Sub-volcanic xenoliths from Raoul Volcano : a window on petrogenic processes in an intra-oceanic subduction system

Gabbroic and tonalitic xenoliths from c. 3 ka pyroclastic deposits on Raoul Island in the intra-oceanic Kermadec arc are samples of shallow crust formed <2 Ma ago. Gabbros are orthocumulates composed of plagioclase, augite and orthopyroxene. Some trace element and isotopic characteristics are similar to those observed in Raoul mafic lavas, but SiO₂ and incompatible trace element abundances are lower. The rare earth element (REE) patterns of the gabbros manifest strong positive Eu anomalies indicative of cumulate plagioclase. Tonalitic xenoliths comprise plagioclase, quartz, magnesiohornblende, magnetite, ilmenite and titanite. Their REE patterns feature strong negative Eu anomalies, which could indicate extensive plagioclase fractionation, but some have orthocumulate textures. In contrast to Raoul dacites, tonalitic xenoliths commonly contain amphibole. They also have higher total REE, and lower K, Ba and Rb abundances, and more evolved Sr isotope compositions. They represent a spectrum from highly fractionated felsic melts, through crystal mushes to felsic cumulates and were generated during an earlier phase in the evolution of the Raoul magmatic system. (author).