. 1982)

for clinopyroxene and orthopyroxene, respectively. Ve ry few inclusion compositions lie above the Kd curves.

slightly higher Ti levels relative to those of other Slamet

clinopyroxenes.

Compared with phenocryst interiors, the rim and

groundmass compositions from Slamet pyroxenes

usually have lower mgf values. Pyroxene inc lusions

occurring in other phenocryst phase s are usually lower

in mg# than that of the pyroxene phenocrysts from the

sam e rock (Fig . 4). A similar relationship is seen for

olivine inclusions and phenocrysts. The abund ance of

Othe r s

comp onents is variable, but is generally lower

than those of the phenocrysts.

As with plagioclase, zoning profiles across Slamet

pyroxenes can vary within single sam ples. Howev er,

mg # values within single pyroxene crystals do not vary

widely.

O l i v i n e s

The fosterite content of olivine cores ranges widely

(Fig.

3).

Rocks with MgO > 6 wt% have olivines ranging

in comp osition from

- Fow s .

Rocks with 6 > MgO

wt% > 4 also display a wide range of olivine Fo contents

(-foscrs~

. The only analysed olivines from rocks w ith

MgO < 4 wt% are from the Legokmene unit. The com-

positions for these olivines are approximately

Fob?.

Olivine rims and groundmass usually have lower mgf

values than coexisting phenocrysts. The Fo content of

rims and groundmass can be as low as

-Fo,,.

Olivine inclusions w ithin other phenocryst phase s are

usually lower in Fo compo nent than are olivine phe-

nocrysts from the same rock. This feature is illustrated

in Fig. 5, in which the range of Fo content for olivine

phenocrysts and inclusions is plotted against the mg#

value of the host rock.

Significant c hemica l zoning, in terms of mg# is rare

and subtle in olivine phenocrysts and can vary from

norma l to reverse zoning w ithin a single rock specime n.

O x ides

The method of Carmichael (1967) was used to calcu-

late Fe,O, in both spinels and hexagonal oxides. The

predominant oxide in Slamet lavas is titanomagnetite. In

terms of Ti, Fe2+ and Fe3+,

titanomagnetite phenocrysts

90

T

80

40 . . . . . ri.,,...a. ;,

45

50

55

60

65

Hg%l rock

Fig. 5.

Fo

contents of olivines vs 100 * Mg/ (Mg + Fe*+) of rock samples (Fe*+ = 0.85*EFe ). Solid symbols

represent range of phenocryst compositions; open symbols represent inclusion com positions. Circles and solid

lines = Muda; squares and lines with short da shes =

Lebaksiu; triangles and lines with long dashes = Tua

(including Sirumiang enclaves). Num bered curves (0.30 and 0.40) denote Kd values.


9/30

Evolution of G. Slamet Volcano Indonesia 143

from Muda lavas contain the highest ulviispinel com-

ponent. Although some Muda lavas have high TiOs

levels suggesting that the amount of ulviispinel in

magnetite is a function of bulk rock comp osition, many

lavas with low Ti02 contain mag netite with relatively

high amounts of ulviispinel (e.g. Keruh unit). Exper-

imental work demonstrates that high-TiOz magnetites

can crystallize under conditions of high pressure

(3 10 kbar; O sborn et al., 1978). This implies that Muda

magm as may have undergone limited crystallization at

high pressures and may have risen and erupted quickly

enough for the preservation of the high-TiOz m agnetites.

Certainly, the rarity of andes ites in the M uda sequence

is in accord with short resident times at shallow crustal

levels for the parental magm as. On the other hand, the

higher &&pine1 component in Muda magnetites may

also reflect precipitation of mag netite unacc ompa nied by

a TiOz-bearing phase, e.g. amphibole. As a result,

interpretation of TiOz contents in Slam et mag netites

remains equivocal without further study.

Very rare ilmenites occur a s inclusions in phenocryst

phase s of more evolved lavas. Geotherm ometers or

barom eters were not applied, for it is highly unlikely that

the ilmenites and m agnetites formed in equilibrium.

Chromium-bearing spinels occur in the more mafic

lavas, particularly as inclusions w ithin Fo-rich olivine

phenocrysts, and range in compo sition from picotite to

chromite. The Cr-rich oxides are probably remna nts

from the early stages of fractionation of mantle-derived

magmas.

Amph i b o l e s

According to the guidelines of Leake (197 8), Slam et

amphiboles are pargasites to ferroan pargasites. A mphi-

boles from Muda lavas are titanian pargasites ( T i > 0.25

atoms per 23 oxygen), whereas those from the Sirumiang

mixed andesite are potassian (K 2 0.25).

25 I

.

b

.

1 2 3 4 5 6 7 0 9

MgoH

Fig.

6.

(a) 1000* Zr/K vs MgO wt%. (b) Zr/Rb vs MgO wt%.

Muda lavas have higher 1000+ Zr/K and Z r/Rb than T ua

lavas. N-MO RB values for 1000 z Zr/K and Zr/Rb are z 120

and 80, respectively (Hofmann, 1988). Symbols as in Fig. 2.

40-

A

30 a'

a 0

XNucm

20. :

AEndnw

f As25 3

5167

0

OHat .

.

10 *I

. .

8 . ' ' 0 .

04 4

1 2 3 4 5 6 7 8 9

MgO_

l -

0.8 .'

9 0.6 . n

P 0. 4 . '

0. 2 a'

B

0

0

szs 0

OS167

.

0

CHmt

-0.

.

.

.

.

.

_

1

2

3 4

5

6 7

a

9

MgOwt%

Fig. 7. (A) Zr/Nb vs MgO w t%. (B) Hf/Nb VSIMgO wt%.

Muda and Lebaksiu lavas have lower Zr/Nb and; Hf/Nb than

Tua lavas . Symbols as in Fig. 2; x = N-M ORE / (Hofmann,

1988); S167 = Keruh; S25 = Cendana.

