The island of Luzon is bounded by a simple system of opposed subduction zones. Contrasting slabs of "oceanic" Phillipine Sea and "marginal" South China Basin lithosphere are injected into the mantle, where they appear to impact distinctive imprints on magma sources. This project will attempt to model subduction- induced source evolution through geochemical/isotopic study of post-Miocene lavas from calc-alkalic Bicol and Bataan volcanic lineaments, and the alkalic Taal-Banahaw group in southern Luzon. Data for major and trace elements, and Be-10, Sr, Nd, Pb, H, and O isotopes will be used to contrast effects of subducting (sparse) pelagic sediment and MORB basement with (voluminous) continental sediment and variable MORB "transitional" basement. Thus it should be possible to asses the influence of subducted sial on the genesis of alkalic magma. Several specific questions will be addressed including: 1. The long term history of the suprasubduction mantle: was this of the MORB, OIB, or sub- continental type? 2. The nature of mantle metasomatism beneath the Bataan and Bicol arcs. 3. The significance of accessory phase stabilities; do they compositionally buffer second-stage metasomatic melts? 4. The role of subduction-derived H2O. The project will involve sampling twelve active or dormant volcanoes, and is coordinated with research by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), and research groups in West Germany and France.
