Geochronology and geochemistry of late Carboniferous–middle Permian I- and A-type granites and gabbro–diorites in the eastern Jiamusi Massif, NE China: Implications for petrogenesis and tectonic setting

Abstract

Late Carboniferous–middle Permian magmatism in the Jiamusi Massif of northeast China, in the eastern segment of the Central Asian Orogenic Belt (CAOB), provides critical evidence regarding the tectonic history and geodynamic processes in the region. The gabbro–diorites of the Longtouqiao pluton and two groups of coeval granite in the study area comprise a bimodal magmatic suite. Precise LA-ICP-MS U–Pb zircon ages indicate that the granitoids and gabbro–diorites were emplaced in the late Carboniferous–middle Permian (302–267Ma). Group I granites have high SiO2 (70.75–77.04wt.%) and K2O (3.65–5.89wt.%) contents, are enriched in LILEs (e.g., Rb, Th, and U) relative to HFSEs and LREEs, and have negative Nb, Ta, P, and Ti anomalies, which collectively indicate affinities with subduction-related magmas. Group II granites are weakly peraluminous (A/CNK=1.03–1.07) and are characterized by enrichment in alkalis (Na2O+K2O=8.22–8.90wt.%), low MgO (0.04–0.09wt.%) and P2O5 (0.01–0.04wt.%) contents, high Zr and Nb contents, high 10,000×Ga/Al ratios, and they are geochemically similar to aluminous A-type granites. All the magmatic zircons in these granitoids have great variations of εHf(t) (+7.89 to −5.60) and two-stage Hf model ages (TDM2) of 0.8–1.7Ga, which suggest that the precursor magmas originated from a heterogeneous source that involved juvenile components derived from a depleted mantle source during magma generation. The aluminous A-type granite magmas were probably derived by high-temperature partial melting of a felsic crustal source, whereas the other granite magmas probably resulted from partial melting of a mafic lower crust. The gabbro–diorites of the Longtouqiao pluton are depleted in Nb, Ta, P, and Ti, and show flat distributions of most LILEs and HFSEs, except for large positive anomalies in Ba, K, and Pb. These features reflect a limited degree of crustal contamination associated with the subduction-related magmatic processes. These data, together with previously reported data and the nature of various rock types along the eastern part of the Jiamusi Massif, suggest that the intrusive rocks were formed in a geodynamic regime that changed from compression to extension during the westwards subduction of the Paleo-Pacific oceanic lithosphere, probably as a result of slab break-off of the subducting plate.