Generation of post-collisional normal calc-alkaline and adakitic granites in the Tongbai orogen, central China

Abstract

Post-collisional granites are generally generated by partial melting of continental crust during orogenic extension. The occurrence of normal calc-alkaline granites following adakitic granites in a collisional orogen is frequently supposed as a sign of tectonic regime transition from compression to extension, which has been debated yet. In this paper, we present a comprehensive study of zircon U–Pb ages, Hf–O isotopes, as well as whole-rock major and trace elements and Sr–Nd isotopes, for Tongbai and Jigongshan post-collisional granitic plutons in the Tongbai orogen. Zircon U–Pb dating yields intrusion ages of ca. 140 and 135Ma for the Tongbai and Jigongshan plutons, respectively, suggesting they are post-collisional granites. These granites are high-K calc-alkaline series, metaluminous to weakly peraluminous with A/CNK ratios of 0.85–1.08. The Tongbai gneissic granites are normal calc-alkaline granite, having variable SiO2 (61.93–76.74wt%) and Sr/Y (2.9–38.9) and (La/Yb)N (1.7–30.1) ratios with variably negative Eu anomalies (0.41–0.92). They have relatively high initial Sr isotope ratios of 0.707571 to 0.710317, and low εNd(t) (−15.74 to −11.09) and εHf(t) (−17.6 to −16.9) values. Their Nd and Hf model ages range from 2.2 to 1.8Ga and 2.3 to 2.2Ga. On the contrary, the Jigongshan granites show higher SiO2 (66.56–72.11wt%) and Sr/Y (30.1–182.0) and (La/Yb)N (27.4–91.4) ratios with insignificant Eu anomalies (0.73–1.00), belonging to adakitic granite. They have Isr=0.707843–0.708366, εNd(t)=−19.83 to −17.59, and εHf(t)=−26.0 to −23.5. Their Nd and Hf model ages vary from ca. 2.5 to 2.4Ga and ca. 2.8 to 2.6Ga. The Tongbai and Jigongshan granites are characterized by mantle-like zircon δ18O values (5.17–5.46‰). These geochemical features suggest that the Tongbai and Jigongshan granites were derived from partial melting of Paleoproterozoic and Archean continental crust, respectively. Fractional crystallization affected the geochemical compositions of the Tongbai gneissic granites, while the compositions of the Jigongshan granites were mainly controlled by partial melting. The adakitic signatures of the Jigongshan granites were either inherited from their source or ascribed to more garnet in their residues. The voluminous post-collisional granites might form by the collapse of the thickened Tongbai orogenic root. The normal Tongbai gneissic granites occurred slightly earlier than the Jigongshan adakitic granites in the Tongbai orogen, suggesting that it is not a mandate to sign the tectonic transition from adakitic to normal calc-alkaline granites in post-collisional settings. Therefore, this study provides another example supporting the burst of voluminous post-collisional granites with different compositions as a consequence of the collapse of the thickened orogenic roots of collisional orogens.