Framework of Mesozoic tectonic evolution in southeastern Hunan and the Hunan-Guangdong-Jiangxi border area

Abstract：Southeastern Hunan and the Hunan-Guangdong-Jiangx border area belong to the northern part of the central segment of the Nanling Range. This region is a favorable window for studying Mesozoic tectonic setting and evolution in South China. According to isotopic chronological, geological and geochemical studies, the time ranges and tectonic settings of granites of three Mesozoic ages are determined, and on that basis, combined with the characteristics and genetic mechanism of continental basins of different Mesozoic ages and volcanic rocks, stages and characteristics of tectonic deformations and ore-forming processes, a preliminary sequence framework of the Mesozoic geological events is defined. Four major stages of Mesozoic tectonic evolution in the region may be distinguished; from early to late times they are the Early Triassic－early Middle Triassic pre-orogenic stage (I), the late Middle Triassic–early Middle Jurassic intracontinental orogenic stage (Ⅱ), the early Middle Jurassic–Late Jurassic post-orogenic stage (Ⅲ), and the Cretaceous intraplate rift stage (Ⅳ). ｏf these，stage II may be further divided into four substages (Ⅱ1–Ⅱ4). The pre-orogenic stage (Ⅰ) was a stable marine sedimentary stage. The late Middle Triassic (Ⅱ1) was an intracomtinental subduction-converge peak stage, when a great deal of mainly NNE-directed thrusts and folds formed, resulting in crustal thickening. In the terminal Middle Triassic–latest Triassic (Ⅱ2）, compression was relatively relax and a great deal of crustal-derived granitic magma formed and was emplaced (233–210 Ma). There was a syn-orogenic uplift-extensional tectonic setting in the terminal Late Triassic–Early Jurassic (Ⅱ3), when the NNE-directed rift coal-bearing basins formed and high-Na and low-K tholeiitic magma was erupted. The initial Middle Jurassic (Ⅱ4) witnessed NNE-directed sinistral convergence-strike-slip orogeny, when thrusts formed and early-formed rift basins underwent inversion to transform to piedmont thrust-contractive basins. In the early Middle Jurassic－Late Jurassic post-orogenic stage (III), voluminous post-orogenic granite was emplaced (at 174－135 Ma), accompanied by extensive mineralization. At the end of the Jurassic there occurred a transient compression event. In the Cretaceous intraplate-rift stage (Ⅳ), there occurred such tectonic associations as basin-range tectonics and metamorphic core complexes, bimodal volcanic rocks and subvolcanic rocks, acid dikes and Shangbao small intrusions with the characteristics of AA-type granite. Finally, the paper discusses the early Yanshanian tectonic environment and the formation mechanism of tectono-magmatic characteristics of different stages in the Mesozoic in South China.