THE ND-ISOTOPIC, SR-ISOTOPIC AND PB-ISOTOPIC CHARACTER OF LAVAS FROM TAAL, LAGUNA-DE-BAY AND ARAYAT VOLCANOS, SOUTHWESTERN LUZON, PHILIPPINES - IMPLICATIONS FOR ARC MAGMA PETROGENESIS

Following the amalgamation of a collage of pre-Neogene terranes largel y by strike-slip and convergence mechanisms to form the Philippine isl ands, volcanic chains, related to oppositely dipping subduction zones, developed along the eastern and western margins of the archipelago. T here is ample field evidence that this volcanic activity, predominantl y calc-alkaline in chemical character, had commenced by the Oligocene. Volcanoes resulting from subduction along the Manila-Negros trench in the west (e.g., Taal, Laguna de Bay and Arayat) form a high-angle lin ear array, trending away from the MORB field on Pb-isotopic covariatio n diagrams; have the highest Sr- and lowest Nd-isotopic compositions, of the two chains (but nevertheless plotting above bulk earth on the S r-87/Sr-86 versus Nd-143/Nd-144 Nd covariation diagram); and exhibit S m/Nd and Rb/Sr values that are lower and higher, respectively, than th e estimated values for bulk earth. While the Sm/Nd and Rb/Sr character istics are common to both chains, volcanoes associated with the Philip pine-East Luzon trench have Pb-isotopic compositions that fall in the Indian Ocean MORB field and that require time-integrated evolution in a high Th/U environment. They also have higher Nd- and lower Sr-isotop ic ratios. The source materials of Philippine volcanoes, therefore, ha ve undergone varied recent enrichments in LILE, as indicated by the de coupling of isotopic and elemental ratios. These enrichments, particul arly for the western volcanoes, cannot be entirely due to small degree s of partial melting in the mantle wedge, considering that they were a ccompanied by elevations in radiogenic Pb. Elevated Pb ratios are best explained by the introduction of subducted, continentally derived sed iments. The sedimentary component in the western volcanoes is probably the South China Sea sediments derived largely from Eurasia. That this component is not available in the Philippine-East Luzon trench is ref lected by the fact that the eastern volcanoes have higher Nd- and lowe r Sr-isotopic ratios as well as less radiogenic common Pb.