Genesis of dacitic magmatism at batur volcano, Bali, Indonesia: Implications for the origins of stratovolcano calderas

Journal of Volcanology and Geothermal Research, 28 (1986) 363-378

363

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

GENESIS OF DACITIC MAGMATISM AT BATUR VOLCANO, BALI, INDONESIA: IMPLICATIONS FOR THE ORIGINS OF STRATOVOLCANO CALDERAS

G.E. WHELLER and R. VARNE

Department of Geology, University of Tasmania, G.P.O. Box 252C, Hobart, Tas. 7001, Australia (Received July 1, 1985; revised and accepted February 4, 1986)

ABSTRACT Wheller, G.E. and Varne, R., 1986. Genesis of dacitic magmatism at Batur volcano, Bali, Indonesia: Implications for the origin of stratovolcano calderas. J. Volcanol. Geotherm. Res., 28: 363-378. Batur is an active stratovolcano on the island of Bali, Indonesia, with a large, wellformed caldera whose formation is correlated with the eruption about 23,700 years ago of a thick ignimbrite sheet. Our study of the volcanic stratigraphy and geochemistry of Batur shows the formation of the caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic. The dacitic rocks are glassy, possess equilibrium phenocryst assemblages, and display compositional characteristics consistent with an origin by crystal--liquid fractionation from more mafic parent magmas in a shallow chamber, possibly at 1.5 km depth and 1000--1070°C. However, although separated by a gap of 6wt.% SIO2, the dacitic rocks are clearly related in their minor- and trace-element geochemistry to those basalts and basaltic andesites erupted after the caldera was formed rather than to the andesites erupted immediately before the dacites first appeared. We infer from this and published experimental modelling of the possible crystallization behaviour of basaltic magma chambers that a magmatic cycle involving caldera formation began independently of the previous activity of Batur by formation of a new, closed-system magma chamber beneath the volcano. Fractional crystallization, possibly at the walls of the chamber, led to the early production of derivative siliceous magmas and, consequently, to caldera formation, while most of the magma retained its original composition. The postcaldera Batur basalts represent the largely undifferentiated core liquids of this chamber. This model contrasts with the traditional evolutionary model for stratovolcano calderas but may be applicable to the origins of calderas similar to that of Batur, particularly those in volcanic island arcs.

INTRODUCTION S t r a t o v o l c a n o calderas, of ' K r a k a t o a ' (Williams a n d M c B i r n e y , 1979) or ' s t r a t o c o n e ' ( W o o d , 1 9 8 4 ) t y p e , are t y p i c a l l y m o d e r a t e - s i z e d (5 t o 15 k m i n d i a m e t e r ) c o l l a p s e c a l d e r a s w h o s e f o r m a t i o n is t r a d i t i o n a l l y c o n s i d e r e d

0377-0273/86/$03.50

© 1986 Elsevier Science Publishers B.V.

