Collaborative Research: Magma Generation and Tectonics in the Early Mariana Arc System Revisited

Lavas from the fore-arc islands of Guam, Rota, and Saipan, and DSDP sites 458 and 459 (Fig. 1) have been analyzed for major element and trace element concentrations, as well as Sr, Nd, Pb, and Hf isotopic compositions, to investigate the evolution of volcanism in the Mariana arc from the initiation of subduction of the Pacific plate beneath the Philippine plate in the Eocene through the Miocene. The oldest “protoarc ” (Pearce et al., 1999) lavas in the Mariana fore-arc region are ca. 48 Ma tholeiite to boninite sequence from DSDP sites 458 and 459. These tholeiites have NMORB-like REE, HFSE, and Th concentrations, but are enriched in LIL elements, Pb, and U. The capping boninite-series glasses have similar slab-derived trace element abundance patterns, but lower and flatter REE contents (1-2 x PUM). 40Ar/39Ar ages obtained on boninite series lavas from Guam stretch back to 44Ma. These lavas have U-shaped REE patterns and HREE concentrations about 3-8 x PUM. La/Nb decreases with increasing Ba/La for both the DSDP and Guam lavas (Fig. 2). Pb isotope values plot within fields defined by Pacific plate lavas and volcanogenic sediments (Fig. 3; Meijer, 1976; Pearce et al., 1999). Hf isotopic compositions display weak correlations with Zr/Sm ratios for lava