Geochemical and isotopic study of a plutonic suite and related early volcanic sequences in the southern Mariana forearc

The forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81‐06‐26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well‐studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48–43 Ma. This is the first report of boninite‐like plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58–1.44, Nb/La: 0.18–0.79) and other features uniquely connected with boninites: TiO2 25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO2, forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low‐velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites. Key Points Rare mafic to felsic plutonic suites were found at two Mariana fore‐arc sites Suites are chemically like the boninite magma formed at subduction initiation Earliest stages of arc crust development can form intermediate to felsic plutons