Amphibole and magma evolution: insights from composite xenoliths from Batan Island, Philippines

Volcanism is the end product of many complex processes occurring beneath volcanic arcs at subduction settings. Processes, such as metasomatism of the mantle, partial melting, crystal fractionation and eruption, can only be inferred from erupted material. Two exceptions are arc terranes and the rare entrainment of mantle xenoliths. Multiple component xenoliths have been erupted in high K, calc-alkaline andesites on Batan Island, the Philippines. However literature has only described the peridotite components, not the amphibole rind or gabbro components. Combining novel and conventional analysis techniques, prior work has been placed in the context of this composite xenoliths formation. Interestingly, amphibole is a major mineral phase in the all xenolith components, but non-existent in the host lava. Recent literature uses trace element abundance in lavas to infer that amphibole fractionation occurs at many arc settings, yet no physical evidence saw the process occurring (e.g. Davidson et al., 2007). This composite xenolith shows amphibole forming under both reaction and cumulate conditions, two processes which amphibole may evolve a magma in arc settings. As such, the evidence shown can be used as a novel analogue to processes occurring in other arc settings.