Geochemistry

The lavas of Slamet can be subdivided on the basis of

Zr/K and Zr/Rb ratios (Fig. 6). Two groups, recognized

on the basis of these ratios, age and incompatible

trace-element abundance have been named l+w and h igh

abundance magmas (LAM and HAM ; Vukadinovic and

Nicho lls, 1989). In terms of the geological units

described above, Tua (Sirumiang, Mendala and Sum-

baga), Lebaksiu and Cendana belong to the LAM

group;

and Mud a (Baturaden, Banyumudal and

Kubah), Keruh, Legokmene and Kalipagu belong to the

HAM group. In general, Muda lavas have greater Zr/K

(N 15) and Zr/Rb (N 5) ratios than those of Tua rocks

(~8 and -2, respectively), and Lebak siu lavas are

transitional between those of Mud a a nd Tua. However,

Cendana amphibole andesites have trace-element con-

tents and ratios that are characteristic of Tda rocks but

have 87Sr/86Srvalues that fall within the Muda range.

Similarly, the Keruh dacite has trace-element levels and

ratios resembling those of Mud a lavas but has *Sr/ Sr

similar to those of Lebaksiu lavas.

Other incom patible trace-element ratios also discrimi-

nate between LAM and HAM rocks, particularly Zr/Nb

and Hf/Nb; both ratios involve elements that are rela-

tively immobile in H,O/rock systems (e.g. Cann, 1970).

LAM rocks have Zr/Nb and Hf/Nb values of approxi-

mately 25 and 0.7, respectively (Fig. 7) whit are similar

to those of MO RB (BVSP, 1981). In HA

lv

lavas these

ratios are lower in value (Zr/Nb w 12; Hf/Nb -0.25)

and resemble those of enriched-MORB (cf. BVSP , 198 1;

le Roex, 1987).

M a j o r - a nd t r a c e- el emen t va r i a t i o n s

The variation of major a nd trace eleme nts, w ith

respect to MgO , in Slamet rocks is illustrated in Fig. 8.

Silica is almost constant (N 50-5 1 wt%) in rocks with

MgO levels ~4.5%. Exceptions include the felsic host

of

the Sirumiang mixed andesite (MgO x 5,


10/30

144


13

11 -

9-

7- i

AA

xf "s" Yx ". +

c*

. A

A

I I

0

0

?--A---

16,

4L . ' *' * ' . ' . ' . ' . ' *

12

3 4

5 6 7 8

9

M90

.

8

I . I .

I .

1 . I .

I .

1 .

12

3 4 5

6 7 8 9

hiO

Fig.

S Caption on p. 147.


11/30

Evolution of G. Slam et Volcano, Indonesia

145

LAM

I

I

.I.

I

.I ., .I ., .

I

600

0

500

t

0

400

i

i

?(

300 b. . ; . xx

t. . . - * * 1..

-

X

200

x x

8

100

:

L .,\x;xa

.

Lub

A

X

300

a

dX

xx x .

2001 . I * ' * ' * ' * ' . ' . ' .

1

2 3

4

5

6 7

8

r-

L

1

2

3 4 5

6 7

8 9

WO

Fig.

I Continu ed. Caption on p. 147.


12/30

146


300 -

LAM

40 -

30 -

. i

20 -

10 -

A

SQ

400 -

8

300 -

+

*ix ""*, x:

200 -

*U# t nl a

100 -

o- ~ ~ . ' ~ ~ ~ ~ ~

400

8

300 -

8

m

200 - t

X

## x

m

l oo -

A

X

x =

A xx

8

+

HAM

d

X

A

*

.

+

ixx

IJ

i,kf

+

x2, x

A xx

X

I

I .,.,.,.I.,.,.

o.aA--lA**..

12

3

4 5 6 7

8

9

MO

12 3 4

5

6

7

8

9

NO

Fig

8 Cont inued. Cap t ion op pos i te on p. 147.


13/30


147

100

80 -

LAM

m

x

x m

60 -

Y

x

' e

X

. z

40 -

A 3X

X

HAM

140

60 -

40 * '

*

' . * ' *' . * ' * I *

I ., , I .I . d . I .

12 3 4 5 6 7 8 9 12

3 4 5 6 7 8 9

w&J

W

LAM

HAM

X Lebaksiu

+ G. Cendana

A Sumbaga

Mendala

0 Sir um. Host

m Sirum. Enclave

0 Kawah

0 Kalipagu

Keruh

+ Legokmene

x Banyumudal

A Baturaden

Fig. 8. Major elements and selected trace elements vs MgO wt% for LAM (left-hand column) and HAM

(right-hand column). Oxides in wt% , others in ppm.

SiO, x 58 wt%) and, to a lesser degree, the Kawa h lavas

(MgO x 4.5, SiO 2w 53 wt%). LAM and HAM rocks

with MgO < 4.5% increase in Si02 with decreasing

MgO . The m ost silicic material is from the pumiceous

Keruh dacite unit (z 63 wt%).

TiO, content increases from -0.9 to 1.4 wt% with

decreasing MgO in LAM rocks with MgO > 6%; this

trend is defined mainly by the Lebaksiu sequence . The

Sirumiang felsic host has anomalously low values of

TiO, (-0.5%) relative to its MgO level. HAM rocks

have marginally higher TiO, abundance than LAM

rocks at comparable MgO levels. HAM lavas with MgO

values between 4 and 5% have TiOz contents ranging

from N 1.2 to 1.9%; the Kawah and Banyum udal units

define the lower and upper limits of this range. Evolved

HAM rocks display a positive correlation between TiO,

and MgO.

In both LAM and HAM basaltic rocks, A& O3

increases from N 15 to 18 wt% with decreasing MgO . In

rocks with M gO < 4%, A&O , remains between N 17.5

and 20% ; however, the Sirumiang felsic host has lower

Al,O, (w 16.5%). The Kalipagu andesites extend the

basaltic trend in which A &O3 increases w ith decreasing

MgO . Generally, A& O3 levels of Baturaden basalts are

slightly higher than those of Banyu mud al lavas.

