SIGNIFICANCE OF SLAB-DERIVED PARTIAL MELTS AND AQUEOUS FLUIDS FOR THE GENESIS OF THOLEIITIC AND CALC-ALKALINE ISLAND-ARC BASALTS - EVIDENCE FROM MT ARAYAT, PHILIPPINES

Mt. Arayat, an inactive stratovolcano, is located behind the volcanic front of the Bataan segment of the Taiwan-Luzon Arc in the Central Valley of Luzon, the Philippines. Volcanism in this arc-segment results from subduction of South China Sea Basin crust along the Manila Trench. Eruption products of Mt. Arayat comprise pyroclastics and porphyritic basalt and andesite lavas of high-K calc-alkaline affinity. Compared to typical oceanic island-arc volcanics, the basalts display slightly elevated abundances of Ta and Nb relative to Zr and Hf, and of Ta, Nb, Zr, Hf and light rare-earth elements (LREE's) relative to Y and heavy rare-earths (HREE's). Their REE distribution suggests mixing between high-HREE low-LREE/HREE and low-HREE high-LREE/HREE components, the latter suggesting the presence of residual garnet in the source region of Mt. Arayat basalts. Enrichment of LREE's relative to HREE's is typical of calc-alkaline and high-K calc-alkaline island-arc basalts, but is usually missing or small in island-arc tholeiites. LREE enrichment is often attributed to a metasomatizing aqueous fluid generated during dehydration of the subducted slab. For this model to be applicable, extremely low abundances in the fluid of agents like F-, OH- and/or CO32- which form hard complexes with the REE's, are a prerequisite. Due to the temperature dependence of the Eu3+/Eu2+ redox potential, divalent Eu predominates in aqueous fluids of low complex-forming capacity in the subduction zone environment whereas the other REE's (with the possible exception of Yb) are still trivalent. A LREE-enriched aqueous fluid generated by slab dehydration should therfore display a positive Eu anomaly. However, LREE-enriched island-arc basalts generally do not show positive Eu anomalies, implying that this LREE enrichment is unlikely to result from REE transfer by an aqueous fluid. Instead, a model is favoured which assumes contributions from both slab-derived aqueous fluids and partial melts. In the source region for island-arc tholeiites the mantle wedge is only affected by a LILE-enriched aqueous fluid, whereas the source region for calc-alkaline and high-K calc-alkaline basalts is additionally metasomatized by small volumes of melt generated by small degrees of partial melting of the subducted slab with garnet as a residual phase. These LREE-enriched partial melts may react with the depleted peridotite of the mantle wedge to produce domains of refertilized garnet-peridotite. Subsequent partial melting produces calc-alkaline melts whose trace-element compositions display signatures of both depleted peridotite and more fertile garnet-peridotite.