First identification of Late Mesozoic intraplate magmatism in the Chinese North Tianshan: Implications for the orogenic architecture and crustal evolution

The formation and dynamics of granitoids in an intra-continental setting are crucial for understanding the architecture and evolution of continental crust. Here, we report geochronological, geochemical, and Sr-Nd-Hf isotopic data for newly discovered late Mesozoic granitic intrusions in the Tianshan belt, northwestern China. These granitoids are I-type granites derived from an igneous precursor and were emplaced during ∼145-132 Ma. They have positive ε _Nd (t) values and young Nd model ages, together with relatively low Sr/Y ratios, indicating that they might have originated from partial melting of the juvenile lower crust. There is a prominent decoupling between zircon Hf and bulk rock Nd isotopes, which may have resulted from the early crystallization of Ti-rich minerals. These granitic intrusions also display subduction-related geochemical characteristics, which are likely inherited from the Paleozoic crustal sources that were metasomatized by subduction-related fluids. We conclude that these late Mesozoic granitoids were emplaced in an intra-continental setting, and were likely triggered by thermal relaxation due to crustal shortening and thickening. These data further imply that the Tianshan has changed into crustal reworking during the Mesozoic from its prominent crustal growth in the Paleozoic.