MANTLE METASOMATIC ENRICHMENT VERSUS ARC CRUST CONTAMINATION IN THE PHILIPPINES - OXYGEN-ISOTOPE STUDY OF BATAN ULTRAMAFIC NODULES AND NORTHERN LUZON ARC LAVAS

Fifty new oxygen isotopic analyses have been obtained on metasomatized peridotite nodules from Batan Island (Philippines) as well as on their host lavas and other upper Tertiary and Quaternary volcanic rocks from the northern part of the Luzon arc. They are used to investigate the possible mechanisms of the crustal component contribution which were previously considered in this arc on the basis of trace-element and Sr-Nd-Pb-Th isotopic variations observed on the same samples. Namely, they allow to distinguish mantle metasomatism by fluids carrying a sedimentary component from wall-rock contamination during magma ascent or storage. Ultramafic nodules display MORB-type deltaO-18-values (+5.4 to +6.1 parts per thousand). Their O-Sr and O-Nd isotopic patterns are consistent with enrichment of a highly depleted-type mantle source by < 3% bulk sediments. Their host lavas display grossly similar isotopic features, although some slightly higher deltaO-18-values (up to +6.8 parts per thousand) may have resulted from minor crustal contamination or fluid exchange. For other lavas in the northern Luzon arc, combined Sr, Nd and O data indicate that both source and crustal contamination are involved to various degrees along the different sections of the arc, varying with local geodynamic conditions. As a whole, the chemical budget of the northern Luzon arc is dominated by source metasomatic enrichment processes, but the effects of crustal contamination are detectable both in its northern part (collision regime) and southern part (subduction of relatively young oceanic crust, presumably devoid of an important sedimentary cover). The precise nature of the sedimentary components involved in mantle enrichment processes is difficult to identify. It is suggested that the imprint of pelagic sediments might have been stored in the mantle source as a result of previous Tertiary subduction stages, before the recent addition of bulk terrigenous sediment. Such a two-stage process may explain the peculiar Nd-Sr-O signatures of northern Luzon ultramafic nodules and host lavas.