Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia

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1 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 PAGES doi:1.193/petrology/egi19 Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia O. REUBI* AND I. A. NICHOLLS VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES, SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, MELBOURNE, VIC. 38, AUSTRALIA RECEIVED NOVEMBER 18, 3; ACCEPTED JANUARY 7, 5 ADVANCE ACCESS PUBLICATION MARCH 11, 5 The Batur volcanic field (BVF), in Bali, Indonesia, underwent two successive caldera-forming eruptions that resulted in the deposition of silicic ignimbrites. The magmas erupted during and between these eruptions show a broad range of compositions from low-sio andesite to high-sio dacite. On the basis of their geochemistry and mineralogy these magmas may be assigned to six groups: (1) homogeneous andesites with phenocryst compositions essentially in equilibrium with the whole-rock composition; () remobilized crystal-rich low-sio andesites with resorbed phenocrysts in equilibrium with the whole-rock composition; (3) mixed low-sio dacite with a relatively large range of phenocryst compositions, with most phenocrysts slightly too evolved to be in equilibrium with the wholerock; () extensively mixed low-sio dacites with a very large and discontinuous range of phenocryst compositions, with most phenocrysts either more Mg-rich or more evolved than the equilibrium compositions; (5) remobilized crystal-rich low-sio dacites with resorbed and euhedral phenocrysts; (6) homogeneous high-sio dacites lacking evidence for magma mixing and showing narrow ranges of phenocryst compositions in equilibrium with the whole-rock composition. This range of silicic magmas is interpreted to reflect a combination of closed- and open-system fractional crystallization, magma mixing and remobilization of cumulate piles by heating. The variety of magmas erupted simultaneously during the caldera-forming eruptions suggests that the magmatic system consisted of several independent reservoirs of variable composition and degree of crystallization. The magmatic evolution of individual reservoirs varied from closed-system fractional crystallization to fully open-system evolution, thereby resulting in simultaneous production of magmas with contrasted compositions and mineralogy. Extensive emptying of the magmatic system during the caldera-forming eruptions led to successive or simultaneous eruption of several reservoirs. KEY WORDS: caldera; ignimbrite; magmatic chambers; magma mixing; petrology; Sunda Arc INTRODUCTION Volcanic activity at supra-subduction zone volcanoes is dominantly characterized by eruption of small to moderate volumes (<5km 3 ) of basaltic to high-silica andesitic magmas. However, eruptions involving large volumes (tens to hundreds of km 3 ) of silicic magmas associated with caldera collapse are not uncommon (e.g. Bacon, 1983; Aramaki, 198; Heiken & McCoy, 198; Druitt et al., 1989; Allen, 1) and can have dramatic outcomes as illustrated by the 1 BC Minoan eruption of Santorini (Heiken & McCoy, 198), the 1815 Tambora (Self et al., 198) and the 1883 Krakatau eruptions (Self & Rampino, 1981). Understanding the processes that control the genesis and evolution of these large-volume silicic magmatic systems at arc volcanoes is important in terms of hazard assessment. The large volume of magma erupted during calderaforming eruptions provides an extensive sample of the subvolcanic magmatic system. In addition, because of the almost instantaneous nature of these eruptions, the magma erupted represents a snapshot of the state of the magmatic system just prior to the eruption. As a result, the deposited ignimbrites are ideal for studying the characteristics of the associated silicic magmatic systems. A large number of studies of this type have been carried out (e.g. Foden, 1986; Bacon & Druitt, 1988; Mandeville *Corresponding author. Present address: Institute of Mineralogy and Geochemistry, University of Lausanne, 115 Lausanne, Switzerland. olivier.reubi@okeano.org # The Author 5. Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@ oupjournals.org

2 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY Ν BATUR VOLCANIC FIELD Batur fall deposits, reworked pyroclastite and soil 1 Bukit Penulisan Batur volcano historical lava flows (period 6) Penulisan and Peneloken fall deposits (period 5) Kintamani Batur 1 Payang tuff cone 1 et al., 1996), establishing the complexity of these systems and the importance of composition, temperature, crystallinity and magma mixing in the evolution of the magmatic systems. Nevertheless, the origin of large volumes of silicic magmas and the role of fractional crystallization of mantle-derived mafic magmas, crustal anatexis and/or remelting of crustal intrusive bodies in their petrogenesis remains a matter of debate. It is generally assumed that these silicic magmas evolve within a single large magma chamber. However, in the light of recent studies that suggest that the magmatic systems at arc volcanoes are essentially open systems, comprising several interconnected small reservoirs (e.g. Gamble et al., 1999; Dungan et al., 1; Streck et al., ; Reubi & Nicholls, 5), this may be questionable. This paper presents a detailed study of the mineralogy and geochemistry of the andesitic to dacitic magmas erupted at Batur volcano, Bali, Indonesia, during two successive catastrophic, caldera-forming, eruptions, and of andesitic to dacitic lava flows that were emplaced 1 JAVA SEA 115 E Batur volcanic field 8 1'S Bratan Caldera Bali km Gunungkawi Ign. Ubud Ign. Agung INDIAN OCEAN 8 5'S S SUMATRA 1E Lake Batur Lake Batur JAVA BALI Gunung Abang 1E 1 1 SULAWESI TIMOR Gunungkawi Ignimbrite (period ) Bunbulan lava-dome complex and Payang tuff cone (period 3) Ubud Ignimbrite (period ) Abang volcano (period 1) Penulisan volcano (period 1) Caldera I rim Caldera II rim Crater rim 1 km Fig. 1. Sketch geological map of Batur volcanic field. Insert shows the distribution of the ignimbrites related to the two caldera-forming eruptions. 1 within the caldera between the two collapse events. The aim is to establish the characteristics of the magmatic system prior to the two caldera-forming eruptions and to investigate the processes that controlled the genesis and evolution of the voluminous silicic magmatic reservoir. GEOLOGY OF BATUR VOLCANIC FIELD The late Quaternary Batur volcanic field (BVF), situated in northern Bali, Indonesia (Fig. 1) is part of the Sunda arc system, which is associated with northward subduction of the Indo-Australian plate beneath the Eurasian plate (Hamilton, 1979). The BVF is located about 15 km above the Benioff zone and adjacent to the active Agung volcano and the extinct or dormant Bratan caldera. The crust beneath Bali is about km thick, with an oceanic velocity structure (Curray et al., 1368

