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Abstract: The Gan-Hang rift, trending at least 450km in a NE–SW direction, is a part of a Mesozoic Basin and Range Province in southeastern China. Detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope data of three granitic plutons and a diabasic dike in the northwest of the Gan-Hang rift, are used to explore the origin of these granites and their relationship to the evolution of the Gan-Hang rift. SHRIMP zircon U–Pb dating shows that the granitic plutons and diabasic dike were emplaced in the Early Cretaceous (122–129 Ma). The Tongshan and Damaoshan plutons, close to the Gan-Hang rift, consist mainly of weakly peraluminous granitic rocks, which show A2 subtype affinity. These granites have initial 87Sr/86Sr ratios of 0.7080–0.7103, εNd (T) values of−1.4 to−5.6 and εHf (T) (in-situ zircon) values of −3.8 to +1.2. Detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized Mesoproterozoic metamorphic basement (including metasedimentary and metaigneous rocks) in the shallow (<30km) crust at a high temperature (~823°C). These pure crustal melts underwent crystal fractionation of biotite, plagioclase and K-feldspar, forming the more felsic end-member of granitic rocks. The Ehu pluton, more distant from the Gan-Hang rift, is composed of two-mica granites, which are strongly peraluminous and are S-type granites. These granites have higher initial 87Sr/86Sr ratios (0.7162−0.7191) and lower εNd (T) (−7.1 to −7.4) and εHf (T) (−6.8; in-situ zircon) values than the Tongshan and Damaoshan granites. Detailed elemental and isotopic data suggest that the Ehu granites were formed by partial melting of Paleoproterozoic metasedimentary basement in the deep (~40–50km) crust at a relatively low temperature (~757°C), with no fractional crystallization and mantle-derived magma mixing in their evolution. The coeval diabasic dike is calc-alkaline and has initial 87Sr/86Sr ratio of 0.7085, εNd (T) value of +0.9 and εHf (T) (in-situ zircon) value of −7.1, suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment-derived melt with monazite in the residual phase. The association of Early Cretaceous (122–129Ma) S- and A-type granites in the northwest of the Gan-Hang rift marks the onset of back-arc extension or intra-arc rift. With ongoing extension the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the Gan-Hang rift, forming the A-type granitic magmas such as Tongshan and Damaoshan, whereas partial melting of metasedimentary rocks in the less thinned crust farther from the Gan-Hang rift formed the S-type granitic magmas such as Ehu. The red sediments with the total thickness more than 10,000m have been successively deposited in the Gan-Hang rift valley since the late Early Cretaceous (~105Ma), suggesting that this region experienced the most back-arc extension. [Copyright &y& Elsevier]

Abstract: Extensive middle to late Jurassic felsic and mafic magmatism occurred in the southern Hunan province, southeast China. SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Hf isotopic data have been determined for these rocks. The results indicate that the middle Jurassic (178–170 Ma) Changchengling and Ningyuan basaltic rocks belong to tholeiitic series and alkaline series, respectively, and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab-released fluids of the subducted Paleo-Pacific plate. The late Jurassic (152–146 Ma) Daoxian basalts and Guiyang lamprophyre dikes are low-Ti, high-Mg potassic rocks. They were derived directly by partial melting of shallower (60–100 km) lithospheric mantle composed of amphibole- and phlogopite-bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc. The middle Jurassic (164 Ma) Tongshanling granitic rocks belong to calc-alkaline series and are strongly peraluminous. They were formed by partial melting of Palaeoproterozoic metasedimentary basement in the lower-crust plus additional input from coeval basaltic magma. The late Jurassic (156 Ma) Jinjiling and Xishan granitic rocks show A2 subtype affinity. They were generated by partial melting of granulitized Palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low-Ti, high-Mg potassic magmas. A moderate degree (56–58%) of fractionation of these pure crustal melts could account for more felsic end-member of granitic rocks. Detailed petrologic and geochemical data for the middle to late Jurassic felsic and mafic rocks from the southern Hunan imply that during the middle Jurassic time SE China on the southeast of the Shi-Hang zone was a continental arc coupled with the subduction of the Palaeo-Pacific plate and since the beginning of late Jurassic time an intra-arc rift has been formed along the Shi-Hang zone as a consequence of slab roll-back. [Copyright &y& Elsevier]

Abstract: The widespread occurrence of Mesozoic granites in the Gan-Hang Belt in Southeast China is associated with similarly widespread, economically important mineralization, but the precise timing, origin, and plate tectonic significance of these granites are not well understood. We have studied two of these (Early Cretaceous) granite bodies, the Yangmeiwan granite and the Daqiaowu granitic porphyry, from the northeastern part of the Gan-Hang Belt in western Zhejiang Province by zircon U-Pb geochronology, major and trace element analyses, and Nd–Hf isotopic analyses. LA-ICP-MS and SHRIMP U–Pb dating of zircon grains from these two granites yield ages of 133–136Ma, representing a Early Cretaceous magmatic event. These granitic rocks are metaluminous to weakly peraluminous and have a pronounced A-type geochemical signature with high Na2O+K2O, Fe2O3*/MgO and Ga/Al ratios. They show low CaO, MgO and TiO2 contents, enrichment in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y), depletion in Sr, Ba, P, Eu and Ti. They also show A2 subtype affinity and were probably formed at a high temperature (~810°C for Yangmeiwan granite and ~850°C for Daqiaowu granitic porphyry). These A-type granitic rocks show bulk rock εNd(t) values in the range of −6.5 to −3.6 and zircon εHf(t) values from −7.8 to −0.9, with Mesoproterozoic TDM c ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these A-type granitic rocks were generated largely by partial melting of granulitized Mesoproterozoic metamorphic basement rocks (including parametamorphic and orthometamorphic rocks), with a possible input of mantle-derived materials and followed by subsequent fractional crystallization. Our new data together with previous published data suggest that the Cretaceous A-type granitic rocks with ages between 137Ma and 122Ma occurred along the Gan-Hang Belt, indicating an important Mesozoic extensional event in Southeast China. This event represents either a back-arc extension or an intra-arc rift due to the roll-back of the paleo-Pacific plate, and it started as early as ~137Ma in Xiangshan and Xinlu basins. It is earlier than ~130Ma as previously suggested in the studied region and later than that along the south part of the Shi-Hang Zone. With ongoing extension during slab roll-back of paleo-Pacific plate, the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere would have triggered crustal rocks to partially melt, generating granitic magmas. Our new geochemical data also suggest that mantle-crust interaction in the formation of the A-type granitic magma along the Gan-Hang Belt gradually intensified from early to late stages between ca. 137 and ca. 122Ma. [Copyright &y& Elsevier]