Total iron, expressed as total Fe0 (FeO*), rang es

from x 11 to 5 wt% and behaves similarly to TiO,. The

Sirumiang felsic host has lower FeO* (N 6%) than that

of the main trend formed by LAM rocks. Note that

inflections on both the FeO * and TiOz vs MgO graphs

occur at identical MgO levels for LAM (-6%) and

HAM (-4.5%), implying magnetite saturation and

fractionation at these levels. Also, in HA M rocks with

MgO between 4 and 5% , FeO* and TiO, are both

anomalously high-particularly in Banyumudal lavas-

suggesting minor magnetite accumulation,.

CaO decreases regularly with decreasin

MgO in both

LAM and HAM rocks. The Cendana and fM

endala units

are slightly richer in CaO , and the Sirumiang felsic host

is poorer, compared with other LAM rocks. The CaO vs

MgO trend for HAM is tighter than that for LAM

rocks. In the HAM group, the Baturaden unit has higher

CaO levels than those of the Banyumudal unit. In HAM

rocks, the trend steepens slightly at MgO < 4 wt%.

Na, 0 contents

of LAM and HAM lavas

(-2.54 wt%) increase with decreasing MgO. Except for

the Lebaksiu unit, LAM lavas have slightly less Na,O

at any given MgO value compared with HAM lavas.

The Sirumiang felsic host has higher Na,O than the

trend defined by LAM rocks. In the HAM group,

Kawah rocks have higher levels of Na, 0 compared with

Baturaden lavas.

The abundance of K,O and Rb increases regularly

and similarly with decreasing M gO. Unlike Na,O, K,O

and Rb levels are similar between LAM and HAM lavas,

with ranges of m l-3 wt% and 20-80 ppm, respectively.

Within the LAM group, K and Rb levels are lower in

Cendana andesites compared w ith Sumb aga andesites.


14/30

148 D. Vukadinovic and I. Sutawidjaja

The less-mafic members of the Kalipagu andesites also Ca l c a l k a l i n e o r t h o l ei i t i c c l a ss l j i c a t i o n o r Sl ame t vo l c a n i c

plot below the main trend.

rocks?

Ba-MgO variation is similar to that of K and R b, and

the range of Ba content is large (N 100-600 ppm). The

Lebak siu basa lts lie above the trend defined by the

Sumb aga andesites and Mendala basalts. In HAM

group rocks with > 4% MgO , Ba increases only slightly

with decreasing M gO; however, at ~ 4% MgO , Ba rises

more sharply.

Both LAM and HAM lavas generally increase in Sr

(-275-350 ppm) with decreasing MgO . The Sirumiang

host and enclave material are both significantly richer in

Sr, compared with other Slamet rocks. The Baturaden

basalts and Kalipagu andesites have higher and more

varied Sr values (between 300 and 400ppm) than do

other units with similar MgO contents.

The definition of the terms calcalkaline and tholei-

itic has changed and diversified through time. The

problem is compo unded by the frequent usage of these

terms in the literature without workers specifying what

they exactly me an by them . Peacoc k (1931) first pro-

posed the term calcalkalic for rock suites with an

alkali-lime index between 56 and 61. Slamet lavas are

calcalkalic according to this classification schem e

(Fig. 9a). Peacocks alkali-lime index is seldom used

these days (except as a historical footnote); instead,

petrologists usually classify subalk alic, subd uction-

related rocks as either calcalkaline or tholeiitic in nature.

Zr levels are considerably lower in LAM

(N 50-l 75 ppm) than in HAM rocks (N 100-350 ppm).

In both groups, Zr content increases regularly with

decreasing MgO . The Lebaksiu basalts are marginally

richer, and the Cendana amphibole andesites poorer, in

Zr than are other LAM lavas at similar MgO contents.

Zr levels clearly distinguish M uda units from each other.

In particular, Baturaden basalts and Kalipagu andesites

are depleted in Zr, relative to Banyumudal basalts and

Legokmene andesites; this is also true for Y. Kawah

basaltic andesites, having Zr values (-200 ppm) inter-

mediate to those of the Banyumudal and Baturaden

basalts . In the Keruh dacite unit, Zr contents rise steeply

with decreasing M gO.

Wager and Deer (1939) defined tholeiitic series as

those showing significant Fe-enrichment relative to Mg

and alkalies during differentiation (e.g. Skaerg aard,

Greenland) and calcalkaline series as those lacking Fe-

enrichment (e.g. Cascades, w estern U.S.A.). AFM dia-

grams (Fig. 9b) are usually used to distinguish between

the two series. According to the definitions set out by

Irvine and Baragar (197 1), Slamet lavas are calcalkaline

but lie close to the dividing line between the series, with

some mafic samples actually plotting in the tholeiitic

field.

Yttrium contents are relatively constant at -25 pp m

throughout the spectrum of LAM lavas. The Cendana

and some of the Sumba ga andesites are noticeably lower

in Y compared with other LAM rocks. HAM rocks are

richer in Y, which increases slowly (N 25-35 ppm) with

decreasing MgO , compared with LAM rocks.

Similarly, Miya shiro (1974) regarded volcanic rocks as

either calcalkaline or tholeiitic in nature on the basis of

FeO*/MgO relative to silica. Gill (1981) used this dia-

gram to define tholeiitic rocks as those hav ing hig h

FeO */Mg O relative to SiOz, regardless of the slope of

the line. On the FeO*/MgO vs SiOr diagram (Fig. SC),

Slam et rocks lie predom inantly in the tholeiitic field as

defined by Gill (1981), even though the trend formed by

Slamet rocks has a shallower slope than that of the

defining line of Fig. 9c.

SC and V levels decrease with decreasing MgO in both

LAM and HAM groups, with slight steepening of the

trends at ~6% and ~5% MgO for LAM and HAM

rocks, respectively. Overall levels of SC and V are similar

between the

two

groups (Sc z 350-10 ppm,

V x 350-50 ppm). The Kalipagu andesites are slightly

richer in SC and V compared with the Legokmene

andesites.