364 to involve differentiation of a basaltic magma chamber over long periods of time, leading eventually to p r o d u c t i o n of a cap of siliceous magma at the top of the chamber (Macdonald, 1972). This trend of increasing SiO2 c o n t e n t of erupted material with growth of the volcano until exhaustion of the dacitic or rhyodacitic magma, possibly accompanied by collapse of part of the volcanic edifice into the partially evacuated magma chamber to form a caldera, is generally taken as a volcanic cycle. In m any volcanoes (e.g. Krakatau), a new cycle appears to begin with the eruption of postcaldera basalts and the growth of intra-caldera volcanoes. Batur volcano, on the island of Bali in the eastern Sunda arc possesses a stratovolcano caldera that was described by van Bemmelen (1949) as " o n e of the largest and finest calderas in the world". In the traditional view, this volcano has evolved through one volcanic cycle. The eruption of numerous basaltic lava flows after caldera f o r m a t i o n indicates that another cycle has begun. However, we show here that the stratigraphy and petrogenesis of the material erupted from Batur do not support the traditional evolutionary model for stratovolcano calderas. Instead, we suggest the Batur data show, when viewed in conjunction with the results of recent modelling of the crystallization behaviour of basaltic magma chambers, that dacitic magmatism may also initiate volcanic cycles, leading to a new model for the origins of stratovolcano calderas. A more detailed geological, geochemical and petrological study of Batur volcanism will be presented elsewhere. TECTONIC AND GEOLOGICAL SETTING The Sunda--Banda volcanic island arc extends for 4 7 0 0 k m from the n o rth er n tip o f Sumatra in the west to the small island of Damar in the east, after which it curves tightly for 600 km to the north (Fig. 1). In Sumatra and western Java, volcanism may have been occurring since Triassic times but in the eastern part of the arc it seems to have begun only in the Middle to Late Miocene (Hamilton, 1979). The basement on which the m oder n volcanism occurs, changes progressively from continental crust 2O--30km thick in Sumatra and Java, through intermediate-type crust near Bali and L o m b o k to crust with oceanic thickness near Wetar and the Banda Islands (Curray et al., 1977; Purdy and Detrick, 1978). The crust beneath Bali is about 18 km thick and has seismic velocities similar to those of oceanic crust. Curray et al. (1977) suggest this crust is old, trapped oceanic crust that may have been thickened by reverse or thrust faulting caused by horizontal compression, but Hamilton (1979) considers it to be the edge of a continental shelf that underlies eastern Java and the Java Sea and which was built largely of subduction melange during the Cretaceous and Early Tertiary. The oceanic Indian--Australian plate is converging in a north-northeasterly direction towards the Sunda arc at 4.9 to 6.0 c m / y (Le Piehon, 1968). The

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seismic zone of earthquake foci beneath the arc reaches to approximately 650 km depth between Java and Flores, with a gap in seismicity between 300 and 500 km depth (Fitch and Molnar, 1970; Cardwell and Isacks, 1978). Beneath 100 km depth the seismic plane dips at about 65 °. Batur volcano lies approximately 160 km above the middle of the seismic plane (Hamilton, 1979). The oldest sedimentary rocks exposed on Bali are Late Miocene calcareous sandstones (Kadar, 1972) and new K--Ar data (Wheller et al., in prep.) show that the oldest-known Balinese volcanic rocks are pillow basalts of Late Pliocene age (Fig. 2). Most of the island is composed of subaerial volcanic sequences which, in the eastern half of the island, were erupted from the extinct Quaternary volcanoes Bratan, Batukau, and Seraja, and the two active volcanoes Batur and Agung (Purbo-Hadiwidjojo, 1971). The southern .4!

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parts of Bali are formed of upraised corm reefs of Pliocene--Pleistocene age (Kadar, 1977). BATURVOLCANO

The southern flanks of Batur dip gently for 60 km to the south coast of Bali but its northern flanks dip steeply to reach sea level within 6 km. The outer rim of Batur caldera is elliptical in shape, 13 km by 10 km, and there is an inner, circular collapse zone 7.5 km in diameter (Fig. 3). The outer caldera rim lies between 300 and 700 m above the level of an intra-caldera lake which covers about one third of the caldera floor. The summit of a new basaltic stratovolcano forming centrally within the caldera has reached approximately 7 0 0 m above the caldera floor, and 1 7 0 0 m above sea level. The summit of Agung volcano occurs at 3 1 4 0 m above sea level and lies 18 km southeast of the new Batur volcano. Y o k o y a m a and Suparto (1970) have shown that a high, positive Bouguer gravity anomaly, possibly caused by a body of excess mass (density > 2.5 g/cm 3) lying beneath the surface, occurs within Batur caldera.