3 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI 1977). Most of the island consists of Tertiary to Quaternary volcanic rocks, with the southern part formed from uplifted coral reefs of Pliocene Pleistocene age (Purbo-Hadiwidjojo, 1971; Kadar, 1977). The BVF comprises two well-formed nested calderas and an active cone built within the smaller caldera (Fig. 1). Volcanic activity within the BVF may be subdivided into six main periods (Fig. 1) (Reubi & Nicholls, ), as follows. (1) Building of a basaltic to dacitic stratovolcano (Penulisan volcano) and a small parasitic cone on the SE flank. This activity started at least 51 kyr BP (Wheller & Varne, 1986). () Collapse of the first caldera (CI), associated with the eruption of dacitic ignimbrite (Ubud Ignimbrite) dated by 1 C at 9 3 years BP (Sutawidjaja, 199). (3) Formation of an andesitic to dacitic lava-dome complex (Bunbulan lava-dome complex) and a small dacitic tuff cone (Payang tuff cone) within CI. () Collapse of the second caldera (CII), again accompanied by eruption of andesitic to dacitic ignimbrite (Gunungkawi Ignimbrite), dated by 1 C at 15 years BP (Sutawidjaja, 199). (5) Andesitic to dacitic explosive activity within CII, producing pyroclastic fall deposits [Peneloken and Penulisan fall deposits of Sutawidjaja (199)]. (6) Building of the historically active, 17 m high, basaltic andesite Batur stratovolcano within CII. Detailed descriptions of the two ignimbrites have been given by Reubi & Nicholls (); only the key characteristic are summarized below. The Ubud Ignimbrite consists dominantly of pyroclastic flow deposits with minor pumice fall deposits. The intra-caldera succession comprises up to 16 nonwelded to densely welded pyroclastic flow units. The outflow succession covers most of southern Bali (Fig. 1) and comprises up to five flow units. The deposits are typically ash-rich, lithic clast-poor and contain 15% of pumices. Welding grades from incipient in distal sections to partially welded in more proximal settings. The pumices show a range of textures from highly vesicular (up to 75 vol. %) to black glassy fiamme but have a consistent mineralogy. There is no stratigraphic correlation between the intracaldera and the outflow successions, suggesting that the latter record an earlier phase of eruption, the products of which are buried beneath the observed succession within the caldera. The total volume of observed deposits is 18 km 3 [13 km 3 Dense Rock Equivalent (DRE)]. However, the volume of the associated caldera suggests that up to 6 km 3 could have been erupted (see Reubi & Nicholls, ). The Bunbulan lava-dome complex occurs in the NE sector of CI and forms the NE wall of CII (Fig. 1). The succession consists of six superimposed lava flows with minor intercalated pyroclastic flow and fall deposits. The dacitic lava flows at the base of the succession are thick (up to 3 m), massive and non-vesicular whereas the higher andesitic flows are thin (5 6 m), non- to moderately vesicular and have brecciated bases. The Gunungkawi Ignimbrite intra-caldera succession consists of interbedded accretionary lapilli-bearing ash surge, ash fall, pumice lapilli fall and thin pyroclastic flow deposits, overlain by a thick and massive pyroclastic flow deposit comprising 3% of pumices. The outflow succession occurs in central, southern, and northern Bali and comprises a single flow unit, which is underlain by intercalated pumice-rich and ash-rich pyroclastic flow deposits in northern Bali. The deposits are non-welded. Two distinct types of pumices are observed. The first type is grey, moderately to highly vesicular (5 85 vol. %), crystal-poor ( 5%) and occasionally banded, with alternating black and grey bands. The second type is black, moderately vesicular (5 65 vol. %) and moderately crystal-rich ( 5%). The total volume of the observed deposits is 7 km 3 ( km 3 DRE) but a volume of up to 9km 3 DRE may be expected from the size of the associated caldera. The Peneloken and Penulisan fall deposits cover the rims of the CI and CII calderas and the SW flank of BVF (Fig. 1) (Sutawidjaja, 199). The Peneloken fall deposits consist of interbedded ash-rich, accretionary lapilli-bearing fall deposits and pumice lapilli to bomb fall deposits. The Penulisan fall deposits consist of interbedded, normally graded pumice lapilli to coarse ash fall deposits. Pumices are highly vesicular (7 85 vol. %), crystal poor (<5%), and white to pink in the Peneloken fall deposits and moderately vesicular (5 6 vol. %), crystal poor (<5%) and dark grey to yellow in the Penulisan fall deposits. ANALYTICAL METHODS Whole-rock major element contents were determined by X-ray fluorescence (XRF) spectrometry on fused glass beads at the University of Melbourne, using a Siemens SRS3 instrument. Trace element contents were determined by inductively coupled plasma-mass spectrometry (ICP-MS) at Monash University on a Finnigan- MAT ELEMENT high resolution instrument. Samples were dissolved by HF HNO 3, HNO 3 and HCl acid digestion. Precision for trace elements is typically better than 5%. Mineral analyses were carried out on a CAMECA SX-5 electron microprobe at the University of Melbourne, using ZAF on-line data reduction and matrix correction procedures. The accelerating voltage used was 15 kv and the beam current 5 na for minerals and 1 na for the groundmass glass, with 1 s counting time. Analytical uncertainty is typically <1% for major elements. 1369

