Early Paleoproterozoic TTG gneisses and potassic granitoids in the southern Trans-North China Orogen: Key constraints on the tectonic setting during the tectono-magmatic lull and the initiation of plate tectonics

Magmatic rocks in the North China Craton (NCC) are crucial to unraveling the tectonic background during the tectono-magmatic lull (TML, 2.45-2.20 Ga) and to constraining the initiation of plate tectonics. In this study, previously published zircon U-Pb-Hf-O data and whole -rock geochemistry are collected from the early Paleoproterozoic felsic intrusive rocks in the southern Trans -North China Orogen (TNCO) and new data presented. The formation ages and petrogenesis of the early Paleoproterozoic felsic intrusive rocks in the southern TNCO are summarized to constrain the tectonic setting during the TML. Zircon U-Pb geochronology of the tonalite-trondhjemite-granodiorite (TTG) gneisses yielded intrusive ages of -2.3 Ga and -2.2 Ga. Geochemically, the gneisses can be classified as -2.3 Ga high -Al and -2.2 Ga low -Al types, and were likely derived from the partial melting of pre-existing lower crust at medium and low pressure, respectively. The heavy rare earth depletion and have high Sr/Y and La/Yb values of the -2.3 Ga TTG gneisses indicates that they were derived from the partial melting of thickening lower crust in the garnet -hornblende phase, which is consistent with a gradual thickening of the crust. The potassic granitoids are geochemically divided into I -type and A -type granites and have early Paleoproterozoic ages at -2.3 Ga and -2.27 Ga. Zircon U-Pb geochronology of the dioritic gneisses yielded intrusive ages of -2.3 Ga. These potassic granitoids and dioritic gneisses were generated by the partial melting of ancient basement rocks, with variable degrees of juvenile contributions. Considering the large variety of zircon epsilon Hf(t) values, and Sr/Y and (La/Yb)N ratios obtained for the early Paleoproterozoic felsic intrusive rocks, the crustal thickening that occurred via tectonic compression prior to -2.3 Ga is most likely due to convergence. The high 818O and low 818O values in the southern TNCO may have been the result of slab subduction, which can effectively transfer surface material directly or indirectly to the lower crust, and the TTGs that formed as a result of the subsequent partial melting inherited the high 818O characteristics to varying degrees. Glacial meltwater is an important source of low 818O fluids and a large number of rift events developed during the 2.5-2.4 Ga interval. The resulting fractured environment provided sufficient space for the infiltration of surface water, and rocks with low 818O characteristics are likely produced by the remelting of crustal rocks that have been thermally altered by surface water or atmospheric hydrothermal processes. Combined with evidence such as the Paleoproterozoic Alaska -type intrusive rocks, diabases, sanukitoids and sedimentary rocks, the TNCO is proposed to have been in a subduction compression setting at -2.3 Ga. Meanwhile, the alkaline and related elements (such as Rb, Th, Ba and Nb), K2O/Na2O ratios, and zircon 818O values of the magmatic rocks increased significantly during the TML, which indicate that plate subduction started prior to -2.3 Ga. The difference in the intensity of the magmatism during the TML may be related to continental freeboards in different regions-