In both LAM and HAM groups, Cr and Ni values

decrease with decreasing MgO levels. Overall C r con-

tents are roughly equivalent between the two groups

(max. w 350 ppm), but Ni levels are higher in LAM lavas

(max. N 80 ppm, cf. -60 ppm in HAM ). Low-MgO

Lebaksiu basalts have higher Cr and Ni abundance

compared with Mendala basalts and some Sumba ga

andesites. Baturaden basalts, particularly those with low

MgO, are richer in Cr and Ni than the other HAM

units.

Alternatively, the continuum between calcalkaline and

tholeiitic rock series can be separated on the basis of

LILE vs SiOz system atics. Due to greater am ounts of

data, KzO values are commonly used as representative

of LILE contents in the belief that the abundan ce of the

former mim ics that of the latter (Gill, 1 981). For

example, Jakes and Gill (1970) demonstrated a positive

correlation between L a/Yb (and La) and KzO in arc

rocks. Ac cording to the K, 0 vs SiOz classification schem e

(Peccerillo an d Taylor, 1976), Slam et volcanic rocks are

calcalkaline to high-K calca lkaline in character (Fig. 9d).

By considering four different m ethods of classifi-

cation, Slamet rocks can be pigeonholed twice as calcal-

kaline, once as tholeiitic, and once as transitional

between calcalkaline and high-K calcalkaline. Clearly,

classification of subduction-relate d igneous rocks is a

moot point. The discrepanc ies between the different

classifications are probably due to their monitoring of

Fig. 9 (opposire). (a ) Peaco cks ( 1931) alkali-lime index applied to Slamet r ocks. Slam et lavas are calcalkalic

according to this classification schem e. Circles = Na,O + K20; squares = CaO . (b) AFM diagram showing

LAM (open circles) and HAM (filled circles). Thin, dashed line with extrem e FeO-enrichment is the

Skaerg aard trend (Wa ger and Brown, 1967); thick, dashed line is the boundary between the tholeiitic (Th)

and calcalkaline fields (Ca) as defined by Irvine and Bar agar (1971). (c) FeO* /MgO vs SiO, for Slamet r ocks.

LAM (circles) and HAM (squares). Dividing line between tholeiitic ( TH) and calcalkaline (CA) fields is from

Gill (1981). (d) K,O vs SiO, for Slamet roc ks. Fields adopte d from Pecce rillo and Taylor (1976). Thick,

sub-horizontal lines define the following fields: I = tholeiitic; II = calcalkaline; III = high-K calcalkaline;

IV = alkaline. Thin, vertical lines define boundaries within the basalt-andesite-dacite compositional spectrum.

Symbols as in (c).


15/30


149

a

2 -- Na20 + K20

A lka l i - l ime

index B 60

01 + : 1

49

52

55

58

61 64

SiO2 wt.

N&O +

K20

w

4-

. c

3 a.

01..

.

-:.

- *:*.

. -:.

. .

I

45

50

55

60

65

SiO2 wt

4

3

P

2

1

0

I

d

hall

45

50 55 60 65

Si02 wt%

Fig

9-Caption opposite.


16/30

150


different aspects of magma genesis. For instance,

FeO*/MgO vs SiOz tells us much about the crystal

fractionation history of a suite, particularly the role

of ferromagn esian mine rals (especially mag netite; see

Osbo rn, 1959), but little about the nature of the source

material of the magm a. On the other hand, a KzO vs

SiO, plot m ay indicate if a suite of rocks were de-

rived from a LILE-enriched source, but it does not

reveal much about the nature of high-level fractionation

processes (unless a K-rich phase is involved, usually a t

higher levels of SiOz, or significant crustal assimilation

has taken place). An extensive survey by Gill (1981)

indicates that the supposed inverse relationship between

Fe-enrichment and K,O contents in arc rocks (Jakes and

Gill, 1970) is . . . crude at best . . . (p. 107).

In summary, one can unambiguously state that Slamet

lavas are subduc tion-related, subalka lic rocks that cover

a wide compositional spectrum. One can also state

(1) that th e rocks are relatively K-rich and (2) that they

show some Fe-enrichment in the mafic end of the com-

positional range. These are two characteristics that may

reflect the nature of different regimes: magma source (cf.

Vukadinovic and Nicholls, 1989) and magma cham ber.

100 k

I

*

E

E

/

Mg016 wt.

?I

11

100 L

I

b

0

x

*

5

4

10

G

E

li

6>Mg0>4 wt.

111

t

I

100 L

I

Mg014 wt.%

a ,i

La Ce Pr Nd

SmEuGdTbDyHoEr Yb

Fig. 10. Chondrite-normalized R EE patterns for HAM and

LAM. Rocks are divided according to MgO wt% content.

Symbols: filled squares = HA M; open squares = LA M. Nor-

malizing values from Taylor and McLennan 1985).

Rare ear th e lements

Figure 10 is a series of conventional chondrite-

normalized REE plots for Slamet volcanic rocks within

one of three MgO ranges (MgO 2 6 wt%; 6 > MgO >

4 wt%; MgO < 4 wt%). Across the spectrum of MgO

values, REE abu ndance is consistently lower in LAM

than in HAM lavas. Significant Eu anomalies

(1.05 < Eu/Eu* < 0.95) occur in Slamet volcanic rocks;

however, Eu/Eu* is generally smaller (i.e. larger down-

ward trough) in the HAM rock group. Furthermore, as

MgO decreases from 26 to 1.05) occurring in Slamet material.

The significance of these anomalie s is difficult to asses s.

Hole et a l . (1984) attributed the occurrence of negative

Ce anomalies in arc magm as to the participation of

subducted sediments that have strong negative Ce

anom alies. Conversely, the presence of negative Ce

anomalies in Lesser Antilles arc basalts and largely

positive ones in the fore-arc sediments (D SDP 543;

White

et al. ,

1985) indicates that Ce decoupling

may be due to relatively o xidizing con ditions in the

magm a-source region (White and Patchett, 1984).

