Geochemical and Isotopic Study of a Plutonic Suite and Related Early Volcanic Sequences in the Southern Mariana Forearc

In the southern Izu-Bonin-Mariana (IBM) arc, rocks associated with the early formation of the arc are found in the submerged forearc and forearc islands. Previous studies of the early arc have focused on volcanic rocks, which were recovered by drilling, dredging, submersible dives and from outcrops on the forearc islands. Intermediate and felsic plutonic arc-related rocks have been recovered in only a few locations across the entire arc and their study has revealed different petrogenetic processes from location to location. The rocks in this study were dredged from two locations southeast of Guam at ~3500m and 7200m during a 1981 cruise by the University of Hawai'i. The plutonic rocks of dredge RD63 (3500m) are normative gabbros, quartz gabbros, diorites, quartz diorites, granodiorites and tonalites. The rocks from dredge RD64 (7200m) are altered basalts, andesites, dacite, diorite and diabase. Rocks from RD63 have extremely low TiO₂ (0.04-0.34%), REE patterns that are MREE (middle rare earth element) depleted, and high Zr/Sm (primitive mantle normalized) values of 2 - 15. Rocks from RD64 have low TiO₂ (0.11-0.39%) but REE patterns are linear, with LREEs slightly depleted or enriched. Our objective is to determine the relationship between the rock suites and previously studied volcanic sequences for which geochemical data have been published. According to Reagan et al., (2010), boninites and "forearc basalts" (FABs) were erupted in the southern IBM in the late Eocene. FABs underlie boninite lavas in drill cores and dive sites and so are thought to be the earliest erupted volcanics of the proto IBM arc. On Sr, Pb, Nd and Hf isotopic diagrams, the rocks from RD63 and RD64 plot in distinct fields that overlap with the early volcanics from the southern IBM. Overall, rocks from RD63 have lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf, and higher ⁸⁷Sr/⁸⁶Sr than rocks from RD64. For Hf versus Nd isotopes, both groups plot within the field of Indian rather than Pacific MORB. Pb isotope ratios for both groups plot close to or beneath NHRL, consistent with a Pacific, rather than Indian MORB character. Compared with volcanics, RD63 rocks are most similar to boninites from the Facpi formation on Guam, but have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd. Rocks from RD64 have higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr than boninites generally, and plot between the FABs and boninites. Based on isotopic data and trace element modeling, we conclude that the plutonic suite from RD63 is derived from a parental magma similar to the early boninite lavas, whereas rocks from RD64 are related to the early FABs or more recent tholeiitic arc lavas.