Intrusive Rocks from the Southern Mariana Trench: Comparison with Volcanic Sequences of the Early Izu Bonin Mariana Arc

Intrusive rocks with felsic to intermediate compositions have been recovered from the southern IBM (Izu Bonin Mariana) trench wall. We studied samples from collections of rocks recovered on cruises of the University of Hawaii and JAMSTEC. Objectives of this study are to determine if those rocks are part of the IBM mid-crust, and to determine if they are geochemically and isotopically similar to volcanic rocks in this system. Preliminary major and trace element analyses yield results that are consistent with island arc magmatism: SiO2 between 48.8% and 76.9%, low Ti (TiO2 < 0.5%), and negative Nb anomalies relative to REE (rare earth elements) and LILE (large ion lithophile elements). Most samples show enrichment in the fluid-mobile elements suggesting moderate interaction with fluids. Two rock types or series are identified. One has extremely low TiO2 and REE patterns that are MREE (middle rare earth element) depleted. These samples also have high Zr/Sm (primitive mantle normalized values of 2 - 7. These features are consistent with a magma source similar to that for boninite volcanic rocks, which have been found on Guam and other Mariana forearc areas. The second series has low TiO2 but REE patterns are linear, with LREEs slightly depleted or enriched. This second series may be related to low-K arc tholeiites or FABs (fore-arc basalts). Preliminary geochemical data suggest that our samples are intrusive equivalents of volcanic rocks that were erupted in the early stage of IBM arc development from 48-40 Ma. They may be fragments of a felsic to intermediate middle arc crust that formed early in the history of the IBM arc and was exposed by faulting or erosion of the trench wall.