4 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY KO FeO andesite dacite high-k medium-k CaO (c) SiO WHOLE-ROCK COMPOSITIONS AND TEMPORAL EVOLUTION A total of 3 samples of the pumices from the two ignimbrites, the Peneloken and Penulisan fall deposits, and the Bunbulan lavas were analysed for their whole-rock compositions. Representative whole-rock major and trace element analyses are listed in Table 1. The complete dataset can be downloaded from the Journal of Petrology website at Selected Harker diagrams are plotted in Fig., and Th, Sr and Ba contents, and La/Yb and Zr/Nb ratios are plotted against SiO in Fig. 3. The analysed samples have tholeiitic major element compositions and trace element compositions typical of subduction-related magma. They range from medium-k andesite to medium- to high-k dacite [classification of Peccerillo & Taylor (1976)] (Fig. a). The Ubud Ignimbrite pumices range in composition from low-sio to high-sio (65 68 wt % SiO ) dacite (on a volatile-free basis). The Gunungkawi Ignimbrite grey pumices are high-sio andesite to high-sio dacite (6 67 wt % SiO ), whereas the black crystal-rich pumices are low-sio andesite to low-sio dacite (56 63 wt % SiO ). The Bunbulan lavas show a wide range of composition, from andesite to rhyolite (58 71 wt % SiO ) (a) (b) Ubud Ignimbrite pumice Bunbulan lava-dome complex lava Payang tuff cone lava Gunungkawi Ignimbrite grey pumice Gunungkawi Ignimbrite black pumice Peneloken and Penulisan fall deposits pumice MgO 1 AlO3 (e) Fig.. Variation of selected major elements vs SiO for Batur silicic pumices and lavas. The K O vs SiO diagram indicates the nomenclature used in this study (Peccerillo & Taylor, 1976). Analyses are normalized on a 1% anhydrous basis. SiO (in the following description and discussion, the only rhyolitic lava, B1, is considered to be part of the high- SiO dacite group). The Peneloken and Penulisan fall deposit pumices are high-sio andesite to high-sio (61 68 wt % SiO ) dacite in composition. Concentrations of compatible elements (e.g. MgO, CaO, P O 5, Sr) decrease rapidly with increasing SiO abundance whereas concentrations of incompatible elements (e.g. K O, Zr, Ba, Th) increase. Al O 3 abundance decreases only slightly with increasing SiO abundance. All samples define linear arrays, except two Bunbulan lavas (B5, B1) that have lower K O and higher MgO and Al O 3 at similar SiO contents. Two pumices from the Ubud Ignimbrite densely welded intra-caldera facies show abnormally high contents of K O that are not correlated with higher contents of incompatible trace elements, except Rb in one of these pumices. These high contents are believed to result from post-depositional vapour-phase alteration. All samples show similar primitive mantle normalized trace element patterns typical of subduction-related magmas (Fig. ; Table 1). The magnitudes of the negative Ti anomalies increase with increasing SiO. Eu and Sr show weak positive anomalies in the low-sio andesites (Eu/ Eu* ¼ 11 1) that progressively become negative (d) 137

5 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI Table 1: Major and trace element contents of representative samples from the Batur silicic magmatic system Ubud Ign. Bunbulan lava-dome Gunungkawi Ign. Sample: B6 #.18 #.38 #.35 B11 B1 B5 B7 B16 B8 pumice pumice pumice pumice lava lava lava lava pumice pumice SiO TiO Al O FeO * MnO MgO CaO Na O K O P O Total mg-no Sc V Cr Co Ni Cu Rb Sr Y Zr Nb Mo Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

6 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 Table 1: continued Gunungkawi Ign. Peneloken fall dep. Penulisan fall dep. Sample: #.1 B59 #.3 #.9 x-r #.31 x-r B79 B8 B81 #.15 pumice c-b pumice c-b pumice pumice pumice pumice pumice pumice pumice SiO TiO Al O FeO * MnO MgO CaO Na O K O P O Total mg-no Sc V Cr Co Ni Cu Rb Sr Y Zr Nb Mo Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U c-b pumice, colour banded pumice; x-r, crystal-rich. *Total Fe given as FeO. 137

7 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI Th Sr 1 Ba (c) rock/primitive mantle SiO Ubud Ignimbrite pumice Bunbulan lava-dome complex lava Payang tuff cone lava Gunungkawi Ignimbrite grey pumice Gunungkawi Ignimbrite black pumice Peneloken and Penulisan fall deposits pumice La/Yb Fig. 3. Variation of selected trace elements vs SiO for Batur silicic pumices and lavas. (a) (b) Zr/Nb (e) SiO andesite 56 wt% SiO (#.31) andesite 58 wt% SiO (B7) dacite 6 wt% SiO (#.19) dacite 68 wt% SiO (#.35) (d) 1 Cs Ba U Ta La Pb Mo P Sm Hf Ti Tb Y Er Yb Rb Th Nb K Ce Pr Sr Nd Zr Eu Gd Dy Ho Tm Lu Fig.. Primitive mantle normalized trace element patterns for representative andesites and dacites from Batur [normalizing values from Sun & McDonough (1989)]. anomalies with increasing SiO content (Eu/Eu* ¼ 9 7 in the high-sio dacites). The dacites have incompatible trace element ratios similar to those of the andesites (Fig. 3), but have slightly lower middle to heavy rare earth element (MREE/HREE) ratios (e.g. Dy/Yb ranges from 1 in the andesite to 15 in the dacite). On a plot of SiO content versus the relative stratigraphic positions of the samples within and between the different units, no clear overall trend toward more or less silicic compositions with time is observed (Fig. 5). Within each unit, the most silicic magmas tend to be erupted during the early phases of activity and generally become 1373

