Petrogenesis Of The Early Paleoproterozoic Low-Delta O-18 Potassic Granites In The Southern Ncc And Its Possible Implications For No Confluence Of Glaciations And Magmatic Shutdown At Ca. 2.3 Ga

The interval of 2.45 to 2.20 Ga has previously been interpreted to contain a plate tectonic shutdown, but newly discovered magmatism of this stage is filling in the early Paleoproterozoic magmatic gap. However, the crust evolutionary scenarios and geodynamic driving mechanisms during this stage are still lacking comprehensive understanding. The present study verifies that the igneous rocks formed during this interval did occur along the southern margin of the NCC. The high-K granites in the Xiong'ershan area are dated at 2.32 to 2.29 Ga. Their low-delta O-18 values of zircons (-0.92 to + 4.78 parts per thousand, 3.26 parts per thousand on average) are reported for the first time, representing the primary record of low-delta O-18 magma in the early Paleoprotemzoic. The high-K granites show transitional features between S-type and normal A- and I-type granites. Zircon epsilon(Hf)(t) (-7.6 to + 1.5) and whole-rock epsilon(Nd)(t) values (-3.8 to + 1.1) with much older T-DM2 (Hf T-DM2 = 3.36 to 2.77 Ga and Nd T-DM2 = 3.08 to 2.63 Ga, respectively) than their formation ages, together with heterogeneous major-element composition point to recycling of continental crust older than ca. 2.77 Ga. Low CaO/(FeOt + MgO), low to moderate Al2O3/TiO2 (36.7 to 292), Rb/Sr (0.51 to 5.67) and Rb/Ba (0.04 to 1.09), variable but mostly high CaO/Na2O (0.05 to 5.17) ratios commonly imply metapsammite-dominated (meta-greywacke) sources with minor pelite-derived and tonalites materials. Considering that zircons lack metamict features and have low but positive and homogeneous delta O-18 values, we prefer a high-temperature hydrothermally altered older crustal source by surface water before or during partial melting. The great diversity of rock assemblages and geochemical features require a thermal anomaly linked to an extensional setting to account for the crustal melting. The occurrence of potassic rocks, and spatially associated gabbros, diorites and ITGs likely indicate no confluence of the Paleoprotemzoic magmatic gap and glaciations during the 2.35 to 2.29 Ga at least in the southern NCC.