GEOCHEMICAL ANALYSIS OF AMPHIBOLITES AND GABBROS FROM THE MARIANA FOREARC

Rocks collected from the Marianas forearc by Shinkai 6500 submersible diving near Guam were analyzed for whole-rock major and trace element compositions, as well as mineral textures and compositions. Major and trace element data were collected using ICP-MS, XRF and ICP-OES analytical techniques. Eight samples were analyzed including three amphibolites, three fore-arc basalts or dikes, and two gabbroic samples. The amphibolites were collected from a location at the south end of the Mariana Trough, in a region dominated by serpentinized peridotites. One of the amphiboltes is almost entirely composed of garnet and hornblende. Our findings suggest that the protolith for this relatively high-grade metamorphic rock was a MORB-like basalt that originated from the Pacific plate and was subducted to at least 1 GPa (Michibayashi, personal communication, 2011) before reemerging at the forearc surface. This sample is strongly enriched in the middle REE compared to light and heavy REE, suggesting that this rock represents a crystal cumulate resulting from high-pressure crystal fractionation or represents the residue after partial melting of a basalt. The other amphibolites also appear to have had basaltic protoliths, but they did not circulate in the wedge as deeply. The garnet amphibolite was also analyzed using LA-ICPMS and SEM to investigate the trace element zoning patterns within the minerals to determine paragenesis and pressure-temperature-time paths. These analyses found Cu- and Ti-rich amphibole veins within the garnet, suggesting amphibole growth continued after garnet growth ceased. In addition, the minerals appear to have been actively reacting with each other before their compositions quenched. The basalts and gabbroic samples were collected on the trench-slope southeast of Guam and were likely associated with initiation of Pacific plate subduction (Reagan et al., 2010, G-cubed). These samples were recovered in association with peridotites suggesting that they were collected in the vicinity of the Moho. The fore-arc basalts exhibit some enrichment of U and Ba which is most likely due to sea water alteration. Geochemical investigations of these samples have gathered important data pertinent to understanding subduction processes from a remote area.