8 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 17 m Penulisan fall deposit 5 #.31 #.3 B59 Peneloken fall deposit Gunungkawi Ignimbrite (.15 Ky BP) 1 B7 B16 B5 Bunbulan lava-dome complex 55 #.18 B11 B Lava flow Pyroclastic flow deposits SiO Pyroclastic fall and surge deposits #.35 progressively more mafic with time. Significant and rapid variations are observed within the two ignimbrites and within the Peneloken fall deposits. GLASS COMPOSITIONS Matrix glass compositions from the Ubud and Gunungkawi Ignimbrite pumices are presented in Table and the variations in MgO and Al O 3 contents relative to SiO are shown in Fig. 6. The glasses show much the same compositional range as the whole-rock compositions. Glasses from the low-sio andesite sample #.31 and the high-sio dacite sample #.35 are systematically more SiO -rich than the whole-rock compositions. The low-sio dacitic pumices (samples #.18, B16, #.3 and B59) have glass compositions that range from slightly lower SiO to higher SiO contents than their respective whole-rock compositions. The two colour-banded, low-sio, dacitic pumices (samples #.3 and B59) show the broadest range of glass compositions. However, no systematic variations in glass composition were observed between the black and grey zones. Ubud Ignimbrite (9.3 ky BP) Fig. 5. Variation in SiO content vs relative stratigraphic position for Batur silicic pumices and lavas. Labels of the samples selected for detailed study of the compositional variation of phenocrysts are shown next to the symbols. PETROGRAPHY Ubud Ignimbrite Pumices in the Ubud Ignimbrite are petrographically homogeneous and contain 8 11% phenocrysts (vesiclefree basis) with plagioclase the dominant phase, followed by olivine, clinopyroxene, Ti-magnetite, orthopyroxene and ilmenite (Table 3). Apatite is a common accessory phase present as inclusions within the phenocrysts. Plagioclase typically displays oscillatory zoning (Fig. 7a). Occasional plagioclases with sieve-textured cores containing abundant large glass inclusions are observed within the pumices from the intracaldera succession (low-sio dacite). Crystal clots comprising plagioclase, clinopyroxene, Ti-magnetite, orthopyroxene or plagioclase, olivine, Ti-magnetite, clinopyroxene are frequent. Bunbulan lava-dome complex The Bunbulan lavas are porphyritic with plagioclase, olivine, clinopyroxene, Ti-magnetite and ilmenite ( 1% phenocrysts) and have microlitic textures. 137

9 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI Table : Representative chemical compositions of matrix glasses from Ubud and Gunungkawi pumices Ubud Ignimbrite Gunungkawi Ignimbrite Sample: #.18 #.18 #.35 #.35 B16 B16 B59 B59 B59 #.3 #.3 #.31 #.31 18g9 18g 35g13 35g5 b16g1 b168g B59gw6 B59gb6 B59g3 3g1-1 3g6-1 31g17 31g5 SiO TiO Al O FeO * MnO MgO CaO Na O K O Total *Total Fe given as FeO MgO Al O SiO B7 whole-rock (Bunbulan lava-dome complex) #.3 glass #.3 whole-rock B59 glass B59 whole-rock Gunungkawi Ignimbrite #.31 glass #.31 whole-rock B16 glass B16 whole-rock #.18 glass #.18 whole-rock Ubud Ignimbrite #.35 glass #.35 whole-rock Fig. 6. Variation diagrams comparing whole-rock and matrix glass compositions for the Ubud and Gunungkawi Ignimbrite pumices. Analyses are normalized on a 1% anhydrous basis. Plagioclase is the dominant phase in all the lavas and generally displays oscillatory zoning (Fig. 7b). Plagioclases with sieve-structured cores containing abundant glass inclusions are rare in the high-sio dacitic lavas, but are more common in the low-sio dacites and andesites. Clinopyroxene is more abundant than olivine except in the high-sio dacites (Table 3). Clinopyroxene phenocrysts in the andesite B7 are commonly mantled by pigeonite. Ilmenite is present only in the high-sio dacites. Apatite is a common accessory phase. Crystal clots comprising plagioclase, clinopyroxene, Ti-magnetite, olivine are common in these lavas. Gunungkawi Ignimbrite The grey, occasionally colour-banded, moderately to highly vesicular, high-sio andesitic to dacitic pumices that dominate in the Gunungkawi Ignimbrite contain 6 1% phenocrysts (vesicle-free basis) with plagioclase the dominant phase, followed by olivine, clinopyroxene, Ti-magnetite and ilmenite (Table 3). The majority of plagioclases are optically unzoned (Fig. 7c) or slightly oscillatory zoned. Plagioclases with sieve-textured cores containing large glass inclusions are occasionally observed. Plagioclase commonly occurs in crystal clots with olivine, Ti-magnetite, clinopyroxene. Apatite is 1375

10 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 Table 3: Modal analyses of representative samples from the Batur felsic magmatic system, quoted on a vesicle-free basis Ubud Ignimbrite Bunbulan lava-dome Gunungkawi Ignimbrite Sample: #.18 #.35 B11 B1 B5 B7 B16 B59 #.31 L-S dacite H-S dacite H-S dacite H-S dacite L-S dacite andesite L-S dacite L-S dacite L-S andesite Matrix Plagioclase Olivine Clinopyroxene Orthopyroxene <.1 <.1 n.o. n.o. n.o. n.o. n.o. n.o. n.o. Fe Ti oxides L-S, low-sio ; H-S, high-sio ; n.o., not observed. Fig. 7. Photomicrographs of the dominant plagioclase phenocryst textures in the Ubud and Gunungkawi Ignimbrites and the Bunbulan lavas. (a) Euhedral oscillatory zoned crystal in low-sio dacitic pumices from the Ubud Ignimbrite (#.18). (b) Euhedral oscillatory zoned crystal in high-sio dacitic lavas from the Bunbulan lava-dome complex (B1). (c) Euhedral crystals from the crystal-poor, colour-banded, high-sio andesitic to dacitic pumices from the Gunungkawi Ignimbrite (B59). (d) Resorbed crystal from the black, crystal-rich, low-sio andesitic to low- SiO dacitic pumices from the Gunungkawi Ignimbrite (#.31). 1376