M an t l e- no rm a l i zed t r ace -elemen t abundan ce d iag ram s

A notable feature of the mantle-normalized diagrams

(Fig. 11 ) is the behavior of Nb in LAM and H AM rocks.

As with REE, Nb, Zr, Hf, Ti and Y levels are consist-

ently lower in LAM than in HAM lavas throughout the

MgO spectrum. As MgO decreases, Zr and Hf become

progressively enriched relative to Sm and Y; however, Ti

develops distinct negative anomalies. This suggests that

as Slamet magm as evolved, Zr and Hf became m ore

incompatible than did Sm and HREE, which in turn

becam e more incompa tible than Ti. The onset of

Ti-magn etite crystallization in basaltic-and esitic and

andesitic magm as will certainly cause Ti depletion in the


17/30

Evolution of G. Slam et Volcano , Ind onesia

151

&@CJs?6wt. %

6>MgOL4wt. %

Fig. 11. Mantle-normalized diagram with rocks grouped as in

Fig. 1 0. Symbols: HAM = filled circles; LAM = open circles;

S167 (Keruh dacite) = filled squares. Normalizing values from

Taylor and McLennan (1985). Shaded a rea in (c) is the range

of HAM values from (b).

liquid, the crystallization of which is in accord with petro-

graphic and major-element observations. It is also well

docum ented that the crystal/liquid partition coe fficients

for REE increase with increasing polym erization of

silicate melts (e.g. Watson, 1976; Ryerson and He ss,

1978; Mahood and Hildreth, 1983; Nash a nd Crecraft,

1985 ; Lesher, 1986). If increased melt polymerization

affects Hf and Zr partition coefficients less than those for

the REE, then enrichm ent of Zr and Hf relative to the

mid-R EE can occur with progressive fractionation.

Strontium levels are similar in LAM and HAM

throughout the MgO spectrum. Negative Sr anomalies

are common in HAM rocks and increase w ith decreas-

ing MgO . LAM lavas generally lack strontium anom-

alies, sugge sting that a greater proportion of plagioclase

existed in the fractionating assemblage of HAM magm as

compared with those of LAM (see Vukadinovic, 1993,

for a discussion on Sr anomalies in arc basalts).

The highly incompatible elements (e.g. Cs, Th, U)

occur in similar quantities in LAM and HkM rocks at

comparative MgO levels. Caesium shows the most scat-

ter, possibly reflecting the difficulty involve

ds

in obtaining

precise ana lyses for this element via S MS :

conse-

quently, the Cs data should be used with caution. Ratios

such as Ba/Rb and Th/U are relatively constant through-

out the whole MgO range for both LAM and HAM

groups, but Lebaksiu magmas have slightly higher

Ba/Rb than that of other Slamet rocks.

Strontium isotopes

The range

of *Sr/%r ratios from Slam et

(0.70478-0.70629) is among the widest known from a

single arc volcano. In general, 87Sr/86 Sr atios de crease

with decreasing age in Slamet volcanic ocks: LAM

rocks have higher 87Sr/86Sr 0.70565-0.7062

b

) compared

with those of HAM (0.70478-0.70578). This trend is

maintained by the Kawah unit, erupted in 1973, which

has the lowest Sr/%r ratio.

*Sr/%r ratios

show considerable scatter with

respect to both MgO a nd SiOz and correlate negatively

with IMITER ratios Zr/K and Zr/Rb (Pig. 12) the

significance of which is considered below.

Magma Evolution Process&

Introduction

Ideas on the origin of andesites during the last 30-40

years are many and varied. Although in the 1950s

numerous workers (following Bowen, 1928) had sug-

gested that basa lts are parental to andesite s v ia crystal

fractionation, and as such . . . the origin of andesite

magm as cannot be discussed independently 1rom that of

basalt mag mas (Kuno, 1968, p. 149), petrologists have

devoted muc h time and effort to attem ptink to demon -

strate a primary origin for andes ites.

25

0. 7045 0.705 0. 7055

0. 706

0. 7065

67w66Y

Fig. 12. 1000 + Zr/K vs *Sr/% r. Error bars at different values of 1000 + Zr/K assuming f 10% fo r Zri and

K. Field enclosed by dotted line = low-*Sr/?Sr Muda lavas; dashed line = high-*Sr/Sr Muda lavas.

Symbols as in Fig. 2 and open diamonds = other units.


18/30

152


On the basis of liquidus experime nts for a range of

calcalkaline co mpositions (high-Al basalt through to

rhyolite), Green and Ringw ood (1968) concluded that

primary andesite mag ma can be derived from anhydrous

melting of quartz eclogite, in turn derived from sub -

ducted basalt. This type of direct origin for andesites was

advanced primarily by Marsh and Carmichael (1974)

and Marsh (1976,1978). Subsequent advances in knowl-

edge of equilibrium trace-element (especially RE E) par-

titioning behavior between liquids and coexisting solid

phases (e.g. Gast, 1968; Shaw, 1970) revealed that

andesitic magm as are unlikely to have been in equi-

librium with significant amounts of garnet either as a

residual phase during source melting or as part of a

fractionating assemblage (e.g. Gill, 1974, 1978; Nicholls

and Harris, 1980). To avoid this problem, Marsh and

co-workers have instead proposed that high-Al basalt

mag ma s can be generated from eclogite by degrees of

melting sufficient to eliminate garnet from residues and

that andesitic magmas are subsequently produced by

fractionation (e.g. Brophy and Marsh , 1986). In spite of

this, support for mode ls of primary andesite (or, for that

matter, arc basalt) generation directly from deep eclogite

melting has been largely abandoned. The present con-

sensus on the main role of the subducted lithosphere is

that it acts as the source of metasomatic agents that

influence the chem istry a nd melting behavior of the

overlying mantle wedge.

Overlying arc crust may also be involved in the

generation of andesite s. Frequently, the formation of

rhyolitic m agm a is attributed to the melting of continen-

tal crustal material alone. Ew art et

al.