Precaldera stage Batur volcano probably grew to 2500--3000m height above sea level before the caldera was formed (Fig. 3), Because of the widespread cover of younger tephra deposited from eruptions leading up to and following calderaformation, mafic lava flows produced during the main constructional stage are now best exposed in the deep barrancos cutting the northern flanks of Batur. A sequence of thin, basaltic lava flows is also exposed locally inside the caldera at the base of the caldera wall. These precaldera flows contain plagioclase and olivine, and rare clinopyroxene and Ti-magnetite as phenocrysts, and generally have glassy groundmasses. Thinly-bedded airfall and pyroclastic flow tephra deposits crop out outside the caldera in several places at both the northern and southern coasts. At Tanah Lot, at the southern coast of Bali and 50 km southwest of Batur, the sequence is at least 1 0 m thick, extending beneath sea level. Rounded, black, pumiceous andesite clasts up to 10--15 cm in size are the d o m i n a n t juvenile material in these deposits. This material is significantly more mafic than dacitic tephra which overlies these pyroclastic deposits and whose eruption appears to have signalled the initial stages of calderaformation. Samples from two flows at the base of one barranco, the Meja River, give K--Ar ages of 0.51 + 0.02 and 0.31-+ 0.04Ma respectively (Wheller et al., in prep.). Samples of two basaltic flows at the base of the caldera walls, inside the caldera, contain no detectable radiogenic At, indicating ages of less than 0.1 Ma. The Batur volcano therefore had been in existence for at least 0.4 Ma before eruption of the dacitic material began.

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Caldera stage The walls of Batur caldera mainly comprise dacitic pyroclastic rocks, including many types of welded and non-welded airfall and pyroclastic flow deposits. These rocks form a cap up to 700 m thick which overlies the predominantly basaltic volcanic products of the precaldera stage (Fig. 3). Mafic volcanic rocks are rare in the caldera walls and crop o u t only near the base, suggesting that the onset of explosive, dacitic eruptions occurred rapidly. Black, glassy, dacitic material is a ubiquitous c o m p o n e n t of the calderawall pyroclastic deposits, occurring as small, angular lithic clasts, as b o m b s and as highly vesiculated pumice. Plagioclase, clinopyroxene, orthopyroxene, olivine, Ti-magnetite and apatite form phenocrysts in these rocks and some contain groundmass pigeonite. Angular, andesitic microxenoliths are abundant in some rocks. Rare dacitic lavas have phenocryst assemblages similar to those of the pyroclastics but have more crystalline groundmasses. Marinelli and Tazieff (1968) correlated the formation of the Batur caldera to the eruption of the Bali ignimbrite, which was mainly deposited to the south of Batur and which extends to almost as far as the coast (Fig. 2). Our field work shows this pyroclastic flow to be locally at least 16 m thick and exposed over approximately 450 km:. The main part of the ignimbrite, the core facies, is composed almost entirely of crystal-poor pumiceous glass, forming a grey-coloured, homogeneous, fine-grained rock which is widely quarried and carved into the distinctive Balinese statues. Crystals of plagioclase, clinopyroxene, olivine, o r t h o p y r o x e n e and very rare hornblende occur in the ignimbrite. Large, dark-coloured pumice clasts become abundant towards the top and distal margins of the deposit, forming a distinctive marginal facies. Many clasts are rounded but none are flattened. The base of the Bali ignimbrite is best exposed at Marga in southern Bali where it rests over reddened soft. There, it is immediately underlain by a 2--3-cm-thick deposit of coarse sand-sized pumice which was probably produced during the initial plinian phase of the ignimbrite eruption (Sparks et al., 1973; Sheridan, 1979). Evidence of this deposit elsewhere has so far n o t been found due, at least in part, to the generally poor exposure of rocks on Bali. New 14C ages (analyses by M. Barbetti, The N.W.G. Macintosh Centre for Quaternary Dating, University of Sydney) of two charcoal clasts from the base of the ignimbrite at Marga are in excellent agreement and give a pooled age of 23,670 + 210 y. B.P. for the ignimbrite, in good agreement with an earlier lac age of 22,000 + 1 5 0 0 y . B.P. from charcoal collected a b o u t 20 km east near the Melangi River (Marinelli and Tazieff, 1968). The ignimbrite is overlain in central Bali by several plinian pumice deposits which are locally up to 1.5 m thick along the Batur caldera rim and which drape erosional gullies inside the caldera. These pumice deposits appear to have been erupted after caldera-formation from a vent (Bukit Pajang) located at the southwest wall of the inner collapse zone of the caldera {Fig. 3).

