Slab melt as metasomatic agent in island arc magma mantle sources, Negros and Batan (Philippines)

Two new cases of association of adakites with 'normal' island arc lavas and transitional adakites are recognized in the islands of Batan and Negros in northern and central Philippines, respectively. The Batan lavas are relate d to the subduction of the middle Miocene portion of the South China Sea ba sin along the Manila trench; those of Negros come from the almost aseismic subduction of the middle Miocene Sulu Sea crust along the Negros trench. Th e occurrence of the Batan adakites is consistent with previous findings sho wing adakitic glass inclusions within minerals of mantle xenoliths associat ed with Batan arc lavas. The similarity of adakite ages (1.09 Ma) and that of the metasomatized xenoliths (1 Ma) suggests that both are linked to the same slab-melting and metasomatic event. Earlier Sr, Pb and Nd-isotopic stu dies, however, also reveal the presence of an important sediment contributi on to the Batan lava geochemistry. Thus, the role played by slab melts, ass umed to have mid-ocean ridge basalts-like (MORB) isotopic characteristics, in enriching the Batan subarc mantle is largely masked by the sediment inpu t. The Negros adakites are present only in Mount Cuernos, the volcanic cent er nearest to the Negros trench. Batch partial melting calculations show th at the Negros adakites could be derived from a garnet amphibolitic source w ith normal-MORB (N-MORB) geochemistry. This is supported by the MORB-like i sotopic characteristics of the Mount Cuernos lavas. The volcanic rocks from the other volcanoes consist of normal arc and transitional adakitic lavas that have slightly higher Sr- and Pb-isotopic ratios, probably due to sligh t sediment input. Mixing of adakites and normal arc lavas to produce transi tional adakites is only partly supported by trace element geochemistry and not by field evidence. The transitional adakites can be modeled as partial melts of an adakite-enriched mantle. Trace element enrichment of non-adakit ic lavas could reflect the interaction of their mantle source with uprising slab melts, as metasomatic mantle minerals scavenge certain trace elements from the adakitic fluids. Therefore, in arcs beneath which thick (up to 2 km) continent-derived detrital sediments are involved in subduction, like i n Batan, the sediment signature can overwhelm the slab melt input. In arcs like Negros where slow subduction could cause a more efficient scraping of thinner (approximately 1 km) detrital sediments, the contribution of slab m elts is easier to detect.