11 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI Table : Representative compositions of plagioclase phenocryst cores Ubud Ignimbrite Bunbulan lava-dome complex Gunungkawi Ignimbrite Sample: #.35 #.18 #.18 B11 B1 B5 B5 B7 B16 B16 B59 B59 #.3 #.3 #.31 SiO Al O Fe O CaO Na O K O Total An % present as inclusions within the phenocrysts. Angular lithic fragments up to 5 mm in size are common within these pumices and comprise coarse-grained granodiorites, microdiorites, intersertal textured andesites and porphyritic andesites. The black, moderately vesicular, crystal-rich, low-sio andesitic to low-sio dacitic pumices contain 3 35% phenocrysts of, in order of abundance, plagioclase, olivine, clinopyroxene and Ti-magnetite (Table 3). In the low-sio andesites, the phenocrysts have globular morphologies indicative of extensive resorption. Plagioclases show particularly pronounced resorption textures and have sieve-structured cores full of very large glass inclusions (Fig. 7d). Euhedral phenocrysts are rare. In the low- SiO dacite, resorbed phenocrysts are also observed but euhedral crystals are dominant. Peneloken and Penulisan fall deposits The high-sio dacitic and high-sio andesitic pumices in the Peneloken fall unit contain <1% phenocrysts of plagioclase, olivine, clinopyroxene, Ti-magnetite, ilmenite. Plagioclase typically displays oscillatory zoning. Occasional plagioclases with sieve-structured cores are observed within the high-sio andesitic pumices. MINERALOGY AND MINERAL CHEMISTRY Two samples of pumice from the Ubud Ignimbrite (samples #.18 and #.35), four from the Gunungkawi Ignimbrite (samples #.31, B16, #.3 and B59), and four samples of lavas from the Bunbulan lava-dome complex (samples B7, B5, B11 and B1) were selected for detailed study of the compositional variation within the phenocryst assemblage. These samples were chosen to typify the ranges of composition and petrography observed within the various units and provide insights into temporal and mineralogical variation within the Batur silicic magmatic system. Plagioclase Representative analyses of plagioclase crystals are presented in Table. The complete dataset can be downloaded from the Journal of Petrology website. Histograms of plagioclase phenocryst core compositions are shown in Fig. 8. Also illustrated in Fig. 8 are ranges of plagioclase compositions in equilibrium with the whole-rock compositions calculated using Ca/Na plag K D min/liq (¼ X Ca liq plag liq X Na/XNa XCa) values in the range 55, based on the experimental studies of Baker & Eggler (1987), Sisson & Grove (1993) and Panjasawatwong et al. (1995). Four distinct composition distribution patterns are observed. (1) The high-sio dacites (#.35, B11 and B1) show essentially single populations centred on Canumber [1 Ca/(Ca þ Na)] 33 36, overlapping the range of equilibrium compositions. Plagioclase phenocrysts are largely normally zoned with superimposed oscillatory zoning. () The low-sio dacites (#.18 and B16) display a broad main population around Ca-number 56, within or slightly more Na-rich than the range of equilibrium compositions, and a few Ca-rich (Ca-number 61 9) outliers. Crystals from the main populations are normally or slightly reversely zoned, with superimposed oscillatory zoning. The Ca-rich outliers are normally or occasionally reversely zoned and have Ca-rich rims (Canumber up to 89). (3) The low-sio dacites (B59, #.3 and B5) show large peaks around Ca-number 31 36, which could clearly not be in equilibrium with melts similar to the whole-rock compositions, a series of small peaks between Ca-number 7 and 61 within the range of equilibrium compositions, and occasional Ca-rich (Canumber 61 9) outliers. Plagioclase phenocrysts show normal or slight reverse zoning with superimposed oscillatory zoning. Crystals that form the small peaks between 1377

12 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 # analyses # analyses #.31 L-S andesite #.3 L-S dacite B59 L-S dacite B16 L-S dacite x Ca/(Ca+Na) plagioclase #.18 L-S dacite #.35 H-S dacite x Ca/(Ca+Na) plagioclase Ubud Ignimbrite Gunungkawi Ignimbrite # analyses B7 andesite L-S low-sio H-S high-sio rim range of compositions in equilibrium with whole rock B5 L-S dacite B1 H-S dacite B11 H-S dacite x Ca/(Ca+Na) plagioclase Bunbulan lava-dome complex Fig. 8. Histograms showing the Ca-number distribution of plagioclase cores and the ranges of phenocryst rim compositions for selected Batur silicic pumices and lavas. Ranges of compositions in equilibrium with each whole-rock composition have been calculated using Ca/Na K D min/liq values of 55 (Baker & Eggler, 1987; Sisson & Grove, 1993; Panjasawatwong et al., 1995) and are represented by grey shades. Ca-number 7 and 61 have rims that are distinctively more Ca-rich than the crystals that form the large peaks. () The low-sio andesite #.31 and the andesite B7 show large and continuous ranges of composition with major peaks at around Ca-number 71 and 51, respectively. The majority of crystals in these samples could be in equilibrium with melts similar to the whole-rock compositions. Plagioclase phenocrysts are generally normally zoned with superimposed oscillatory zoning. Occasional slightly reversely zoned crystals showing an increase of up to 6 Ca-number from core to rim are also observed. In the low-sio andesite #.31, the rare euhedral plagioclases show a similar range of composition to the crystals with globular morphologies. Olivine Ranges of olivine phenocryst core and rim compositions are shown in Fig. 9a, and representative analyses are included in Table 5, together with the calculated ranges 1378