370

Postcaldera stage The new Batur stratovolcano has erupted m a n y basaltic lava flows across the caldera floor. These are relatively small-volume, vesicular flows which contain olivine, clinopyroxene, plagioclase and Ti-magnetite as phenocrysts. Nine flows have been erupted since 1849 over the western part of the caldera floor (Stehn, 1928; Neumann van Padang, 1951; Simkin et al., 1981) and several older flows occur in the eastern part. The youngest lava was erupted in 1974. The most voluminous historical flow was produced in 1963 when TABLE 1 Major- a n d t r a c e - e l e m e n t analyses of r e p r e s e n t a t i v e volcanic r o c k s f r o m B a t u r v o l c a n o Precaldera 67325

Number: SiO2 TiO2 A1203 Fe203 MnO MgO CaO Na20 K20 P2Os LOI H20rest total

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Postcaldera

67336

67342

67331

67273

67294

67238

67266

48.83 1.02 17.62 12.12 0.22 5.78 10.33 2.79 0.60 0.14 0.39 0.34 0.15

55.18 1.29 16.09 10.76 0.25 2.95 6.92 3.89 2.11 0.47 --0.28 0.21 0.17

58.58 1.33 15.29 9.40 0.23 1.80 5.06 3.51 3.38 0.61 0.01 0.50 0.20

61.69 0.79 16.09 6.64 0.21 1.43 3.98 5.23 2.32 0.31 0.55 0.35 0.14

64.59 0.77 16.18 5.83 0.22 1.16 3.25 5.50 1.60 0.28 0.03 0.07 0.13

65.83 0.55 15.25 4.72 0.18 0.56 2.05 5.16 2.66 0.14 1.65 0.54 0.14

52.94 1.02 18.16 10.20 0.21 3.62 8.78 3.63 1.29 0.23 --0.57 0.42 0.14

54.86 1.03 18.57 9.23 0.20 2.68 8.20 3.81 0.94 0.27 --0.41 0.08 0.15

99.55

100.01

99.90

99.73

99.61

99.43

100.07

99.61

Ba Rb Sr Zr Nb Y La Ce Nd Sc V Ni Cr

178 8 417 41 2 16 7 15 10 33 383 25 35

369 49 362 159 9 40 19 45 27 30 230 5 11

581 97 326 318 17 66 38 88 49 26 26 3 7

444 47 295 153 9 36 18 43 25 17 42 2 2

437 49 280 156 10 33 18 34 22 16 18 2 4

518 74 192 220 13 41 24 5O 26 16 6 2 2

232 22 428 78 5 22 16 24 15 27 265 9 9

268 23 431 89 6 26 14 26 18 27 228 6 8

Mg/(Mg+ Fe)

0.486

0.352

0.275

0.299

0.283

0.190

0.413

0.365

H 2 0 - a n d LOI (loss o n i g n i t i o n ) d e t e r m i n e d b y w e i g h t loss a f t e r h e a t i n g to 110 a n d 1 0 0 0 ° C , respectively. All o t h e r e l e m e n t s d e t e r m i n e d b y X R F . Rest is trace e l e m e n t s expressed as wt.% oxides.

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Fig. 4. Variation of MgO and SiO2 contents in wt.% with Zr contents in p.p.m, for volcanic rocks from Batur volcano (filled triangles = precaldera lavas, filled circles = postcaldera lavas, open circles = dacitic bombs and lavas, stars in circles----pumiceous andesite elasts, filled star = pumice clast in Bali ignimbrite (sample 67294 in Table 1), filled diamond = post-Bali ignimbrite pumice clast). CaO, FeO, A1203, TiO2, P2Os, MnO, Sr, Sc, V, Ni and Cr follow similar trends to MgO, and Na20 and Ba trends are similar to that of SiO2. Analyses normalized to total 100%, volatile-free, for plotting.

