Petrography and Geochemistry of Volcanic Rocks from Ungaran, Central Java, Indonesia Richard Claproth University of Wollongong

University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 1989 Petrography and geochemistry of volcanic rocks from Ungaran, Central Java, Indonesia Richard Claproth University of Wollongong Recommended Citation Claproth, Richard, Petrography and geochemistry of volcanic rocks from Ungaran, Central Java, Indonesia, Doctor of Philosophy thesis, Department of Geology, University of Wollongong, 1989. http://ro.uow.edu.au/theses/1398 Research Online is the open access institutional repository for the University of Wollongong. For further information contact Manager Repository Services: [email protected]. PETROGRAPHY AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM UNGARAN, CENTRAL JAVA, INDONESIA. A thesis submitted in partial fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY from THE UNIVERSITY OF WOLLONGONG UNIVERSfTY OF | by WDU.ONGON3NGG I LIBRARY-J RICHARD CLAPROTH, Ir. (ITB) DEPARTMENT OF GEOLOGY 1988 1(uaBdik_m untuk. istrilQi tercinta, Marisa; anafcatiaklQi, %&ma, 9hya & Joshua; guru, dan sahaBatH_i, TauC. Except where otherwise acknowledged, this thesis represents the author's original research which has not previously been submitted to any institution in partial or complete fulfilment of another degree. Richard Claproth ABSTRACT The development of Java Island which forms part of the Sunda Arc is due to subduction of the northward-moving Indian-Australian Plate beneath the Eurasian Plate. Ungaran volcano, Central Java, is situated 197 km above the Benioff Zone dipping at 55°, and forms part of the second of three cycles of volcanism recognized on Java Island. The volcano which was active between the Late Pliocene and Late Pleistocene is characterized by three stages of growth, interrupted by two episodes of cone collapse and the products of eruption can be grouped into four major units comprising Oldest Ungaran, Old Ungaran, Parasitic Cones and Young Ungaran. Lavas consisting of basalts, basaltic andesites and andesites are porphyritic with either a holocrystalline or hypocrystalline groundmass. Plagioclase, clinopyroxene, Fe-Ti oxide and amphibole are the major phenocrystic phases but biotite and olivine occur in some samples. In holocrystalline samples the phenocrysts are set in a fine-grained groundmass of feldspar, clinopyroxene, Fe-Ti oxide, and accessory apatite. Plagioclase phenocrysts range in composition from oligoclase to anorthite and some grains have thin rims of K-feldspar. Compositions of phenocryst rims and-coexisting groundmass plagioclase are similar but the groundmass grains have a more restricted range in compositions. With increasing age the plagioclase phenocrysts in all'rock types from Ungaran become less calcic. Clinopyroxene (diopside, augite and salite)Hs the only pyroxene in lavas from Ungaran and small changes in the compositions of phebocrysts in the series basalt to basaltic andesite to andesite are attributed to increasing silica activity and decreasing pressure. Magnetite is the only Fe-Ti oxide in all lavas from Ungaran. Most of the amphibole grains are magnesian-hastingsite and aggregates of "black amphibole" indicate conditions of rapid cooling at pressures less than 9 Kb. Biotite is a common accessory phase in basaltic andesites and andesites but is absent from basalts. Most of the fresh olivine occurs in basalts from Old Ungaran and it has a compositional range from F059 to F079. Lavas from Ungaran exhibit a continuum of compositions which range from 48.95% to 60.80% Si02. On the basis of K20 and Si02 contents, most of the basalts are shoshonites whereas most of the basaltic andesites and all andesites are high-K calcalkaline. Shoshonitic rocks dominated the early stages of magmatic activity whereas high-K calcalkaline rocks were produced during later stages. Compared with most rocks of similar SiC>2 content, the lavas from Ungaran are characterized by high contents of AI2O3 and total alkalies, high ferric/ferrous iron ratio, high contents of incompatible elements and low MgO contents. Mafic rocks from Ungaran range from Ne- normative to Q-normative depending on the ferric/ferrous iron used in the calculation. Most of the basalt samples, however, are saturated if an assumed ratio of 0.2 for Fe203/FeO+Fe203 is used but all are relatively evolved with a maximum Mg-number of 0.55. The low Mg-numbers indicate that these basalts crystallized from derivative melts , and do not represent primary, mantle-derived magma. Trace element modelling on the basis of published distribution coefficients and possible source compositions suggests that the rocks from Ungaran are generated by 5 to 10% melting of spinel lherzolite or amphibole lherzolite which had been previously