Along-arc geochemical and isotopic variations in Javanese volcanic rocks: 'crustal' versus 'source' contamination at the Sunda arc, Indonesia

Understanding the genesis of volcanic rocks in subduction zone settings is complicated by the multitude of differentiation processes and source components that exert control on lava geochemistry. Magma genesis and evolution at the Sunda arc is controlled and influenced by 1) along arc changes in the composition and thickness of the overriding Eurasian plate, 2) the variable age of the subducting oceanic crust and, 3) changes in the type and amount of sediment deposited on the subducting plate. Along-arc changes in geochemistry have long been recognised in the Sunda arc (Whitford, 1975), but debate still prevails over the cause of such variations and the relative importance of shallow (crustal) versus deep (subduction) contamination at the Sunda arc, Indonesia. Detailed study of individual Sunda arc volcanic centres is, therefore, a prerequisite in order to establish the relative importance and contributions of various potential source components and composition modifying differentiation processes at individual volcanoes, prior to an along arc comparative petrogenetic investigation. We present new radiogenic isotope data for Javanese volcanoes, which is combined with our recently published (Handley et al., 2007; Handley et al., 2008, Handley et al., 2010; Handley et al., 2011) geochemical and isotopic data of Javanese volcanic rocks along with data from other detailed geochemical studies to establish whether variable contributions from the subducting slab, or a change in crustal architecture of the overriding plate, best explain along-arc variations in isotope ratios and trace element characteristics. In West and Central Java Sr isotope ratios of the volcanic rocks broadly correlate with inferred lithospheric thickness implicating a shallow level control on isotopic composition. However, key trace element ratios combined with Hf isotope data indicate that the subducted slab and slab thermal regime also exert major control on the composition of the erupted Javanese lavas. This information presented will help elucidate the nature of the transition between the continental and oceanic basement to the arc, which is expected to lie between Sumatra and East Java. Whitford, D.J. (1975) Strontium isotopic studies of the volcanic rocks of the Sunda arc, Indonesia, and their petrogenesis. Geochim. Cosmochim. Acta, 39: 12871302. Handley, H.K., Macpherson, C. G., Davidson, J. P., Berlo, K. & Lowry, D. (2007). Constraining Fluid and Sediment Contributions to Subduction-Related Magmatism in Indonesia: Ijen Volcanic Complex. J. Petrol. 48, 1155-1183. Handley, H.K., Davidson, J.P., Macpherson, C.G. & Stimac .J.A. (2008). Untangling differentiation in arc lavas: constraints from unusual minor and trace element variations at Salak Volcano, Indonesia. Chem. Geol. 255, 360-376. Handley, H.K., Macpherson, C.G., Davidson, J.P. (2010). Geochemical and Sr-O isotopic constraints on magmatic differentiation at Gede Volcanic Complex, Java, Indonesia. Contrib. Mineral. Pet. 159, 885-908. Handley, H.K., Turner, S., Macpherson, C.G., Gertisser, R., Davidson, J.P. (2011) Hf-Nd isotope and trace element constraints on subduction inputs at island arcs: limitations of Hf anomalies and Sm/Hf ratios as input indicators. Earth Planet. Sci. Lett. 304, 212-223.