Geochemistry of volcanic rocks from the Sunda Arc

Abstract

Geochemical analyses of igneous rocks can provide valuable information about processes, element fluxes, and rock lithologies not evident at the surface. This is particularly important in subduction zone settings where complex interactions between the subducting plate, mantle wedge and arc crust cannot yet be measured by alternative methods.
The Sunda arc, in SE Asia, provides an ideal opportunity to study the effects of subduction in a complex tectonic setting where the basement is poorly exposed and understood. However, in order to do so, magma compositions modified during differentiation in the arc crust must be effectively distinguished from those modified at the source. This study includes a detailed major- and trace element and isotopic (Sr-Nd-Hf-Pb) examination of volcanoes from west Java (Papandayan, Patuha and Galunggung), Central Java (Sumbing), east Java (Kelut) and Bali (Agung), the result of which provides greater insights into petrogenesis both across and along the arc.
Contamination in the arc crust is more extensive than previously recognised, particularly in west and central Java where few volcanoes can be used in order to identify subduction and source contributions. In west Java, volcanoes such as Papandayan and Patuha show significant enrichments in isotope ratios above mantle values (e.g. 87Sr/86Sr ~ 0.706, 143Nd/144Nd ~ 0.5125, 208Pb/204Pb ~ 18.91 and 176Hf/177Hf ~ 0.2827) which indicates a terrigneous crustal contaminant. At Sumbing volcano, most magma compositions are similar to those at Merapi and Merbabu, and show strong evidence for the assimilation of carbonate-rich lithologies with some magmas becoming enriched in CaO, Sr and 87Sr/86Sr.
Differentiation in volcanoes from east Java and the western part of the Lesser Sunda Islands (Bali, Lombok and Sumbawa) is dominantly controlled by fractional crystallisation, which provides better controls on source compositions. At Kelut, one group of samples show the most ‘depleted’ magma compositions yet discovered on Java, which contain MORB-like values for 143Nd/144Nd and 176Hf/177Hf (0.5130 and 0.2831 respectively). These samples represent the depleted (asthenospheric) mantle and are situated towards the front of the arc in east Java. It is likely that the progressive enrichment further back on the arc (i.e. Leucititic compositions at Ringgit-Besar) include more of an enriched (lithospheric) mantle (SCLM) component derived from the NW margin of Australia during the breakup of Gondwana.