Petrogenesis And Geodynamic Setting Of Magmatic Rocks From Uranium-bearing Volcanic Basins,Gan-Hang Belt, Southeast China

The Gan-Hang Belt is an important volcanic rock-hosted uranium mineralization belt in China. Abundant volcanic rock-hosted uranium deposits in the Gan-Hang Belt are mainly concentrated within four volcanic fault basins from southwest to northeast:Xiangshan basin, Shengyuan basin, Dazhou basin and Xinlu basin. However, the precise timing, petrogenesis and geodynamic setting of these magmatic rocks are not well understood, or controversial. In this paper, we integrate new geochronological data, major and trace element geochemical data, and whole-rock Sr-Nd and in situ zircon Hf isotopic data for the magmatic rocks from the Xiangshan, Xinlu and Dazhou volcanic basins. We review and discuss the significance of the Late Mesozoic magmatic rocks in the Gan-Hang Belt in an attempt to better understand the Late Mesozoic magmatic and tectonic evolution of the region.Zircon U-Pb dating yield ages of137-132Ma for Xiangshan volcanic-intrusive complex,136-133Ma for Xinlu volcanic-intrusive complex, and126Ma for Dazhou rhyolite, representing an Early Cretaceous magmatic event for felsic rocks from volcanic rock-hosted uranium deposits in the Gan-Hang Belt.These felsic rocks have a pronounced A-type geochemical signature, such as high contents of Na2O+K2O, high Fe2O3*/MgO ratios, enrichment in REE, HFSE and Ga. They show low CaO, MgO and TiO2contents, enrichment in some LILEs, depletion in Sr, Ba, P, Eu and Ti. These felsic rocks were formed at a high temperature, have high Ga/Al ratios and high contents of Zr+Nb+Ce+Y. In the discrimination diagrams of A-type granites, they plot in the A-type granite field, showing an A-type affinity.Some Early Cretaceous granites have also been previously identified along the Gan-Hang Belt by other scholars, such as Baijuhuajian (126Ma), Damaoshan (126-122Ma), Tongshan (129Ma), Sanqingshan (135Ma), and Lingshan (132-131Ma) granites. These granites also have an A-type affinity. All of the studies indicate an Early Cretaceous A-type granitic belt (137-122Ma) occurs along the Gan-Hang Belt.The Xiangshan volcanic-intrusive complex shows a narrow range of εNd(t) values (-6.9to-8.7), and their zircon grains are characterized by negative initial εHf(t) values, with most values concentrated in the range of-7to-9, and Mesoproterozoic TDMc ages for both Nd and Hf isotopes. The negative and unimodally distributed εNd(t) and εHf(t) values indicate the same magmatic source for all rocks of the complex and predominantly crust-derived material. The Xiangshan volcanic-intrusive complex was probably derived from partial melting of Mesoproterozoic metamorphic rocks, without significant additions of mantle-derived magma. The occurrence of quartz-amphibole schist xenolith enclosed within a mafic microgranular enclave (MME) in the Xiangshan volcanic-intrusive complex, indicate that the hybridization processes for the major elements of the pristine mafic magma may have been contaminated by crustal rocks to form its present dioritic composition of MME before mafic magma injection. The formation of nodular tourmaline in Xiangshan porphyroclastic rhyolitic lava was most likely triggered by liquid immiscibility during highly evolved magmatic differentiation, and tourmaline crystallization typically occurred during the magmatic stage transitional from latter solidus to early subsolidus crystallization. The boron isotope analyses of tourmaline reveal that the nodular tourmalines have the δ11B values concentrated at about-12%o, which is close to the average δ11B value of continental crust, indicating the boron was derived from crustal material. The δ11B values of the tourmaline also indicate that the Xiangshan porphyroclastic rhyolitic lava was derived from crustal material, without significant additions of mantle-derived magma.The Yangmeiwan granite and Daqiaowu granitic porphyry from the Xinlu volcanic basin show bulk rockd(t) values in the range of-6.5to-3.6and zircon εHf(t) values from-7.8to-0.9, with Mesoproterozoic TDmc ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these two A-type granitic rocks were generated largely by partial melting of Mesoproterozoic metamorphic basement rocks. The Yangmeiwan granite and Daqiaowu granitic porphyry generally have slightly higherd(t) and εHf{t) values and younger TDMc