CONSTRAINING FLUID AND SEDIMENT CONTRIBUTIONS TO SUBDUCTION-RELATED MAGMATISM IN INDONESIA: IJEN VOLCANIC COMPLEX

Abstract

Ijen Volcanic Complex (IVC) in East Java, Indonesia is situated on thickened oceanic crust within the Quaternary volcanic front of the Sunda arc. The 20 km wide calderas complex contains around 22 post-caldera eruptive centres, positioned either around the caldera-rim (CR) or along a roughly NE–SW lineament inside the caldera (IC). The CR and IC lavas exhibit separate differentiation histories. Major element and trace element modelling shows that fractionation of olivine, clinopyroxene, Fe–Ti oxide ± plagioclase is important in the CR group, whereas plagioclase is the dominant fractionating mineral in the same assemblage for the IC group. Spatial controls on magmatic differentiation highlight important structural controls on magma storage and ascent at IVC. Mantle-like δ18O values, restricted ranges in Sr, Nd and Hf isotope ratios, and a lack of correlation between isotope ratios and indices of differentiation in IVC lavas are consistent with magmatic evolution through fractional crystallization. Furthermore, the small ranges in isotopic ratios throughout the complex indicate little heterogeneity in the mantle. IVC lavas possess higher 176Hf/177Hf and 143Nd/144Nd isotope ratios than other volcanoes of Java, representing the least contaminated source so far analysed and, therefore, give the best estimate yet of the pre-subduction mantle wedge isotopic composition beneath Java. Trace element and radiogenic isotope data are consistent with a two-stage, three-component petrogenetic model for IVC, whereby an Indian-type mid-ocean ridge basalt (I-MORB)-like fertile mantle wedge is first infiltrated by a small percentage of fluid, sourced from the altered oceanic crust, prior to addition of <1% Indian Ocean sediment dominated by pelagic material.