The newly discovered ca. 1.35 Ga metamafic rocks in the Oulongbuluke Block, NW China, and its record for transition from the Columbia to Rodinia supercontinent

The Oulongbuluke Block is an integral part of the Columbia and Rodinia supercontinents, but the lack of rock records from the transitional period between the Columbia and Rodinia supercontinents during the mid−late Mesoproterozoic has impeded our understanding of the tectonic relationship of the Oulongbuluke Block, which lies between the Columbia and Rodinia supercontinents. In this contribution, we present a systematic petrographic, geochemical, and zircon U-Pb-Hf investigation of newly discovered metamafic rocks in the Oulongbuluke Block. The results show that the metamafic rocks have a protolith age of ca. 1.35 Ga and an arc-related metamorphic age of ca. 1.11−1.09 Ga. The metamafic rock samples are geochemically characterized by relatively high FeO^T/MgO and FeO^T and low SiO₂, MgO, and K₂O + Na₂O, which shows tholeiitic affinity. These metamafic rocks exhibit slight light rare earth element (LREE) depletion and flat heavy rare earth element (HREE) content with no obvious Eu anomalies and slightly negative Nb, Sr, and Zr anomalies. These conditions are similar to those of enriched mid-oceanic-ridge basalt (E-MORB) and normal mid-oceanic-ridge basalt (N-MORB). The metamorphic rocks studied also have positive zircon ε_Hf(t) values (2.96−7.04). Hence, the protoliths of the metamafic rocks may have been produced by variable degrees of melting of spinel-phase lherzolite mantle in a mid-oceanic ridge setting that was probably induced by a mantle plume. The presence of metamafic rocks indicates that the Oulongbuluke Block experienced the final breakup of the Columbia supercontinent at ca. 1.35 Ga, and the ca. 1.11−1.09 Ga arc-related metamorphism coincided with the convergence of the Rodinia supercontinent. The tectonic setting of the Oulongbuluke Block changed from a mid-oceanic ridge setting to an arc setting during the mid−late Mesoproterozoic, which was likely a response to the transition from the Columbia supercontinent to Rodinia supercontinent.