13 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI (a) Ubud Ig. Bunbulan L. Gunungkawi Ig. #.31 #.3 B59 B16 B7 B5 B1 B11 #.18 #.35 equilibrium with whole rock core rim L-S andesite L-S dacite L-S dacite L-S dacite andesite L-S dacite H-S dacite H-S dacite L-S dacite H-S dacite Ubud Ig. Bunbulan L. Gunungkawi Ig Fo% olivine (b) #.31 #.3 B59 B16 B7 B5 B1 B11 #.18 #.35 equilibrium with whole rock mg# pyroxene core rim cpx opx pig Fig. 9. Ranges of olivine (a) and pyroxene (b) composition in each sample. Grey shades represent the ranges of compositions in equilibrium with whole-rock compositions calculated using Fe/Mg K D min/liq values of 9 68 for olivine (Roeder & Emslie, 197; Ulmer, 1989; Sisson & Grove, 1993; Kilinc & Gerke, 3) and 3 3 (Sisson & Grove, 1993) for pyroxene and assuming Fe þ ¼ 9Fe in the magma. of olivines in equilibrium with the respective whole-rock compositions. The complete dataset can be downloaded from the Journal of Petrology website. The distribution coefficient Fe/Mg K D min/liq ranges from 9 to 3 for basaltic to basaltic andesite compositions (Roeder & Emslie, 197; Ulmer, 1989; Sisson & Grove, 1993), but increases to 68 for silica-rich (rhyolite) compositions (Kilinc & Gerke, 3). The calculations were performed using Fe/Mg K D min/liq 9 68 and assuming Fe þ ¼ 9Fe in the melt. As for plagioclase, the studied samples can be divided into four groups. (1) The high-sio dacites (#.35, B11 and B1) show small ranges of olivine composition from Fo to Fo 3, within the range of equilibrium compositions. () The low-sio dacites (#.18, B5 and B16) show olivine composition from Fo 56 to Fo 35 overlapping the range of equilibrium compositions. B16 also contains more Mg-rich outliers to Fo 58. (3) The low-sio dacites B59 and #.3 contain olivines clearly too Fe-rich (Fo 3 3 ) to have been in equilibrium with melts similar 1379

14 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 Table 5: Representative compositions of olivine phenocryst cores Ubud Ignimbrite Bunbulan lava-dome complex Gunungkawi Ignimbrite Sample: #.35 #.18 #.18 B11 B1 B5 B7 B16 B16 B59 B59 #.3 #.3 #.31 SiO FeO MnO MgO CaO NiO Total Fo % Table 6: Representative compositions of pyroxene cores Ubud Ignimbrite Bunbulan lava-dome complex Gunungkawi Ignimbrite Sample: #.35 #.35 #.18 #.18 B11 B1 B5 B7 B7 B16 B16 B59 B59 #.3 #.3 #.31 SiO TiO Al O Cr O Fe O FeO MnO MgO CaO Na O Total En Wo Fs mg-no T ( C) (a) (b) Temperatures calculated using the two-pyroxene thermometers of (a) Kretz (198) and (b) Lindsley (1983). Fe O 3 is calculated on the basis of four cations and six oxygens; mg-number ¼ Mg/(Mg þ Fe þ ) 1. to their host rocks and they also contain Mg-rich olivines (Fo 76 6 ). () The low-sio andesite #.31 shows a continuous range (Fo ) similar to the expected equilibrium compositions. The rare olivines in the andesite B7 are Fo 55 in composition. In all samples, olivine phenocrysts are normally zoned. Pyroxene Representative pyroxene phenocryst analyses are presented in Table 6. The complete dataset can be downloaded from the Journal of Petrology website. The mg-numbers [mg-number ¼ Mg/(Mg þ Fe þ ) 1] for pyroxenes are shown in Fig. 9b, along with mg-number 138

