Geochemical and Sr-Nd-Hf-O isotopic constraints on volcanic petrogenesis at the Sunda arc, Indonesia

Abstract

The Sunda island arc of Indonesia formed as a result of the northward subduction of the Indo- Australian Plate beneath the Eurasian Plate. Along-arc variations in the composition and thickness of the overriding Eurasian plate and variation in the type and amount of sediment deposited on the subducting plate create differential effects on Sunda arc lava geochemistry. Detailed study of volcanic rocks from Salak, Gede Volcanic Complex (GVC) in West Java and Ijen Volcanic Complex (IVC) in East Java was carried out in order to establish the relative importance and contributions of various potential source components and composition-modifying processes at individual volcanic centres, prior to investigating petrogenetic variation along the arc. Differentiation processes play a major role in modifying the geochemical composition of Sunda arc magmas. However, the relative importance and traceable impact of the different processes varies at each volcanic centre. Fractional crystallisation of a typical Javan island arc mineral assemblage exerts the largest control on major and trace element composition of the volcanic rocks. Distinct intra-volcanic complex differentiation trends at rvc and Salak are spatially controlled and are explained by independent conduits and multiple magma reservoirs at different depths in the crust - linked to sub-volcanic structure. Shallow level contamination by typical upper-crustal continental material is insignificant during magmatic differentiation at Salak, GVC and IVC. However, at Salak there is some evidence for assimilation of material similar in composition to the volcanic rocks. Deep fractionation of a phase in which HFSE and HREE are compatible (e.g. amphibole) is inferred in the evolution of most Javan magmas. Magmatism at Salak, GVC and IVC is the product of shallow, relatively homogeneous, fertile, Indian Ocean MORB-like mantle that has been enriched by slab- derived component(s) sourced from the altered oceanic crust and subducted sediment. Hf and Nd isotope ratios of Javan lavas show that the subducted sedimentary source component is heterogeneous and reflects spatial variations in sediment compositions on the down-going plate along the Java Trench. A progressive eastward increase in Sr isotope ratio of volcanic rocks across West and Central Java broadly correlates with inferred lithospheric thickness. A significant change in crustal architecture (i.e. thickness) occurs between Central and East Java. This transition may represent the south-eastern boundary of Sundaland (pre-Tertiary arc basement).