Petrochemistry of the Sorol and Ayu Troughs: implications for crustal accretion at the northern and western boundaries of the Caroline plate

Major element and trace element analyses of samples dredged from six sites along extensional plate boundaries around the Caroline Sea in the western equatorial Pacific define three separate igneous suites. Three dredge sites were located in the Sorol Trough along the Caroline-Pacific plate boundary and three sites were located in the Ayu Trough along the Philippine-Caroline plate boundary. Within the Sorol Trough, fresh transitional-to-alkalic pillow basalts were dredged from a seamount, weathered oceanic tholeiites were sampled at the base of the northern margin, and fresh, Ti-rich ferrobasalts and ultramafics were recovered from a deep valley in the western section. Pyroxenites, high-Mg basalt and a lherzolite, chemically related to the ferrobasalts in the western section of the trough, include schistose fragments and visibly tectonized ultramafics. High-grade metamorphic features observed in these specimens suggest that a significant component of shear accompanies crustal accretion in the Sorol Trough. Fresh pillow-lava fragments from the axial rift and weathered pillows from the eastern margin of the Ayu Trough have compositions similar to Mid-Ocean Ridge basalts (MORBs) and indicate that the Ayu Trough formed through crustal accretion processes. Additionally, an intriguing assemblage of meta-basalts, meta-gabbros and silicic plutonic rocks were recovered from a shallow scarp along the western margin of the Ayu Trough. The chemical characteristics of the meta-basalts and meta-gabbros suggest that they may be related through crystal fractionation. The chemistry of the silicic plutonics, however, indicates that they may not be derived by fractionation of primary magmas with chemical compositions similar to those of the metamorphosed oceanic crustal rocks. Alternatively, the silicic plutonics may represent an assemblage of intrusive island-arc tholeiitic rocks which formed during the initial stages of magma genesis in the island arc that was rifted apart by seafloor spreading in the nascent Ayu Trough.