U-SERIES, SR-ND-PB ISOTOPE AND TRACE-ELEMENT SYSTEMATICS ACROSS AN ACTIVE ISLAND ARC-CONTINENT COLLISION ZONE - IMPLICATIONS FOR ELEMENT TRANSFER AT THE SLAB-WEDGE INTERFACE

We present U-series, Sr-Nd-Pb isotope and trace-element results of a r egional study of geochemical systematics across an island are-continen t collision zone in the East Sunda Are of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K th oleiitic to ultrapotassic, but all are characterised by high Sr-87/Sr- 86 (0.7053 - 0.7067), radiogenic lead isotope ratios (Pb-206/Pb-204 = 18.99-19.15), low (Th-230)/(Th-232) (0.66-0.85), and low Nd-143/Nd-144 (0.51255-0.51272), except for high Nd-143/Nd-144 (>0.51286) at the vo lcanic front. Low (Th-230)/(Th-232) ratios are also found in terrigeno us sediments in front of the are, which, in combination with Sr-Nd-Pb isotopic constraints, indicates that subducted continental material co ntributes to magma sources in this are sector. The volcanoes close to the trench show a large excess of U-238 over Th-230 (UP to 80%) and of Ra-226 over Th-230 (UP to 800%). In addition, they are enriched in el ements thought to be mobile in hydrous fluids during slab-wedge transf er, such as Ba, Pb, and Sr. In contrast, U-Th-Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the are-front l avas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher c oncentration levels and less interelement fractionation form conspicuo us differences with the front volcanics.Our combined isotopic and trac e element data are consistent with three-component mixing whereby a sl ab-derived hydrous fluid and a siliceous melt are both added to the su b-are mantle source. The hydrous fluid largely controls the input in t he shallow part of the subduction zone, whereas the siliceous melt dom inates the flux at deeper levels. Sedimentary material is considered t o be the primary source of both. The large U-Th-Ra disequilibria at th e front suggests that element transfer is a currently active process a ssociated with present-day subduction of continental material. Copyrig ht (C) 1997 Elsevier Science Ltd.