Geochronological, geochemical, and Sr–Nd–Pb–Hf isotopes of Cretaceous gneissic granite and quartz monzonite in the Tongbai Complex: Record of lower crust thickening beneath the Tongbai orogen

Abstract

A comprehensive study of zircon U–Pb ages, major and trace elements, whole‐rock Sr–Nd–Pb isotopes and zircon Hf isotopes of the gneissic granite and quartz monzonite in the Tongbai Complex is conducted in an attempt to unravel their petrogenesis and shed new light on the tectonic evolution and crust thickening process of the Tongbai orogen. Zircon U–Pb dating indicates that the emplacement of gneissic granite was early at 144.2 ± 1.3 Ma, whereas the quartz monzonite formed later at 130.2 ± 1.7 Ma. Both the gneissic granite and quartz monzonite samples share many chemical similarities, such as high Al2O3, K2O, Sr contents and low MgO contents, together with high Sr/Y, high La/Yb ratios, suggesting an C‐adakitic affinity and possibly derived from partial melting of the thickened lower crust. The later quartz monzonites may have involvement of much more proportion of mantle‐derived materials (such as the metamorphosed mafic rocks, ecologite) in the lower crust during partial melting for its petrogenesis, due to its higher MgO, Cr, Ni, and V contents than the gneissic granites. The formation of the gneissic granite and quartz monzonite records the evolution history for the lower crust thickening beneath the Tongbai orogen at the Cretaceous. Nearly all granitoids in the Tongbai orogen currently found have an emplacement age of >130 Ma and have higher Sr/Y ratios and lower MgO, Cr, Ni, and V contents with respect to the granitoids in the Dabie orogen which occurs in two stages of granitic magmatism of >130 Ma and <130 Ma. The Tongbai orogen lacks the granitoids formed by significant crust–mantle mixing that occurred in the Dabei area at <130 Ma. These results indicate that the lower crust was continuing thickening but probably not delaminated beneath the Tongbai orogeny during the Early Cretaceous.