Progress in the study of Mesozoic tectono-magmatism and mineralization in the central segment of the Nanling Mountains-Summary of major achievements of the 1∶250,000 geological survey in southeastern Hunan

Abstract：Mesozoic tectono-magmatism and mineralization in southeastern Hunan in the central segment of the Nanling Mountains have been systematically studied. The following ideas are put forward. (1) The period from the late Middle Triassic to early Middle Jurassic (early Mesozoic) was an intracontinental orogenic stage. The late Middle Triassic witnessed strong intracontinental subduction-convergence in a regional WNW-ESE compressional regime, which resulted in formation of large numbers of E-dipping, NNE-verging thrusts and folds. The formation mechanism of the trough-like folds in an uplift area east of the Chaling-Chenzhou fault were "thick-skinned" rather than "thin-skinned". NW-trending basement hidden faults underwent strong sinistral strike-slip movement under WNW compression, which led the NNE-directed lineament to rotate counterclockwise, thus forming the Anren “y”-type structure and the N-S-trending Shuikoushan-Xianghualing tectonic zone. The strong compression made the crust thicken substantially and caused the temperature of the deep crust rise continuously. In the terminal Middle Triassic-latest Triassic (233-210 Ma), the regional stress weakened, the heated middle crust melted and mantle-derived basic magma underplated; as a result, large-scale granitic magmatism took place in a post-collisional setting. Down-faulted basins formed by syn-orogenic uplift-extension in the terminal Late Triassic-Early Jurassic. Piedmont thrust-compressional basins and thrusts and NW-trending dextral strike-slip faults occurred in a NNE-trending sinistral convergence and strike-slip regime at the beginning of the Middle Jurassic. The dynamic mechanism of early Mesozoic intracontinental orogeny is mainly related to far-field compression effects of plate convergence. (2) The early Middle Jurassic-Cretaceous (late Mesozoic) was a post-orogenic-intracontinental rifting stage. In the early Middle Jurassic-Late Jurassic (174-135 Ma), there occurred large-scale granitic magmatism and mineralization due to lithospheric delamination. The passive emplacement mechanism of the plutons, occurrence of mafic microgranular enclaves, Sr-Nd isotopic features, predominance of high-K calc-alkaline and calc-alkaline rocks in the plutons, geochemical discriminations of structural environment, large-scale nonferrous metal mineralization and tectonic evolution setting suggest a post-orogenic environment in the stage. The Cretaceous was a strong intracontinental extensional stage, when basin-range tectonics and related divergent strike-slip faults formed, various kinds of dikes were widely developed and small AA-type granite bodies and basic volcanic rocks originated locally. Thermochronological data suggest that the evolution of the basin-range tectonics progressed through the tectonic denudation stage and weathering erosion-sedimentation stage. (3) The cause for the fact that the ore-forming capacity of the early Yanshanian granites in southeastern Hunan was far larger than that of the Indosinian granites is mainly attributed to the differences in tectonic setting and tectonic regime. The ore materials in magma of granites could spread and be precipitated more easily in an Early Yanshanian post-orogenic extensional regime; on the contrary, the ore materials were confined in the pluton in an Indosinian post-collisional weakly compressional setting. The second cause of the difference in ore-formation capacity is due to the difference in geochemistry of granites, i.e. the tectono-magmatic evolution and introduction of deep ore-forming fluids caused the early Yanshanian granites to have better ore-forming geochemical conditions than the Indosinian granites. (4) The formation of the W-Sn-polymetallic and Pb-Zn-polymetallic deposit assemblage is probably mainly related to such factors as the structure (or thickness) of the lithosphere, intensity of thermal perturbations in the deep interior of the Earth and corresponding scale of magmatism and depth of pluton emplacement.