Geochronology and geochemistry of Mesozoic intrusive rocks in the Xing'an Massif of NE China: Implications for the evolution and spatial extent of the Mongol–Okhotsk tectonic regime

Abstract

This study presents new zircon U–Pb–Hf and whole-rock geochemical data for intrusive rocks in the Xing'an Massif of NE China, with the aim of furthering our understanding of the evolution and spatial influence of the Mongol–Okhotsk tectonic regime. Zircon U–Pb dating indicates that five stages of Mesozoic magmatism are recorded in the Xing'an Massif, namely during the Middle Triassic (~237 Ma), the Late Triassic (~225 Ma), the Early Jurassic (~178 Ma), the Middle Jurassic (~168 Ma), and the late Early Cretaceous (~130 Ma). The Middle Triassic–Early Jurassic intrusive rocks in the Xing'an Massif are dominantly granodiorites, monzogranites, and syenogranites that formed from magma generated by partial melting of newly accreted continental crust. Geochemistry of the Middle Triassic–Early Jurassic granitoid suites of the Xing'an Massif indicates their formation at an active continental margin setting, related to the southwards subduction of the Mongol–Okhotsk oceanic plate. The Middle Jurassic monzogranites in the Xing'an Massif are geochemically similar to adakites and have εHf(t) values (+3.8 to +5.8) and Hf two-stage model ages (TDM2; 979–850 Ma) that are indicative of derivation from magma generated by partial melting of thickened juvenile lower crust. The Middle Jurassic monzogranites formed in a compressional setting related to the closure of the Mongol–Okhotsk Ocean. The late Early Cretaceous intrusive rocks in the Xing'an Massif are dominated by A-type granitoids that are associated with bimodal volcanic rocks, suggesting their formation in an extensional environment related to either (i) delamination of a previously thickened region of the crust, associated with the Mongol–Okhotsk tectonic regime; (ii) the subduction of the Paleo-Pacific Plate; or (iii) the combined influence of these two tectonic regimes.