Subduction components and the generation of arc-like melts in the Zambalesophiolite, Philippines: Pb, Sr and Nd isotopic constraints

New Pb, Sr, and Nd isotopic data on mineral separates from the crustal and mantle rocks of the Zambales ophiolite (Luzon, Philippines), were obtained to help constrain the origin of its are-like sections, which include island are tholeiites as well as minor boninitic rocks. Previous work using major and trace elements showed that the ophiolite consists of similar volumes o f laterally contiguous MORE-like crust and are-like crust, and that the are -like sections are less than similar to 1 Ma younger than the MORE-like sec tions. It has been suggested that the are-like magmas were derived from a m antle source that saw previous melt extraction, i.e., they are at least 'se cond stage' melts, whereas the MORE-like magmas are the products of 'single stage' melting. The isotopic data presented here exhibit elevated Sr-87/Sr -86 and Pb-207/Pb-204, hut identical Nd-143/Nd-144, in the are-like section s relative to the MORE-like sections, which have Pb, Sr, and Nd isotopic co mpositions indistinguishable from typical MORE. This suggests that an enric hed, aqueous, subduction fluid acted as a flux for the second stage melting that generated the are-like crust. Possible scenarios for the multiple mel ting events and addition of the subduction component include: (a) influx of the subduction fluid directly into the melting zone beneath the MORE-like lithosphere, thus triggering further melting, (b) hydration of young lithos phere by the subduction fluid followed by reheating (by a propagating sprea ding center, for example), and (c) decompression melting of a passive 'nugg et' of depleted and subsequently hydrated mantle as it is tapped by a sprea ding center. (C) 1999 Elsevier Science B.V. All rights reserved.