372 neighbouring Agung volcano also erupted, but more explosively (Zen and Hadikusumo, 1964; Rampino and Self, 1982). GEOCHEMISTRY The precaldera (49--60 wt.% SiO2) a n d postcaldera (52--56 wt.% SiO:) mafic rocks form coherent but separate compositional groups. The compositions of basalts in each group are similar, with chemical features, such as high A1203, low TiO:, and moderate abundances of K, Rb, Ba, and LREE, that are typical of low- to medium-K calcalkaline magmas (Table 1). However, the more evolved rocks of both groups define divergent geochemical trends. For example, decreasing MgO and increasing SiO2 contents along the trend defined by the postcaldera mafic volcanics are accompanied by only a mild increase in Zr contents (Fig. 4). Similar changes in MgO and SiO2 contents in the group of basalts, basaltic andesites, and pumiceous andesites that define the precaldera trend are accompanied by a much more marked increase in Zr contents. The dacites also form a coherent compositional group (62--69wt.% SiO:) that is distinct from both the precaldera and the postcaldera volcanics (Table 1, Fig. 4). The compositional variations within this group, which include decreasing MgO, CaO, TiO2 and P:Os contents and Mg/(Mg + Fe) values with increasing Zr contents, are consistent with crystal fractionation of the observed phenocrysts. Many of the larger dacitic b o m b s are prominently colour-banded. The light-coloured bands are cryptocrystalline and the dark-coloured are glassy. Chemical analysis by X R F shows no compositional differences between the bands. MINERALOGY The presence of complexly-zoned and inclusion-riddled plagioclase crystals in arc volcanics is generally taken as evidence of magma mixing. In the Batur dacites the plagioclase crystals have intermediate compositions (An60-31), are free of inclusions and show only weak zoning. Also, glass inclusions in pyroxene crystals are dacitic and the compositions of coexisting pyroxene and olivine solid solutions show sympathetic variations that are appropriate to the bulk compositions of the enclosing glasses (Fig. 5). These features suggest that the phenocryst assemblages in the Batur dacites are essentially in equilibrium with the rocks in which they occur. Analyses of plagioclase phenocryst cores in a typical dacite b o m b show a unimodal compositional distribution (Fig. 6), also indicating that mixing of different magmas has not occurred. Estimates from coexisting pyroxenes in the dacites (Lindsley, 1983) suggest magmatic temperatures of 1000-1070°C. Coexistence of olivine with both o r t h o p y r o x e n e and clinopyroxene in

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many of the dacites is significant. The Batur dacite olivines are Fe-rich (Fos6-32) and are unusual in that to our knowledge olivines of similar composition are rare in subalkaline volcanic rocks. In other examples of dacitic volcanic rocks with this assemblage, at Glass Mountain for example, the olivine is relatively Mg-rich and apparently crystallized from basaltic magma that then mixed with rhyolite to form dacite (Eichelberger, 1975). Chemical analyses of representative, coexisting phenocrysts in a Batur dacite are given in Table 2.

374 TABLE 2 Chemical analyses, by microprobe, of representative, coexisting minerals in a dacite bomb (sample 67331 in Table 1) from Batur volcano

SiO, TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K2O Si Ti AI Fe 3+ Fe 2+ Mn Mg Ca Na K

ol

opx

pig

epx

Ti-mag

34.27

52.31

52.00

50.70 0.59 2.46 1.55 10.44 0.58 13.93 19.75

18.15 1.86 32.42 45.40 0.94 1.23

38.71 1.37 25.68

0.977

0.923 0.033 1.090

0.77 2.35 19.77 0.83 22.43 1.53

1.950

0.42 0.83 24.09 1.26 18.71 2.70

1.979

0.034 0.066 0.616 0.026 1.246 0.061

0.019 0.024 0.767 0.040 1.062 0.110

1.907 0.017 0.109 0.044 0.328 0.018 0.781 0.796

total oxygens

3.023 l

3.999 6

4.001 6

4.000 6

Mg/(Mg + Fe 2*) Ca/(Ca + Na)