(1968) and Ewart

and Stipp (1968 ) called upon pa rtial fusion of

greywack e-argillite basem ent to produce the liquids

giving rise to the rhyolitic volcanic rocks of the North

Island, New Zealand. Blattner a nd Reid (1982) have

opposed this interpretation on the basis of oxygen

isotope data, concluding that the magm as in question

were originally m antle derived but underw ent extensive

greywacke con tamination during their ascent to the

surface. In addition, crystallization experime nts by

Conrad et al. (1988) demonstrated that a peraluminous

source (such as the greywacke comprising the North

Island basement) for the metaluminous North Island

rhyolites was unlikely. Nonetheless, some exam ples of

rhyolites derived solely by crustal m elting appa rently

exist. For example, a crustal melting origin wa s con-

sidered on the basis of 87Sr/86 Sr atios obtained for the

Sumatran Toba ignimbrite (W hitford, 1975b) and for

both the rhyolites an dandesites of the Padang area, West

Sum atra (Leo et al., 1980). However, such models of

simple crustal fusion are rarely invoked for the genesis

of andes ites, p articularly for those from intra-oceanic

arcs. Exceptions include unusual andesite occurrences

such as the peralum inous, cordierite-bearing lavas from

Ambon, Indonesia (W hitford and Jezek, 1979).

From the preceding accoun t, it is evident tha t andes-

ites are unlikely to represent purely prima ry ma gm as

from any type of source, exc ept in unusu al circum-

stances. Thus, ideas on andesite petrogenesis have come

full circle: most m odels prese nted in the current litera-

ture call largely upon crystal fractionation from parental

basalts (see Gill, 1981,

p.

272). The aspect in which

current mode ls diverge is in the openness of their

magm a cham bers. For example, wha t are the relative

proportions of crustal assimila nt to crystallizing min-

erals involved in AFC (e.g. Briqueu and Lancelot, 1979;

DePaolo, 1981)? Does bulk assimilation occur, or do

only partial melts of the assimilan t mix with the crystal-

lizing ma gma (e.g. Patche tt, 1980)? If the latter ap plies,

does the partial melt mix thoroughly with the differenti-

ating magma, or are selective processes involved (see

Grove et

al.,

1988, for an example of many of the above

processes)? In addition to the above proce sses, are

mag ma cham bers pe riodically replenished by fresh infl-

uxes of magm a and tapped via eruption while continuing

to crysiallize (e.g. OHara and M atthews, 1981)? The in-

ability to answer conclusively the above questions allows

for only semi-qu antitative and equivocal m odelling.

Assumptions

In view of the above discussion, the modelling of

Slamet andesitic magm as was based on the initial

premise that they are, for the most part, simple differen-

tiates of the basalts, the origins of which were discussed

by Vukadinovic and Nicholls (1989). The major-element

trends and least squares-line ar regression calculations

semi-qua ntitatively support crystal/liquid fractionation.

The primary evidence for the operation of ma gm a

mixing at Gunung Slamet is the occurrence within phe-

nocrysts of pervasive mineral inclusions with chemistry

indicating that they were precipitated from liquids more

evolved than that which produced the phenocrysts them-

selves. Man y of the phenocrysts also show reverse zoning.

The preservation of mine ral inclusions derived from

more evolved liquids and the lack of similar inclusions

0.704 I

45 50 55

60 65

sw ) tw t

0 .704

J :

,

0 0 .5 1 1 .5 2

2.5 3

K2OWtlb

0.707 I

Fig. 13. SiOz, K,O, and U vs Sr/%r. Lack of correlation

suggests that the range of *Sr/% r ratios is not due to &ustal

assimilation. Symbols as in Fig. 12.


19/30


153

from less evolved liquids tha n those from w hich the host

phenocrysts precipitated implies that the more mafic

(and presumably hotter) endm ember involved in mixing

wa s probably at or above its liquidus tempe rature. If a

magm a at depth (e.g. basalt) rises adiabatically its

temperature may eventually rise above its liquidus in

P-T space. Mixing between a slightly superheated mafic

magm a and a body of cooler, more salic magm a would

give rise to mineral relationships, as seen in Slamet lavas.

Although *Sr/%Jr ratios for Slamet lavas span a

considerable range, positive correlations between

87Sr/86S r and elem ents that a re typically enriched in

continental crustal materials (e.g. SiO,, K,O, U) are

absent (Fig. 13 ). These types of positive correlations are

normally cited as evidence for assimilation of crustal

materials by magm as (e.g. Briqueu and Lancelot, 1979;

Thorpe et al., 1984; Graham and Hackett, 1987). Note

that the extremes of Sr-isotope comp ositions of Slam et

rocks are represented amongst the most mafic mem bers,

with M gO > 7 wt%. Although this evidence does not

exclude the occurrence of minor crustal assim ilation by

Slamet m agmas, it implies that the range and trends of

isotope ratios an d trace-element contents can be ex-

plained w ithout invoking such processes. Consequently,

the evolution of Slamet m agmas has been modelled

initially without invoking any AFC mechanisms.

Gerlach et

al.

(1988) described a similar situation with

lavas from the Puyehue-C ordon Caulle region, Chile.

Evolutionary routes for Slamet m agmas

Simple crystal/liquidfractionation.

Major- and minor-

element variations (except MnO) for Slam et basalt-

basaltic andesite-andesite series with both coherent

87Sr/86S r and incom patible trace element ratios (e.g.

Zr/K; Fig. 1 2) were modelled by crystal fractionation

using the least squares-line ar regression program XL-

FRA C (Stormer and Nicholls, 1978). Fractionating

phase s entered as input were restricted to those that are

observed as phenocrysts in either the parent or daughte r

compo sition for each fractionating step. The phase

compositions used in the modelling are the averages of

analyses obtained by electron microprobe from phe-

nocryst cores in the parent or other s imilar rocks. O n

occasion, pheno crysts from the daugh ter aom position

were used if data from the parent were unavailable or the

minera l in question is not present in the parent.