15 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI Table 7: Representative compositions of iron titanium oxide compositions Ubud Ignimbrite Bunbulan lava-dome complex Gunungkawi Ignimbrite Sample: #.35 #.35 #.18 #.18 B11 B11 B1 B1 B5 B7 B16 B16 B59 B59 #.3 #.3 #.31 SiO TiO Al O Cr O FeO * MnO MgO Total Fe O FeO Total X ilm 1 X usp T ( C) Cation assignment according to Stormer (1983). Temperatures were calculated using the geothermometer of Ghiorso & Sack (1991). *Total Fe given as FeO. ranges for pyroxenes in equilibrium with whole-rock compositions, calculated using Feþ/Mg K Dmin/whole-rock ¼ 3 3 (Sisson & Grove, 1993) and assuming Fe þ ¼ 9Fe in the magma. These values were obtained from experiments on basaltic to basaltic andesite melt compositions producing highly aluminous clinopyroxenes and may not be appropriate for dacitic melts and clinopyroxenes with low Al O 3 contents. Clinopyroxenes are augites with 5 35% Wo and 3 9% En. The orthopyroxene crystals found in Ubud pumices are hypersthenes with 3% Wo and 61 5% En. The low-ca clinopyroxene rims in the Bunbulan andesite B7 are pigeonite with 11 9% Wo and 5 5% En. The high-sio dacites #.35, B11 and B1 show clinopyroxene compositions with mg-number 67 53, too Mgrich to have been in equilibrium with melts similar to their host rocks. The orthopyroxene in sample #.35 has mg-number 5 5, within the range of equilibrium compositions. The low-sio dacites #.18 and B16 show broad clinopyroxene (and orthopyroxene in #.18) composition ranges from mg-number 83 to 51. Most crystals are more Mg-rich than the calculated equilibrium compositions. The low-sio dacites B59, #.3 and B5 contain two distinct groups; the principal one consists of Fe-rich clinopyroxenes (mg-number 61 5), the second one comprises occasional Mg-rich clinopyroxenes (mg-number 83 76). The low-sio andesite #.31 and the andesite B7 show large ranges in clinopyroxene composition with mg-number Most crystals are more Mg-rich than the equilibrium compositions. The pigeonite rims in B7 have lower mg-number, Pyroene phenocrysts are mostly normally zoned in all samples except sample #.31. However, occasional slightly reversely zoned phenocrysts showing an increase of up to three in mg-number from core to rim are also observed. Assuming the Fe 3þ calculated by stoichiometry, the high mg-number of the clinopyroxene rims in sample #.31 reflects increase in Fe 3þ contents from the core to the rim. High mg-numbers of clinopyroxenes reflect, in part, their high Fe 3þ contents (assuming the Fe 3þ calculated by stoichiometry). Nevertheless, even if total FeO contents are considered in the calculation of mg-numbers, clinopyroxenes remain more Mg-rich than the calculated compositions in equilibrium with the host rocks compositions. Fe Ti oxides Representative compositions of Ti-magnetite and ilmenite are presented in Table 7. The complete dataset can be downloaded from the Journal of Petrology website. Timagnetite in the high-sio dacites #.35, B11 and B1 ranges in composition from 69 to 55 mol % ulvöspinel, whereas the ilmenite content in the rhombohedral phase ranges from 96 to 93 mol %. In the low-sio dacites 1381

16 JOURNAL OF PETROLOGY VOLUME 6 NUMBER 7 JULY 5 #.18, #.3, B59, B5 and B16, the composition of Ti-magnetite varies from 66 to 7 mol % ulvöspinel and the composition of the rhombohedral phase varies from 98 to 93 mol % ilmenite. The low-sio andesite #.31 and the andesite B7 show ranges of titanomagnetite composition of 5 3 and 69 3 mol % ulvöspinel, respectively. ESTIMATION OF INTENSIVE PARAMETERS Temperature and f O Temperatures were calculated for coexisting orthopyroxene and clinopyroxene cores in the Ubud Ignimbrite pumices (Table 6). Temperatures calculated are between 11 and 9 C in the high-sio dacite #.35 and between 98 and 88 C for the low-sio dacite #.18. Compositions of pigeonite rims and adjacent clinopyroxene outer zones in the andesite B7 suggest temperatures between 18 and 11 C. Two-oxide thermometry calculations for the dacitic pumices and lavas are presented in Table 7. Ilmenite is present only in small quantities in the investigated samples and all analysed crystals were groundmass crystals. Only a few contiguous pairs were found. Most temperatures were calculated using non-contiguous groundmass ilmenites and magnetites that are in equilibrium according to the criterion established by Bacon & Hirschmann (1988). The ranges of temperature obtained always bracketed the temperatures obtained from contiguous pairs. Temperatures calculated for coexisting ilmenite and magnetite are substantially lower (up to 15 C) than estimates derived from two-pyroxene thermometry. The lower temperatures suggested by these methods are likely to reflect the xenocrystic origin of the pyroxenes (see below). For the Ubud Ignimbrite high-sio dacite #.35, the estimated temperature range is C. Temperatures between 78 and 736 C were obtained for the Bunbulan high-sio dacite lava B11. Fe Ti oxides in the Gunungkawi low-sio dacites B59 and #.3 yielded temperatures within the ranges and C, respectively. Temperatures for the low-sio dacite B16 range from 867 to 853 C. The pumice or fiamme from the Ubud Ignimbrite densely welded intracaldera facies (sample #.18) and two of the Bunbulan lavas (B1 and B5) gave temperatures <65 C, indicative of postemplacement low-temperature re-equilibration of the Fe Ti oxides. Estimated oxygen fugacities are between 3 and log units below QFM (quartz fayalite magnetite) for the high-sio dacite #.35, between and 6 for the low-sio dacite B16 and between 7 and 11 for the low-sio dacites B59 and #.3. Volatile contents The H O contents of the magmas that erupted to produce the two ignimbrites were estimated by the plagioclase melt hygrometer of Housh & Luhr (1991) for plagioclase rims and groundmass glass. Calculations were carried out using the ranges of temperature estimated from Fe Ti thermometry that are considered to be representative of the conditions that occurred late in the crystallization history (i.e. crystallization of the phenocryst rims) and pressures of 15 kbar (see below). Only plagioclase rims and adjacent groundmass glass compositions in equilibrium according to their Ca-numbers (for Ca/Na K D min/liq values between and 55) were used in the calculations. Nevertheless significant differences (up to 1 wt % H O) were observed between the average melt H O contents calculated from the independent anorthite and albite exchange reactions. The Ubud Ignimbrite high-sio dacite #.35 yielded average values of and 5 wt % for the Ab and An solution models, respectively. The Gunungkawi Ignimbrite low-sio dacites B59 and #.3 gave average values of 5 and 59, whereas average values of 8 and 55 wt % were obtained for the low-sio dacites B16. Despite the large uncertainties, these results suggest that the Ubud Ignimbrite magmas had lower H O content ( 5 wt %) than the Gunungkawi Ignimbrite magmas ( 5 6 wt %). Pressure No suitable mineral barometer assemblage is present in the Batur silicic rocks. The only constraint comes from the absence of amphibole, indicating that these magmas achieved water saturation at pressures below the lower stability limit of amphibole under water-saturated conditions in dacitic melts (1 kbar at 85 C; Gardner et al., 1995). DISCUSSION Magmatic processes Magma mixing The mineralogical and mineral chemical data presented above suggest that a large part of the silicic magmas at Batur volcano were the products of magma mixing or mingling. The low-sio dacites B59 and #.3 show the most compelling evidence for an origin by magma mixing, i.e. very large and discontinuous ranges of phenocryst compositions with most phenocrysts either too evolved or too mafic to be in equilibrium with melts similar to the whole-rock compositions, and large ranges of groundmass glass compositions extending from more mafic to more silicic than the whole-rock compositions. The low-sio dacite B5 also has a large and discontinuous range of plagioclase compositions suggesting an origin by magma mixing. The low-sio dacites #.18 and B16 show large ranges of phenocryst and groundmass 138

