Petrogenesis of the early Paleoproterozoic low-δ18O potassic granites in the southern NCC and its possible implications for no confluence of glaciations and magmatic shutdown at ca. 2.3 Ga

Abstract

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-δ18O values of zircons (-0.92 to + 4.78‰, 3.26‰ on average) are reported for the first time, representing the primary record of low-δ18O magma in the early Paleoproterozoic. The high-K granites show transitional features between S-type and normal A- and I-type granites. Zircon εHf(t) (-7.6 to + 1.5) and whole-rock εNd(t) values (-3.8 to + 1.1) with much older TDM2 (Hf TDM2 = 3.36 to 2.77 Ga and Nd TDM2 = 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 andhomogeneous δ18O 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 TTGs likely indicate no confluence of the Paleoproterozoic magmatic gap and glaciations during the 2.35 to 2.29 Ga at least in the southern NCC.