Zircon ages and Hf isotopic compositions of Permian and Triassic A-type granites from central Inner Mongolia and their significance for late Palaeozoic and early Mesozoic evolution of the Central Asian Orogenic Belt

This work presents zircon ages and Hf-in-zircon isotopic data for Permian and Triassic A-type granitoids and reviews the evolution of central Inner Mongolia, China, during the early Permian and Late Triassic. SHRIMP U-Pb dating of zircons of peralkaline granites yielded Pb-206/U-238 ages of 294 +/- 4Ma and 293 +/- 9Ma that reflect the time of Permian magmatism. Zircon ages were also obtained for Late Triassic granites (226 +/- 4Ma, 224 +/- 4Ma). Our results, in combination with published zircon ages and geochemical data, document distinct magmatic episodes in central Inner Mongolia. The Permian peralkaline granites show typical geochemical features of A-type granites, which also have highly positive zircon epsilon(Hf(t)) values (+4.9 - +17.1), indicating a significant contribution of an isotopically depleted source, likely formed from mantle-derived magmas. Late Triassic A-type granitoids, however, in central Inner Mongolia show large variations and mostly positive in zircon epsilon(Hf(t)) values (-1.3 - +13.5), suggesting derivation from a mixture of crust and mantle or metasomatized lithospheric mantle with crustal contamination. The geochemical characteristics of the Permian peralkaline granites and Late Triassic A-type granitoids are consistent with a post-collisional setting and were likely related to asthenosphere upwelling during the evolution of the Northern Block and Central Asian Orogenic Belt (CAOB).