Crustal assimilation versus subducted sediment input in west Sunda arc volcanics: an evaluation

New geochemical and Sr, Nd, and Pb isotopic analyses of Quaternary to Creta ceous sediments from the northeastern Indian Ocean are used to estimate the composition of the sedimentary material subducted along the Sunda Trench, and to evaluate the effects of crustal contamination versus subducted sedim ent input in the Quaternary volcanics of the west Sunda are. Two sediment e ndmember components are identified: siliceous-clastic (SS) and calcareous-o rganogenic (CS); the latter can be regarded as SS strongly diluted by organ ogenic material, mainly CaCO3. Siliceous-clastic sediments are characterise d by a component isotopically and geochemically similar to "typical" averag e upper crust. Consequently, LILE and LREE concentrations, LILE/LILE and LI LE/LREE values, and Sr, Nd, and Pb isotopic ratios in West Sunda are volcan ics cannot easily distinguish between assimilation of crustal material by u prising magmas and contamination of their mantle source by bulk sediments. Post-Miocene siliceous-clastic sediments sampled in the vicinity of the Sun da are are largely derived from the are itself, and therefore should not be used to evaluate the extent of sediment contamination of Indonesian are vo lcanics. In addition, geophysical evidence suggests that post-Miocene sedim ents are largely accreted rather than subducted, and existing Be-10 isotopi c data imply that post-Miocene sediments are not recycled by present-day vo lcanism. As the least contaminated are volcanics occur in the eastern secti on of the west Sunda are, where the highest sediment fluxes have been calcu lated, subduction of pre-Miocene northeastern Indian Ocean sediments or inc orporation of fluids released from them into the sources of west Sunda are volcanics seem unable to reproduce the range and spatial distribution of Sr , Nd, and Pb isotopic values in the are volcanics. By contrast, assimilatio n of crustal material by uprising melts derived from Indian Ocean-type mant le wedge, up to a maximum of approximately 10% for the most contaminated ar e volcanics, seems better able to account for the Sr, Nd, and Pb isotope sy stematics of the are volcanics, and is consistent with the variations in cr ustal thickness and composition along the are, and with the spatial distrib ution of Sr, Nd, and Pb isotope values in mafic are volcanics. These conclu sions are also supported by the low Be-10 coupled with high B/Be values, an d by the positive B/Be-SiO2 and B/Be-Sr-87/Sr-86 correlations in calc-alkal ine Sunda are volcanics.