17 REUBI AND NICHOLLS SILICIC MAGMATIC SYSTEM EVOLUTION, BALI glass compositions, suggesting that they are the products of magma mixing. Nevertheless, a significant part of phenocrysts in these samples are in or near equilibrium with melts similar to the whole-rock compositions, suggesting either that the differences in composition of the two magmas that mixed were small or that only small volumes of more mafic magma were involved. In contrast, the high-sio dacites (#.35, B11 and B1) are characterized by narrower ranges in plagioclase and olivine compositions that are in equilibrium with the host lava compositions, and groundmass glasses that are more evolved than the whole-rock compositions or, in the case of the B11 and B1 lavas, groundmass plagioclases that have similar to lower Ca-number than the phenocrysts. These features suggest that the high-sio dacites represent homogeneous magmas. However, most pyroxenes in these samples are clearly more mafic than the olivines and the compositions in equilibrium with their host rocks (Fig. 9). Although the apparent lack of equilibrium with the host-rock compositions may result from calculations using distribution coefficients inappropriate for dacitic melts, the large offset in mg-numbers between the olivines and pyroxenes suggests that they did not crystallize in the same melts, implying a xenocrystic origin for the pyroxenes. Considering the absence of evidence for magma mixing, it is likely that pyroxenes represent xenocrysts carried by the dacitic melts, probably restite crystals inherited from an early phase of differentiation. The low-sio andesite #.31 and the andesite B7 also have groundmass glasses or groundmass plagioclases more evolved than the whole-rock or plagioclase phenocryst compositions, respectively, and show phenocrysts dominantly in equilibrium, suggesting that the low-sio andesites and andesites also represent essentially homogeneous magmas. However, the relatively large phenocryst composition ranges and phenocryst resorption textures observed in the Gunungkawi Ignimbrite, low-sio, crystal-rich andesite #.31 suggest that the melt did not evolve along a liquid line of descent. Causes of phenocryst resorption in the Gunungkawi Ignimbrite crystal-rich pumices Phenocryst resorption textures are ubiquitous in the Gunungkawi Ignimbrite black crystal-rich pumices (samples #.31 and #.9). Plagioclase shows the most extensive resorption features, but olivine and clinopyroxene also commonly show rounded shapes, suggesting that subliquidus crystal compositions were taken out of their domains of stability late in the evolution of these magmas. Resorbed plagioclases did not crystallize a rim following the resorption event, indicating that it happened just prior to the eruption. This may reflect several mechanisms: (1) mixing with a more mafic magma; () heating; (3) addition of volatiles to the melt; () decompression. As discussed above, magma mixing may be ruled out, in this case, on the basis of the evolved composition of the groundmass glass and by the absence of compositional contrast between resorbed and euhedral phenocrysts. Decompression affects the whole volume of magma and is, therefore, likely to result in resorption of all the phenocrysts present, which is not consistent with the presence of euhedral crystals in these samples. Couch et al. (1) have shown that heating of a partially crystallized magma body, for example by intrusions of hot mafic magma in the vicinity, can result in the development of a thermal gradient and convection. Convection occurs through generation of localized plumes, preventing complete homogenization and resulting in juxtaposition of crystals with varying thermal histories. This self-mixing process could explain the occurrence of both resorbed and euhedral phenocrysts as well as the large ranges of phenocryst compositions observed in these samples. Increase in melt volatile content may occur if mafic magma accumulates at the base of the magma body and cools. Volatiles exsolved during crystallization of the mafic magma are then transferred to the resident melt. However, this process is normally accompanied by heating and will, therefore, occur only in conjunction with the self-mixing process, enhancing remelting. In summary, the resorption textures observed in the Gunungkawi crystal-rich pumices are thought to result from heating and self-mixing in a crystal-rich magma body. A similar process is believed to have strongly influenced the evolution of the historical basaltic andesite magmas at Batur volcano (Reubi, ; Reubi & Nicholls, in preparation) and has also been proposed as the origin of several crystal-rich magmas erupted at arc volcanoes (e.g. Matthews et al., 1999; Murphy et al., ; Couch et al., 1). Fractional crystallization On the basis of the evidence presented in the previous sections, we conclude that magma mixing was involved in the generation of all magmas except the andesite and high-sio dacite magmas, which are considered likely to be products of simple fractional crystallization. It should be noted that the high-sio dacites contain unequilibrated pyroxenes interpreted as xenocrysts and are therefore not strict products of fractional crystallization. However, as a result of the very low modal proportion of pyroxene (Table 3), whole-rock compositions may be considered as representing homogeneous magmas. Consequently, only these two compositions have been used in our modelling. Previous studies of the overall magmatic evolution of Batur volcanic field have shown that the silicic magmas belong to the same suite as the basaltic andesite magmas erupted during the last 15 years (Wheller & Varne, 1986; Reubi & Nicholls, 5). Oxygen, strontium and thorium isotope ratios of Batur dacites and historical basaltic andesites are very 1383