Adakites without slab melting: high pressure differentiation of island arc magma, Mindanao, the Philippines

Abstract

New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr / Y and depletion of the heavy rare earth elements. 87Sr / 86Sr and 143Nd / 144Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth elements, the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge. The wide range of SiO2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature. (1) Adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing garnet melted to yield adakitic magma. In either case the basaltic precursor was generated from fluid-modified mantle then differentiated within the garnet stability field. In Surigao this requires differentiation within mantle. The Surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth. This has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks.