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Sr‐Nd‐Pb isotope systematics of the Banda Arc, Indonesia: Combined subduction and assimilation of continental material

1993; American Geophysical Union; Volume: 98; Issue: B12 Linguagem: Inglês

10.1029/93jb01716

2156-2202

Pieter Z. Vroon, Manfred J. van Bergen, William M. White, Johan C. Varekamp,

Geochemistry and Elemental Analysis

We present Sr, Nd, and Pb isotope results and SiO 2 , Rb, Sr, Sm, Nd, U, Th, and Pb data for six active volcanoes and one extinct volcanic island distributed over the whole length of the Banda Arc. Rock types range from low‐K tholeiitic in the NE to high‐K calc‐alkaline in the SW. The volcanoes in the NE have “normal” arc signatures ( 87 Sr/ 86 Sr = 0.7045–0.7055, 143 Nd/ 144 Nd = 0.51273–0.51291, and 206 Pb/ 204 Pb = 18.66–18.75), whereas those in the SW have extreme values ( 87 Sr/ 86 Sr = 0.7065–0.7083, 143 Nd/ 144 Nd = 0.51252–0.51267, and 206 Pb/ 204 Pb = 19.28–19.43). Serua, situated in the central part, is the most anomalous volcano with regard to its Sr and Nd isotopic composition ( 87 Sr/ 86 Sr = 0.7075–0.7095 and 143 Nd/ 144 Nd = 0.51240–0.51260) but not with regard to Pb isotopes ( 206 Pb/ 204 Pb = 19.02–19.08). The inactive island of Romang in the SW overlaps the Serua trends. The volcanoes display variable within‐suite ranges in 87 Sr/ 86 Sr and 143 Nd/ 144 Nd. Large ranges (e.g., at Nila) are consistent with assimilation (10–20%) of carbonate‐bearing sediments from the arc crust. Despite the evidence for assimilation, it cannot explain all of the Sr‐Nd isotopic trends found, and Banda Arc magmas 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 subducted 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 Banda Arc, to an Indian Ocean mid‐ocean ridge basalt (I‐MORB) source can explain the isotopic trends; both Serua and Romang require > 5% sediment. The Pb isotopes (e.g., 207 Pb/ 204 Pb ‐ 208 Pb/ 204 Pb) 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 208 Pb/ 204 Pb 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 subducted end‐members were incorporated in the final products.