ORIGIN AND GEODYNAMIC IMPLICATION OF THE DUPAL ISOTOPIC ANOMALY IN VOLCANIC-ROCKS FROM THE PHILIPPINE ISLAND ARCS

Newly acquired Sr, Nd, and Pb isotope data, combined with other recent ly available data in the literature, clearly show that Philippine are volcanic rocks do not have a true Dupal isotopic anomaly, Instead, mos t of the volcanic rocks must have come from a mantle source with a ''D upal-like'' Indian Ocean- to enriched ocean-island-basalt composition. This mantle source is present beneath the Philippine Sea plate and th e South China Sea plate on either side of the Philippine are systems, The isotopic signature of Philippine are volcanics is most probably in herited from the previous southern locations of the different geologic terranes that compose the modern Philippine island arcs, The new data also show that the isotopic signature of Philippine are volcanics, si milar to volcanic rocks from other western Pacific island-arc-back-arc settings, is mainly a function of the mantle source and was only rece ntly modified by a component from subducted sediments and crust.