The role of continental fragments in the formation of intra-oceanic arcs: Constraints from Sr-Nd-Hf-O isotopes of gabbro from the Jiamusi Block, NE China

Intra-oceanic arcs are potential candidates that generate Earth's continental crust, and serve as active sites for the mixing of juvenile arc and mature continental material. However, whether continental fragments occur beneath intra-oceanic arcs, and how continental materials are involved in island arc magmas remain equivocal. Here we report zircon U-Pb-Hf-O isotopes and whole-rock elemental and Sr-Nd isotopic data on the Silurian island arc gabbro from the southeastern Jiamusi Block in NE China. The gabbro was emplaced at ca. 435–429 Ma associated with the northward intra-oceanic subduction of the Paleo-Asian Ocean, and belongs to the eastern extension of the Bainaimiao arc system. Elevated zircon δ18O (+5.5‰ to +7.9‰) and variable εHf(t) (-2.1 to +4.0) values, and negative whole rock εNd(t) (-1.1 to −1.6) values, suggest an enriched mantle component generated by pelagic and terrigenous sediments input into the mantle wedge. The contribution of terrigenous sediments, together with the recognition of Paleoproterozoic crustal rocks, proves for the first time that continental fragments existed beneath the intra-oceanic arc. In conjunction with published data, we propose that that Tarim Craton-type scattered continental fragments occurred within the Bainaimiao arc system, and that they not only facilitated the initiation of oceanic-oceanic subduction through increasing buoyancy, but also account for the old continental zircon and enriched isotopic variations of the young arc rocks. Our study highlights a fossil example that could help better quantify the scale and role of continental fragments in the origin and evolution of intra-oceanic arcs.