SR-ND-PB ISOTOPE SYSTEMATICS OF THE BANDA ARC, INDONESIA - COMBINED SUBDUCTION AND ASSIMILATION OF CONTINENTAL MATERIAL

We present Sr, Nd, and Pb isotope results and SiO2, Rb, Sr, Sm, Nd, U, Th, and Pb data for six active volcanoes and one extinct volcanic isl and distributed over the whole length of the Banda Arc. Rock types ran ge from low-K tholeiitic in the NE to high-K calc-alkaline in the SW. The volcanoes in the NE have ''normal'' arc signatures (Sr-87/Sr-86=0. 7045-0.7055, Nd-143/Nd-144=0.51273-0.51291, and Pb-206/Pb-204=18.66-18 .75), whereas those in the SW have extreme values (Sr-87/Sr-86=0.7065- 0.7083, Nd-143/Nd-144=0.51252-0.51267, and Pb-206/Pb-204=19.28-19.43). Serua, situated in the central part, is the most anomalous volcano wi th regard to its Sr and Nd isotopic composition (Sr-87/Sr-86=0.7075-0. 7095 and Nd-143/Nd-144=0.51240-0.51260) but not with regard to Pb isot opes (Pb-206/Pb-204=19.02-19.08). The inactive island of Romang in the SW overlaps the Serua trends. The volcanoes display variable within-s uite ranges in Sr-87/Sr-86 and Nd-143/Nd-144. Large ranges (e.g., at N ila) are consistent with assimilation (10-20%) of carbonate-bearing se diments from the arc crust. Despite the evidence for assimilation, it cannot explain all of the Sr-Nd isotopic trends found, and Banda Arc m agmas must have already obtained a ''continental'' signature at depth before they reached the arc crust. Within-suite trends of Pb isotopes are virtually absent. We found an extreme range in the volcanics along the arc which coincide with a similar trend in sediments in front of the arc and consider this as strong evidence for the contribution of s ubducted continent-derived material to magma sources. Bulk addition of 0.1-2% of local sediment in the NE Banda Arc, and of 1-3% in the SW B anda Arc, to an Indian Ocean mid-ocean ridge basalt (I-MORB) source ca n explain the isotopic trends; both Serua and Romang require > 5% sedi ment. The Pb isotopes (e.g., Pb-207/Pb-204-Pb-208/Pb-204) also suggest changes in the mantle end-member from I-MORB to oceanic island basalt (OIB) source type. The latter becomes more conspicuous toward the SW and has the high Pb-208/Pb-204 characteristic of Indian Ocean (Dupal) OIBs. We hypothesize that mixing of magmas in the mantle wedge and/or in the arc crust was an important mechanism by which mantle and subduc ted end-members were incorporated in the final products.