Year

1989

Degree Name

Doctor of Philosophy

Department

Department of Geology

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) is 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 SiO2 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 M g O contents. Mafic rocks from Ungaran range from Nenormative 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% SiO2 have a wider range and higher mean 87Sr/86Sr ratio than rocks with >53% Si02 and the available isotopic data are consistent with derivation of Ungaran lavas from heterogeneous OIB-type source. Depletion of Ta, Nb and Ti relative to 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.

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