0.541

0.669

0.581

0.704

plag 57.20 27.05 0.29 9.49 5.69 0.28 2.566

0.506 0.081 0.906 1.409 0.030 0.068

3.000 4

1.430 0.011

0.456 0.494 0.016 4.973 8

0.480

All analyses are from cores of phenocrysts, except for pigeonite which is a groundmass phase. Fe203 calculated from stoichiometry. A p p l i c a t i o n o f t h e ' g e o h y g r o m e t e r ' o f M e r z b a c h e r a n d Eggler ( 1 9 8 4 ) suggests t h a t t h e B a t u r d a c i t e s m a y h a v e c o n t a i n e d 4 - - 5 wt.% H 2 0 p r i o r t o e r u p t i o n . Using t h e t y p i c a l B a t u r d a c i t e p h e n o c r y s t a s s e m b l a g e o f p l a g + o p x + c p x + T i - m a g + oliv a n d t h e i r p y r o x e n e c r y s t a l l i z a t i o n t e m p e r a t u r e s o f 1 0 0 0 - - 1 0 7 0 ° C , t h e P--T--H20 e x p e r i m e n t s o f M e r z b a c h e r a n d Eggler ( 1 9 8 4 ) o n M o u n t St. H e l e n s d a c i t i c l a p i l l i suggest t h e B a t u r d a c i t e s f o r m e d at v e r y l o w p r e s s u r e s , p r o b a b l y a b o u t 0.5 k b a r { a b o u t 1.5 k m d e p t h ) . T h e B a t u r d a c i t e s d i f f e r in c o m p o s i t i o n f r o m t h o s e used in t h e e x p e r i m e n t s , h o w e v e r , b y c o n t a i n i n g s l i g h t l y less n o r m a t i v e q u a r t z , a n o r t h i t e a n d e n s t a t i t e and more normative alkali feldspar. PETROGENESIS

T h e t r e n d d e f i n e d by t h e p o s t c a l d e r a m a f i c v o l c a n i c s , i n c l u d i n g h i s t o r i c a l

375 lavas, is clearly colinear with the dacitic pyroclastics trend (Fig. 4), implying a genetic connection between the two groups. We have found no field, petrographic or geochemical evidence that magma-mixing has occurred on a scale large enough to explain this relationship, nor is there any evidence that the dacites were produced directly by partial melting of mafic or ultramafic crust or mantle, another mechanism that could in theory generate this differentiation trend. Preliminary 180/160 analyses of some Batur dacites, which show 51So = 6.4--7.0°/00, and postcaldera basalts, in which ~ilSo= 5.8--6.9°/00, are consistent with derivation of both groups from a c o m m o n parental magma, with no evidence for assimilation of crustal country rocks. Strontium isotope compositions of a dacite and the 1974 basaltic lava flow from Batur are also very similar, 0.70407 (Whitford, 1975) and 0.70404 (J.D. Macdougall and M.O. Tanzer, pers. commun., 1985) respectively, and further support a magmatic differentiation model for the production of the dacites. Numerical modelling based on the Raleigh differentiation equation indicates the dacites : could have been produced by 80--90% closed-system fractional crystallization of a parental magma having a composition similar to that of the least fractionated of the Batur basalts. However, the traditional concept of a differentiating magma chamber, in which basaltic magma undergoes fractional crystallization as it solidifies causing the residual liquid to become progressively more SiO2-rich with time, is plainly inadequate to account for the Batur volcanism. That concept does not explain the compositional gap in the postulated differentiation series, or the continuing appearance of mafic members of the differentiation series up to 23,700 years after the last of the dacitic magma was erupted, nor would it have predicted the markedly divergent geochemical evolution of the Batur magmatism after the eruption of the precaldera pumiceous andesites (see Fig. 4). Recent experimental and theoretical analyses of evolving magma chambers seem to account better for the volcanic stratigraphy and geochemistry of Batur volcano. Cooling around the margins and r o o f of a closed-system magma chamber would lead to fractional crystallization in the sheath of magma near the walls (McBirney, 1980; Turner, 1980) but would not significantly affect the composition of the main mass of magma in the core of the chamber (Sparks et al., 1984). Fractionated liquids would escape upwards by boundary layer convection, because their density is less than the unfractionated magma, and collect near the r o o f to form a highly differentiated cap available for eruption. This process could have taken place soon after emplacement of the magma chamber (Sparks et al., 1984). The Batur dacites could have been produced in this way in a shallow basaltic magma chamber beneath the volcano. We suggest that the explosive eruptions of highly fractionated dacitic liquids that culminated in the major ignimbrite eruption a b o u t 23,700 years ago are being followed by periodic strombolian eruptions of the basaltic magma that forms the core of this

