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Publication details for Prof Jon DavidsonHandley, H.A., Macpherson, C.G. & Davidson, J.P. (2010). Geochemical and Sr-O isotopic constraints on magmatic differentiation at Gede Volcanic Complex (GVC), West Java, Indonesia. Contrib. Mineral. Petrol 159: 885–908
- Publication type: Journal papers: academic
Author(s) from Durham
The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of mul- tiple volcanic centres and eruptive groups with complex magmatic histories. We present new petrological, miner- alogical, whole-rock major and trace element and Sr–O isotopic data to provide constraints on the relative impor- tance of fractional crystallisation and magma mixing in petrogenesis, as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence for the (late-stage) inter- action of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning observed in plagioclase phenocrysts of all eruptive groups (up to *20 mol% An) may be attributed to physical changes in magma properties (e.g. P, T, and PH2O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are incon- sistent with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested to account for the significant scatter in some element variation diagrams. No correlation is
observed between textural complexity and/or mineral dis- equilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisa- tion involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene provide strong control on the geochemical evolution of GVC rocks. Two-pyrox- ene geothermobarometry provides pre-eruption crystalli- sation temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to *13–24 km depth beneath the volcanoes (mid- to lower crust). Low, mantle-like clino- pyroxene d18O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the moderately thick West Javan arc crust is relatively immature compared to typical con- tinental crust. Trace element ratios and strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary, is distinct from the other GVC groups.