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

Abstract

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 arc-like sections, which include island arc tholeiites as well as minor boninitic rocks. Previous work using major and trace elements showed that the ophiolite consists of similar volumes of laterally contiguous MORB-like crust and arc-like crust, and that the arc-like sections are less than ∼1 Ma younger than the MORB-like sections. It has been suggested that the arc-like magmas were derived from a mantle source that saw previous melt extraction, i.e., they are at least `second stage' melts, whereas the MORB-like magmas are the products of `single stage' melting. The isotopic data presented here exhibit elevated 87 Sr/ 86 Sr and 207 Pb/ 204 Pb , but identical 143 Nd/ 144 Nd , in the arc-like sections relative to the MORB-like sections, which have Pb, Sr, and Nd isotopic compositions indistinguishable from typical MORB. This suggests that an enriched, aqueous, subduction fluid acted as a flux for the second stage melting that generated the arc-like crust. Possible scenarios for the multiple melting events and addition of the subduction component include: (a) influx of the subduction fluid directly into the melting zone beneath the MORB-like lithosphere, thus triggering further melting, (b) hydration of young lithosphere by the subduction fluid followed by reheating (by a propagating spreading center, for example), and (c) decompression melting of a passive `nugget' of depleted and subsequently hydrated mantle as it is tapped by a spreading center.