ISOTOPE AND TRACE-ELEMENT EVIDENCE FOR 3 COMPONENT MIXING IN THE GENESIS OF THE NORTH LUZON ARC LAVAS (PHILIPPINES)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in Sr-87/Sr-86 (0.70327-0.70610), Nd-143/Nd-144 (0.51302-0.51229) and Pb-208*/Pb-206* (0.981-1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high DELTA8/4, and to a lesser extent high DELTA7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5 - 5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS 'subduction component' interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (< 5%) of subducted S China Sea terrigenous sediment. The Sr-87/Sr-86 ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with Nd-143/Nd-144 and Pb-208*/Pb-206*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.