In the literature, results from such mode lling are

typically evaluated by the sum of the squares iof residuals

W).

Mod elling of Slam et rocks usually yielded

Cr2 0.25).

The solutions in Table 2 represent a stepwise transition

from basalt (S154 and S39) to basaltic andesite (S161) to

andesite (S75) and dacite (S167). Ma gnetite fraction-

ation is not required in the early stag es of mag ma

evolution (S154 to S39), but the later stages are depen-

dent up on it to generate the low TiOz contents of the

andesite s. A lthough they do not belong to the Lebak siu

Table 3. XLFRAC models of selected high-*Sr/?Sr Muda lavas

Parent Daughter Parent Daughter

s112

SlOl

Calculated Phase Wt% SlOl s149

Calculated

Phase Wt%

SiOz 50.45

51.03 51.04

51.03 51.35

51.31

TiO,

1.31 1.46 1.44

1.46 1.52

1.53

Al, 0, 15.72 16.73 16.76 16.73 18.48 18.47

FeO* 10.36

10.30 10.39 olv

3.49 10.30 9.73

9.73 olv

1.02

MgO

7.70 5.98 5.97 cpx

7.50 5.98 4.57

4.55 cpx

10.59

CaO 10.51

9.90 9.92 PI

3.01 9.90 9.26

9.28 Pf

-0.56

Na,O

2.88 3.36 3.26

3.36 3.66

3.71

R,O

1.06 1.26 1.24

1.26 1.43

1.42

Zr = 0.0210

%xtls removed = 14.00

Zr = 0.0046


Parent Daughter

Parent Daughter

Sl

S156 Calculated Phase

Wt% s29 s25 Calculated

Phase Wt%

Si02 51.60

55.44 55.29

54.32 58.28

58.20

Ti02

1.50 1.14 1.22

1.18 0.57

0.50

A&Q 18.93

18.43 18.35 cpx

9.80 18.68 18.87

18.88

FeO*

9.53 8.41 8.25

opx

2.81 8.80 6.67

6.62

MgO

4.28 3.43 3.46 Pl

25.04 3.64 2.54

2.56

CaO

9.46

PI

7.44

7.81 7.72 mt

3.77 8.45 8.06

1.89 hb

10.39

Na20

3.30 3.63 3.77

3.50 3.55

3.68 mt

3.24

R,O

1.39 1.71 1.94

1.45 1.44

1.66

Zr = 0.1407


Zr =

0.1086



21/30

Evolution of G. Slamet Volcano, Indonesia 155

sequence, samples S161, S75 and S167 were modelled

from Lebaksiu parental m agmas. Flat HREE patterns

indicate that am phibole played, at most, a minor role

in generating these three andesites-dacites and is in

accord with petrographic observations; consequently,

XLFR AC calculations were carried out without

amphibole fractionation.

XLFR AC results on high-87Sr/86Sr Mud a lavas

(Fig. 12) are given in Table 3. Aga in, m agnetite fraction-

ation is not nec essary in early s tages of differentiation.

This result is consistent with petrographic observations;

a magnetite-free assemblage persists into compositions

with MgO as low as -4.5 wt% (SlOl-S149). Note

that the high Al,O, content of S149 requires minor

plagioclase accumulation in the SlOl-to-S149 step, and

that the Na,O content of S149 is too high to allow it to

be parental to other high-*Sr/@Sr Muda andesites

such as S 156 and S178; therefore, S 1 was selected as an

appropriate parental comp osition. Amp hibole fraction-

ation is not required to produce high-87Sr/86Sr Muda

andesites.

Deriving Cendana hornblende andesites (e.g. S25)

from Muda-type magm as has thus far been unsuccessful.

The XLFRAC example shown in Table 3 has reasonable

Cr* (~0.1); however, K,O is poorly reproduced. A

suitable parental magm a to S25 (i.e. with similar *Sr/*jSr

and appropriate trace-element contents) w as not found

during the course of this study.

Good overall XLFR AC results (low Zr*) were

obtained for low-87Sr/86Sr Mud a lavas (Fig. 12).

Table 4 presents XLFRAC solutions showing stepwise

transitions from a mafic basalt (S104) to basalt (S117)

then branching in three directions towards a high-TiO,

basalt (S146), a low-TiO, basalt-basaltic andesite (S87),

and a basaltic andesite (S71). These results demonstrate

that the generation of a wide range of Ti02 contents in

the basa lt to basaltic-and esite transition can be gener-

ated with variable magne tite crystallization. Cond itions

for magn etite stability a re not well understood, but it is

believed that ma gnetite appea rs on the liquidus only if the

j-0, of the ma gm a is higher than the NNO buffer (cf. Gill,

1981 , p. 197), implying that f0, values were generally

lower in primitive Muda magm as than in Tua ma gmas.

Trace-e lement mode l l ing .

Simple trace-element mod-

elling of Slamet rocks was accomplished by utilizing the

results from XLFRA C to calculate crystal/liquid bulk

distribution coefficients (D) and proportions of residual

liquid (F) and then by applying these via the Rayleigh

Fractionation Law

C ,= C P-) (Shaw, 1970).

