Crustal architecture of the southern Tongbai orogen, central China: Insight from migmatites and post-collisional granites

Abstract

The crustal structure of collisional orogen is critical for deciphering the crustal evolution and geodynamics of an orogen. Metamorphic rocks and post-collisional magmatic rocks are the key subjects to probe the crustal structure. In this study, we conducted an integrated study of zircon UPb age, Hf and O isotopes for migmatites and post-collisional granites in the southern segment of the Tongbai orogen. Zircon U−Pb dating yields protolith ages of 755 ± 55 to 713 ± 6 Ma and metamorphic ages of 162 ± 11 Ma to 138 ± 2 Ma for the migmatites and intrusion ages of 137 ± 2 Ma for the post-collisional granites. The inherited magmatic cores in the migmatites exhibit similar Hf isotopic compositions to the metamorphic zircons. Their εHf(t) values range from 13.5 to −9.5 with crust Hf model ages of 787 Ma to 2233 Ma, indicating that the protoliths were derived from reworking of Paleoproterozoic crust with the addition of juvenile components during the Neoproterozoic. Zircon δ18O of inherited magmatic cores are of −1.45‰ to 5.90‰, signifying the protoliths were formed by remelting of hydrothermally altered low δ18O rocks. By comparison, the post-collisional granites mostly show normal mantle-like O zircon isotope compositions and have lower εHf(t) and εNd(t) values than the migmatite rocks. These features suggest that the granites were formed by fusion of the ancient crust that might have suffered insufficient water-rock interaction. The discrepancies in O–Hf–Nd isotopic compositions of the studied migmatites and granites might be inherited from their precursors. It is thus inferred that the precursors of the granites might lie at the shoulder of the Neoproterozoic rift and thus might have experienced slight water-rock hydrothermal alteration and crust-mantle interaction; whereas the protoliths of the metamorphic rocks might be located at or near the rift center and thus had suffered intensive water-rock hydrothermal alteration and crust-mantle interaction. Prior to the late Mesozoic magmatism, the Tongbai orogen would have an uncoupled double-layered crust, where the upper layer consists of the metamorphic rocks and the lower is the sources of the post-collisional granites. They were stacked by the Permian-Triassic orogeny. The final architecture of the Tongbai orogen might be constituted by the emplacement of the voluminous granites from the deeper crust and the formation of the migmatites in the Tongbai complex. This configuration might widespread for other collisional orogens.