Geochemistry of lavas from Negros Arc, west central Philippines: Insights into the contribution from the subducting slab

Abstract

[1] The Negros Arc in west central Philippines is comprised of six Pliocene to Quaternary stratovolcanoes that have erupted calc-alkaline to slightly shoshonitic basalts to dacites. Petrographic, major and trace element, and Sr, Nd, and Pb isotope data suggest that crustal level differentiation processes have likely generated the range in composition of lavas. Several silicic lavas from the southern volcanoes have high Sr and moderate Sr/Y similar to melts presumed to be derived from subducted basaltic crust (adakites). However, these silicic lavas have low La/Yb unlike typical adakites, and have other incompatible trace element ratios (e.g., Nb/Ta, Ba/La, Pb/Ce) which are opposite to those expected of adakites. Modeling results also indicate that partial melting of the subducting Southeast Sulu Sea crust cannot satisfactorily reproduce the adakite-like characteristics or account for the other incompatible trace element ratios. Fractionation of amphibole (± apatite) together with variable fractionation and accumulation of plagioclase have likely imparted the adakite-like features. Trace element and isotopic variations indicate a three-component mixing, whereby variable proportions of melted sediment and aqueous fluids derived from basaltic crust were added to the subarc MORB-source type mantle to form the source of Negros Arc lavas. Southern Negros lavas reflect the strongest fluid contribution whereas northern Negros lavas best exhibit the sediment melt contribution.