(I)

C, and C,, represent the concentrations of element i in

the daughter and parental liquids, respectively. A range

of publishe d mine ral/melt partition coefficient values

(Kd) , pertinent to the basalt-andesite spectrum, w as

used (Table 5). The following rules were used to deter-

mine the range of

K d

values applicable to any particular

model: (1) if the M gO content of the daughber magm a

being modelled was > 4 wt% (i.e. broadly basaltic), then

the K d values used were those between the minimum and

med ian values of Table 5; (2) if the Mg O content of the

daughter magm a w as


22/30

156


Table 5. Mineral/melt partitioning coefficients used for modelling evolved compositions

Kds

cs

Rb Ba Sr U Th Zr

Hf

Nb

olv(min) 0.00043 0.000179 0.00011 0.000191 0.0001 0.0004 0.0047 0.0038 0.0003

olv(max) 0.05 0.04 0.03 0.02 0.04 0.07 0.06 0.04 0.03

plg(min) 0.0248 0.01 0.02 I.2 0.002 0.004 0.0094 0.0092 0.01

plg(max) 0.13 0.2 0.59 3.2 0.06 0.05 0.03 0.03 0.025

cpx(min) 0.00035 0.001 0.001 0.0014 0.0003 0.006 0.05 0.05 0.005

cpx(ma x) 0.64 0.04 0.3 0.21 0.05 0.04 0.36 0.36 0.3

opx(min) 0.00009 0.0003 0.0003 0.01 0.0002 0.0009 0.02 0.02 0.0005

opx(ma x) 0.45 0.03 0.23 0.1 0.22 0.22 0.22 0.22 0.35

mnt(min) 0.0001 0.0001 0.0001 0.0001 0.008 0.02 0.02 0.02 0.4

mnt(max) 0.08 0.47 0.4 0.68 0.44 0.55 1.7 1.7 I

amph(min) 0.05 0.05 0.08 0.19 0.005 0.017 0.08 0.13 0.1

amph(m ax) 0.5 1.9 6.4 0.59 0.15 0.25 1.789 1.731 1.3

ap(min) 0.00002 0.00003 0.01 1 .3 0.46 0.94 0.005 0.015 0.002

ap(max ) 0.01 0.01 0.03 2 0.46 1.3 0.636 0.73 0.636

Kds

Y

La Sm Eu Gd

DY

Ho

Yb

olv(min) 0.002 0.0005 0.0019 0.0019 0.0019 0.0019 0.002 0.004

olv(max) 0.0308 0.008 0.0088 0.0096 0.0108 0.0148 0.0308 0.0468

plg(min) 0.01335 0.0348 0.0132 0.0221 0.0125 0.01 I2 0.0134 0.0155

plg(max) 0.0454 0.3017 0.1024 3.2 0.0665 0.0498 0.0454 0.041

cpx(min) 0.195 0.02 0.14 0.09 0.18 0.19 0.195 0.2

cpx(ma x) 2.2 0.4 1.3 1.4 1.7 1.9 2.2 1.4

opx(min) 0.0089 0.0005 0.0028 0.0036 0.0046 0.0072 0.0089 0.029

opx(ma x) 0.56 0.3 0.43 0.42 0.48 0.56 0.56 0.56

mnt(min) 0.049 0.005 0.009 0.007 0.016 0.038 0.049 0.072

mnt(max) 0.55 0.45 0.55 0.42 0.62 0.58 0.55 0.47

amph(min) I.1 0.14 0.8 0.83 0.96 1.15 I.1 0.8

amph(m ax) 3.7 0.7219 2.6 2.95 3.35 3.7 3.7 2.1

ap(min) 3.5 2.5 5.5 1.3 5 3.7 3.5 2.3

ap(max) 23 11.5 29.3 31.5 31 25.6 23 13.1

Kds

SC v Cr Ni Zn

olv(min) 0.02 0.03 0.3 4 1.2

olv(max) 0.37 0.09 34 58 I.5

plg(min) 0.01 0.01 0.01 0.01 0.04

plg(max) 0.15 0.07 0.08 0.25 0.25

cpx(min) 1.6 0.03 1.9 I.5 0.31

cpx(ma x) I7 I8 245 II.7

I2

opx(min) 0.53 0.025 2 I.1 2.6

opx(max) 7.5 7.2 143 24 4.4

mnt(min) 0.8 0.11 1 1.4 3.1

mnt(max) 3.3 67 620 77 I3

amph(min) 6 6 0.04 0.5 5

amph(max) I3 45 90 I6 8.7

ap(min) 0.029 0.01 0.048 0.2 0.2

ap(ma x) 0.22 0.01 0.2 2.3 0.2

Sources:

Nagasawa (1973); Hart and Brooks (1974); Shimizu (1974); McCallum and Charette (1978); Pearce and

Norry (1979); Luhr and Carm ichael (1980); Nicholls and Harris (1980); Watson (1980); Gill (1981); Villemant et al.

(1981); Watson and Green (1981); Shervais (1982); Day (1983); Irving and Frey (1984); Fujimaki et al. (1984); Ewart

and Haw kesw orth (1987); Green and Pearson (1987); Watson et al. (1987); Green er al. (1989); Wyers and B arton

(1989). Kds are interchanged between Cs and Rb, Th and U, Zr and Hf, Y and Ho , and SC , V, Cr, Ni and Zn when

data is lacking for one of these elements. T he minimum values are considered realistic for basalts, where as the maximum

values are appropriate for andesite-dacites.

observed liquids (Fig. 14b) may bc quantitatively repro-

duced by periodic m agma replenishment, tapping, and

fractionation.

Rep len i shmen t , t app i ng and f r ac t i ona t i on (RTF ) .

Steady-state RTF systematics, developed by OHara

(1977) and OHara and Matthew s (1981), were expanded

upon by Cox (1988) by randomizing the amounts of

crystallization, eruption and replenishm ent in each cycle,

represented by the variables x, y and z respectively. Cox

found that randomized RTF can produce results from

successive lava flows that are opposite to that expected

from simple parent/dau ghter relationships. For exam ple,

he was able to model two successive flows to have a

positive correlation for Zr vs Ni (incomp atible vs com-

patible). However, if the entire sequenc e of flows were

plotted, the overall correlation is negative (as is

expected) b ut with pronounced scatter, very similar to

the Zr-Ni relationship for Slam et lavas (Fig. 15). As Cox

points ou t, there is no unequivoc al evidence that R TF

processes take place in nature; but the idea is intuitively

acceptab le, particularly in subduc tion environm ents

where there is ample proof that m agmas rise and extrude

through specific points on the Earths su rface for

relatively prolonged periods of time.

Calcu lations in this study were carried out on Micro-

soft EXCEL spreadsheets, which h

1995 geology, mineralogy and magma evolution of gunung slamet volcano, java, i.pdf

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