Post-collisional reworking of subducted continental crust: Insights from late Paleozoic granites in the North Qaidam orogen, northeastern Tibet

Abstract

Post-collisional granites are widely distributed in continental collision orogenic belts, and deciphering their petrogenesis has great bearings for understanding the deep crustal compositional and tectono-thermal evolution of collisional orogens. Here we carried out a combined study of whole-rock major-trace elements and Sr–Nd–Hf isotopes, and zircon UPb ages and LuHf isotopes for late Paleozoic granites from the North Qaidam orogen, northeastern Tibet. The results indicate that they were derived from partial melting of subducted continental crust at high thermal gradients due to post-collisional rifting orogeny. Zircon UPb dating of the granites yields weighted mean 206Pb/238U ages between 377 ± 3 Ma and 384 ± 5 Ma, which postdate the ultrahigh-pressure (UHP) metamorphic ages (ca. 438–420 Ma) of deeply subducted continental crust in the North Qaidam orogen. Therefore, they are products of post-collisional magmatism. Most relict zircons in the granites have concordant Neoproterozoic (793–972 Ma) and early Paleozoic (405–442 Ma) UPb ages, generally consistent with the protolith and metamorphic ages of the UHP metaigneous rocks in the orogen, respectively. The granites are commonly calc-alkaline to high-K calc-alkaline with total alkali contents of 6.89–8.56 wt%, and weakly to moderately peraluminous with A/CNK values of 1.00–1.18 [A/CNK = Al2O3/(CaO + Na2O + K2O), in molar]. They exhibit arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with whole-rock (87Sr/86Sr)i ratios of 0.7074–0.7104, ɛNd(t) values of −9.0 to −4.4 and ɛHf(t) values of −4.2 to +2.9, and syn-magmatic zircon ɛHf(t) values of −7.6 to +7.7 (except one analysis of −15.0). These granites mostly have comparable SrNd isotope compositions to the UHP orthogneiss and continental eclogite in this collisional orogen, indicating their derivation from the subducted continental crust. Using the average compositions of the orthogneiss and continental eclogite in the orogen as the starting materials, phase equilibrium modelling results suggest that the whole-rock major elements of the post-collisional granites can be explained by the mixed melts mainly derived from these two lithologies, which were partially melted at high thermobaric ratios of 1156–1867 °C/GPa associated with different water contents (1–5 wt% for continental eclogite, 1–3 wt% for orthogneiss). Therefore, this study provides integrated geochemical constraints on the source rocks of the post-collisional granites and physicochemical conditions for their genesis.