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 ²⁰⁶Pb/²³⁸U ages of 294 ± 4 Ma and 293 ± 9 Ma that reflect the time of Permian magmatism. Zircon ages were also obtained for Late Triassic granites (226 ± 4 Ma, 224 ± 4 Ma). 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 <i>ε</i>_Hf(<i>t</i>) 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 <i>ε</i>_Hf(<i>t</i>) 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).

本研究报道了中国内蒙古中部二叠纪与三叠纪A型花岗岩类（A-type granitoids）的锆石年龄及锆石铪同位素数据，并综述了该区域早二叠世至晚三叠世的地质演化历程。对过碱性花岗岩开展的SHRIMP锆石U-Pb定年（SHRIMP U–Pb dating）结果显示，其²⁰⁶Pb/²³⁸U年龄分别为294±4 Ma与293±9 Ma，对应二叠纪岩浆作用的发生时限。同时还获得了晚三叠世花岗岩的锆石年龄数据（226±4 Ma、224±4 Ma）。本研究结果结合已发表的锆石年龄与地球化学数据，明确了内蒙古中部存在两期清晰的岩浆活动事件。二叠纪过碱性花岗岩具备A型花岗岩典型的地球化学特征，其锆石εHf(t)值介于+4.9至+17.1之间，整体呈强烈正值，指示其源区具有显著的同位素亏损特征，极可能起源于幔源岩浆。但内蒙古中部晚三叠世A型花岗岩类的锆石εHf(t)值变化范围较大，多数为正值（-1.3至+13.5），表明其源区为地壳与幔源物质的混合体，或是受地壳混染的交代岩石圈地幔。二叠纪过碱性花岗岩与晚三叠世A型花岗岩类的地球化学特征均与碰撞后构造环境相符，且可能与北地块及中亚造山带（Central Asian Orogenic Belt, CAOB）演化过程中的软流圈上涌作用相关。