Protracted post-collisional magmatism during plate subduction shutdown in early Paleoproterozoic: Insights from post-collisional granitoid suite in NW China

Tectonic regimes display different styles worldwide during the early Paleoproterozoic ranging from plate tectonic slowdown or shutdown to vigorous subduction-related magmatism. Here we report a suite of early Paleoproterozoic granitoids from the Quanji Massif in northwestern China that provides potential insights on the tectonic regime in this period. The granitoids formed at 2.37–2.34Ga as evidenced by LA-ICP-MS zircon U-Pb ages. These dioritic to granitic rocks with amphibole and biotite as the major mafic minerals display a wide range of SiO2 (55.8–66.2wt%), low to moderate MgO (1.4–4.1wt%), low to high K2O (1.6–5.9wt%) and Al2O3 (16.3–21.5wt%), with Mg# values in the range of 36–47 and relatively low Cr (7.7–79.2ppm) and Ni (2.3–19.3ppm) concentrations. Geochemically they correspond to high‑potassium calc-alkaline I-type granitoids, with some showing dominantly adakitic but minor A-type affinities. Both the I-type and A-type granitoids show moderately fractionated LREE to HREE and moderately to weakly negative Eu anomalies. However, the adakitic granitoids display strongly fractionated REE patterns and weak negative to positive Eu anomalies. All the rock types from this suite show enrichment in LILEs and LREEs and depletion in HFSEs (Ta, Nb). The Nd-Hf isotope data suggest that the granitoids were sourced from Mesoarchean to Neoarchean metamafic rocks in the lower crust. The coeval nature of the complex genetic types of rocks in this suite suggest that their precursor magmas formed in an extensional tectonic regime similar to those previously reported for the ~2.39–2.37Ga granitoids in central to eastern domains of the Quanji Massif. Our new results in conjunction with those from previous studies suggest protracted post-collisional magmatism lasting for >50Myr along the periphery Quanji Massif following the cessation of plate subduction in the early Paleoproterozoic.