376 chamber. This model is the reverse o f the traditional volcanic evolutionary cycle: the more SiO2-rich liquids are erupted first and the basaltic magmas last. This pattern is similar to that p r o d u c e d within relatively short periods o f time during eruption o f pyroclastic rock deposits zoned compositionally from SiO2-rich at the base to SiO2-poor at the t op (e.g. Ritchey, 1980; Worner and Schminke, 1984}. If our interpretation is correct, the catastrophic caldera-forming eruptions o f Batur may not have been an inevitable stage in the evolution of the volcano, because it had already built a substantial cone over at least 0.4 Ma before conditions became suitable for the establishment of the magma c h a mb er which pr oduc e d the dacites. OTHER CALDERAS Most stratovolcano calderas occur, like Batur, in volcanic island arcs (Wood, 1984), but very few of them have been the subject of detailed petrogenetic studies. For example, we have identified seventeen young calderas in Indonesia alone, from descriptions given in N eum ann van Padang (1951), which are associated with basaltic to andesitic lavas and dacitic pyroclastics. Many of these, particularly Krakatau, have erupted rocks which have very similar chemical and mineralogical characteristics to those at Batur and some have also erupted postcaldera mafic lavas. In Papua New Guinea also, calderas with similar characteristics to Batur occur. The best know n of these is Rabaul, located at the eastern tip of New Britain island, which was t h o u g h t to be near eruption in 1983/84 (McKee et 70

I

I

1

~

t

i

65

0/ / I 60

Batur calderapostcaldera trend /

//

~

~

~fl

55

•

~

/

Batur precatdera trend

50

450

I

50

I

100

I

I

150

200

ppm

I

250

I

300

350

Zr

Fig. 7. Variation of wt.% SiO2 with p.p.m. Zr in volcanic rocks from Rabaul volcano (filled circles), New Britain (Heming 1974), compared to those from Batur.

377 al., 1985). Caldera f o r m a t i o n at this v o l c a n o o c c u r r e d relatively r e c e n t l y , a b o u t 1 , 4 0 0 y. B.P. ( H e m i n g , 1 9 7 4 ) , a n d residual dacitic activity m a y still be o c c u r r i n g ( M c K e e et al., 1 9 8 5 ) a l t h o u g h it has e r u p t e d p o s t c a l d e r a andesites ( H e m i n g , 1974). T h e g e o c h e m i c a l c h a r a c t e r i s t i c s o f t h e R a b a u l volcanic r o c k s are a l m o s t identical to t h o s e o f B a t u r (Fig. 7) a n d t h e range o f r o c k s e r u p t e d f r o m R a b a u l c o n t a i n s , like Batur, a p r o n o u n c e d gap b e t w e e n 55.5 a n d 6 0 w t . % SiO2 ( H e m i n g , 1974). We suggest t h e e v o l u t i o n o f K r a k a t a u a n d R a b a u l calderas, in particular, m a y be v e r y similar to t h a t o f Batur. ACKNOWLEDGEMENTS This research is p a r t o f a regional s t u d y o f y o u n g volcanic r o c k s in t h e e a s t e r n S u n d a arc f u n d e d b y t h e Australian R e s e a r c h G r a n t s S c h e m e and t h e University o f T a s m a n i a . F i e l d w o r k in I n d o n e s i a is a p p r o v e d b y t h e I n d o n e s i a n I n s t i t u t e o f Sciences a n d t h e Provincial G o v e r n m e n t o f Bali a n d s p o n s o r e d b y t h e U n i v e r s i t y o f G a d j a h Mada. GEW received a C o m m o n w e a l t h Postgraduate Research Award.

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