ages of Nd and Hf isotopes than the Xiangshan volcanic-intrusive complex, implying a possible input of mantle-derived materials. Besides, the Yangmeiwan granite has slightly higher εNd(t) values than the Daqiaowu granitic porphyry, indicating the difference of ratios and/or property for mantle-derived materials in these two granitic rocks.The Dazhou rhyolites show bulk rock εNd(t) values in the range of-6.4to-5.5and zircon εHf(t) values from-6.0to-2.9(mostly centered between-5and-6), with Mesoproterozoic TDMc ages for both Nd and Hf isotopes. The Dazhou rhyolites have high contents of SiO? which vary in a narrow range (between74%and77%), with narrow range of sNd(t) values and εHf(t) values, indicating that the magma was principally of crustal origin without the involvement of mantle materials. Dazhou rhyolites have distinctive high-Zr contents (802-1145ppm), with the calculated zircon saturation temperatures extremely high at1000℃. The zirconium in the Dazhou rhyolite distributes into a paucity of large zircon crystals and into the zircon (1～10μm) and baddeleyite (<1μm to5μm) crystallites in the fine-grained groundmass. This indicates that the zirconium precipitation in the Dazhou rhyolite was developed in the late stage of the magmatic evolution. The high-temperature suppression of zircon crystallization and the different mechanisms of magmatic evolution in the magma brought about the distinctive high-Zr contents of the Dazhou rhyolite. The distinctive high-Zr characteristic of the Dazhou rhyolite was a result of high-temperature suppression of zircon crystallization by ambient mafic magmas correlative with the upwelling of the asthenospheric mantle. The Dazhou high-Zr rhyolite confirmed that the upwelling of the asthenospheric mantle was occurred along the Gan-Hang Belt during the Early Cretaceous.Most of the A-type granitoids along the eastern part of the Gan-Hang Belt have slightly higher εNd(t) and εHf(t) values and younger TDMc ages of Nd and Hf isotopes than the Xiangshan volcanic-intrusive complex in the western part of the Gan-Hang Belt, implying a possible input of mantle-derived materials. These Nd-Hf isotopic data also suggest that mantle-crust interaction in the formation of the A-type granitic magma along the Gan-Hang Belt gradually intensified from western to eastern part, and from early to late stages between ca.137and ca.122Ma.These A-type granitic rocks along the Gan-Hang belt show Mesoproterozoic TDMc ages for both Nd and Hf isotopes. Notably, the Nd isotopic data for these A-type granitic rocks, as a whole, lie entirely between the isotopic evolution envelops defined by the rocks of the parametamorphic and orthometamorphic basement rocks in Southeast China, respectively. The Cretaceous A-type granitic rocks with ages between137Ma and122Ma occurred along the Gan-Hang Belt indicate 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. 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 the 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, to generate these A-type granitic rocks.The Early Cretaceous A-type granitic rocks in the Gan-Hang Belt indicate that an A-type granitic belt occurs along the Shi-Hang Belt. The tectonic and magmatic evolution of Southeast China in Late Mesozoic is characterized as a tectonic transition from a subduction-related tectonic setting to an extensional setting due to the roll-back of the paleo-Pacific plate. However, the roll-back of the paleo-Pacific plate was discontinuous throughout the Jurassic and Cretaceous. The tectonic transition from a subduction-related tectonic setting to an extensional setting in the South Shi-Hang Belt occurred at-163Ma. Our study suggests the timing of transition between the two tectonic environments can be constrained to-137Ma in the Gan-Hang Belt. The110-90Ma A-type granites were formed along the coastal region in eastern Zhejiang and Fujian Provinces, indicating the tectonic transition in the coastal region happened at-110Ma.The available data for the Late Mesozoic A-type granitic rocks in Southeast China indicate irregular and episodic roll-back of the paleo-Pacific plate that led to localized extension in the Shi-Hang Belt and the coastal region. The roll-back of the paleo-Pacific plate firstly happened in South Shi-Hang Belt, then the Gan-Hang Belt and finally in the coastal region, showing a younger trend from interior to coastal area.