enriched in incompatible elements. Subsequent to generation, 31.5 to 39.5% fractionation of early formed olivine and clinopyroxene in a ratio 30/70 produced the most mafic rocks in Ungaran. Rocks with < 53% S-O2 have a wider range and higher mean 87Sr/86Sr ratio than ^rocks with > 53% SiC_ and the available isotopic data are consistent with derivation of ^Ungaran lavas from heterogeneous OIB-type source. Depletion of Ta, Nb and Ti relative fto LILE cannot be attributed to a residual Ti-rich phase in the source. Geochemical data are consistent with enrichment of LILE in the mantle wedge by the process of zone • refining or mantle metasomatism, or from a fluid derived from the subducted slab. Comparison between Sr isotopic ratios and contents of HFSE and LILE in Ungaran basalts and the crust of the eastern Indian Ocean suggests that the model involving derivation of Ungaran lavas from a mantle wedge contaminated by a fluid from the subducted slab is plausible. Many observed geochemical variations in Ungaran lavas, particularly in 87Sr/86Sr ratios, reflect heterogeneity in an OIB-type mantle wedge. ACKNOWLEDGEMENTS I wish to express my appreciation to my supervisor Dr P.F. Carr for his help, guidance, patience and encouragement throughout this project. Professor A.C. Cook generously provided access to all facilities in the Department of Geology. Several staff from the CSIRO particularly Dr D.J. Whitford and J. Fardy who carried out the isotopic and instrumental neutron activation analyses deserve special recognition. I also wish to express my appreciation to N. Ware from the Research School of Earth Sciences, Australian National University and Dr B.E. Chenhall from the Department of Geology, University of Wollongong, who provided invaluable assistance with the electron microprobe and whole-rock X-ray fluorescence analyses. I have benefitted from fruitful discussions with many people, especially Drs R. Cas, R.A. Day, J. Foden, F.A. Frey, M. McCulloch, LA. Nicholls, G.E. Wheller and D.J. Whitford I am grateful to Dr C. Gray for his permission to use his unpublished data and for his comments on heterogeneity of the mantle. Numerous other friends provided advice during informal discussions, particularly M. Barsdell, R. Sukhyar and D. Vukadinovic. Special appreciation is addressed to a fellow postgraduate student, Carol Simpson, who tirelessly helped in editing and correcting the English expression of my earlier draft. Technical assistance from staff of the University of Wollongong, especially D.A. Carrie, A.M. Depers, D. Martin, L. Morris, J. Paterson, M. Perkins and R. Varga is gratefully acknowledged. Associate Professor G. Doherty and T. Ratkolo are thanked for generously providing access to computing facilities. Financial aid was provided by the Australian International Development Assistance Bureau and many people from this organisation, particularly N. McPherson, K. Moran, W. Rush, P. Schnelling and D. Wise, have my gratitude. Finally I would like to acknowledge the support of my family especially my mother and sisters, and my spiritual parents Jess and Walter, for their constant assistance and encouragement during the preparation of this thesis. TABLE OF CONTENTS Abstract Acknowledgements Page CHAPTER 1 INTRODUCTION 1.1 Introduction 1 1.2 Previous work 2 1.3 Aim of study 3 1.4 Thesis organisation 3 1.5 Data presentation 4 1.6 Rock nomenclature 5 CHAPTER 2 TECTONIC AND GEOLOGICAL SETTING 2.1 Introduction 7 2.2 Sunda Arc 8 2.3 Java Island 9 2.3.1 BenioffZone 9 2.3.2 Java Trench 10 2.3.3 Outer arc, ridge 10 2.3.4 Outer arc basin 10 2.3.5 Magmatic arc 11 2.3.6 Back arc 11 2.3.7 Stratigraphy 11 2.4 Central Java 12 2.4.1 Magmatic evolution 13 2.4.2 BenioffZone dip 14 2.4.3 Rate of convergence 15 2.5 Ungaran 16 2.5.1 Oldest Ungaran 16 2.5.2 Old Ungaran 17 2.5.3 Parasitic Cones 18 2.5.4 Young Ungaran 19 2.5.5 Dating and correlation 19 2.6 Summary CHAPTER 3 PETROGRAPHY AND MINERAL CHEMISTRY 3.1 Introduction 23 3.2 Oldest Ungaran 23 3.2.1 Basalt 23 3.2.2 Andesite 25 3.3 Old Ungaran 26 3.3.1 Basalt 26 3.3.2 B asaltic andesite 27 3.3.3 Andesite 29 3.4 Parasitic Cones 30 3.4.1 Basalt 30 3.4.2 Basaltic andesite 31 3.4.3 Andesite 33 3.5 Young Ungaran 35 3.5.1 Basalt 35 3.5.2 Basaltic andesite 37 3.5.3 Andesite 38 3.6 Summary and discussion 39 3.6.1 Plagioclase 40 3.6.2 Pyroxene 42 3.6.3 Fe-Ti oxide 44 3.6.4 Amphibole 44 3.6.5 Mica 45 3.6.6 Olivine 46 3.6.7 Order of crystallisation 47 3.6.8 Pressure and temperature of crystallisation 49 CHAPTER 4 TOTAL-ROCK GEOCHEMISTRY 4.1 Introduction 53 4.2 Geochemical features and variation 53 4.2.1 Oldest Ungaran 54 4.2.2 Old Ungaran 56 4.2.3 Parasitic Cones 59 4.2.4 Young Ungaran 61 4.3 Summary and discussion 63 4.3.1 Major elements 63 4.3.2 Strontium 64 4.3.3 Rubidium 65 4.3.4 Th, Pb, Zr, Hi, Y, Nb, Ta and Ti

Load more