1995 Geology, mineralogy and magma evolution of Gunung Slamet Volcano, Java, I.pdf
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Transcript of 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
D. Vukadinovic and I. Sutawidjaja
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.
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D. Vukadinovic and I. Sutawidjaja
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.
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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.
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142
D. Vukadinovic and I. Sutawidjaja
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.
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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,
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144
D. Vukadinovic and I. Sutawidjaja
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.
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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.
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146
D. Vukadinovic and I. Sutawidjaja
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.
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Evolution of G. Slam et Volcano , Indonesia
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.
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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).
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Evolution of G. Slam et Volcano , Indonesia
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.
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150
D. Vukadinovic and I. Sutawidjaja
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
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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.
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152
D. Vukadinovic and I. Sutawidjaja
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.
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Evolution of G. Slam et Volcano , Indonesia
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
%xtls removed = 11.61
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
%xtls removed = 41.42
Zr =
0.1086
%xtls removed = 21.07
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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
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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