Timing of the formation of the Tianhuashan Basin in northern Wuyi as constrained by geochronology of volcanic and plutonic rocks
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The Marwar Supergroup (NW Peninsular India) is thought to be of Ediacaran-Cambrian age, based on previous paleontological and geochronological studies. However, direct constraints on the onset of sedimentation within the Marwar basin are still scarce. In this study, we report U–Pb zircon, LA-ICP-MS, and SIMS ages from the Chhoti Khatu felsic volcanic rocks, interlayered with the Jodhpur Group sandstones (Lower Marwar Supergroup). The cathodoluminescence images of the zircons indicate complex morphologies, and core-rim textures coupled with the wide range of ages indicate that they are likely inherited or in the case of thin poorly indurated ash-beds, detrital in origin. The age spectra of 68 zircon analyses from our sampling display a dominant 800–900 Ma age peak corresponding to the age of basement "Erinpura granite" rocks in the region. The youngest inherited zircon from a felsic ash layer yielded a U–Pb age of 651 Ma ± 18 Ma that, together with previous studies and paleontological evidence, indicates a post-Cryogenian age for the initiation of Marwar sedimentation following a ~125 Ma hiatus between the end of Malani magmatism and Marwar deposition.
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The low-grade metamorphic andesite, rhyolite and tuff are found in the Shuangqiaoshan Group in the Qimen-Pingli area at the Anhui-Jiangxi border region. In this study, we present LA-ICP-MS U-Pb zircon dating and geochemical analysis results for the volcanic rocks found in the Shuangqiaoshan Group. The LA-ICP-MS zircon U-Pb dating shows that these volcanic rocks were formed at (822.4±6.0) Ma, (821.2±4.6) Ma and (830.4±4.9) Ma, which indicates an early Neoproterozoic age of the Shuangqiaoshan Group. In geochemical composition, these volcanic rocks are mainly moderately potassium calc-alkaline, relatively LREE enriched, HREE and HFSE high field strength elements (Nb, Ta and Ti) depleted, and likely were formed in the island arc environment. Whole rock Nd isotope study shows that the andesite was derived by partial melting of subducted oceanic crust and mantle-derived materials contributed to generation of rhyolite and tuff. Combining with the regional geological information, we believe that the volcanic rocks in the Shuangqiaoshan group at the Anhui-Jiangxi border region geodynamically coincide with freguently occurred magmatic activities in South China for the same period (838 Ma-820 Ma). Another word, these volcanic rocks were the results of island arc magmatism at that time.
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Large igneous province
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Research subject. Zircons from the Saf’yanovskoe Cu-Zn deposit rhyolite (Middle Urals). For the first time, zircon U-Pb dating for the rhyolite of the ore-bearing volcanic-sedimentary rocks of the Saf’yanovskoe deposit was performed. The volcanites are characterized by an andesite-rhyodacite composition and are localized at the southern edge of the Rezhevskaya structural-formation zone (SFZ) of the Eastern Ural megazone. A number of publications assign these rocks either to the basalt-rhyolite formation of the Middle Devonian, or to the basalt-andesite-dacite-rhyolite formation of the Lower-Middle Devonian. Aim. To estimate the age of the ore-bearing volcanic rocks under study using the U-Pb SHRIMP-II isotop ic system of zircon from the rhyolite of the eastern side of the Saf’yanovskoe deposit. By its chemical composition, the rhyolite belongs to the silicic varieties of subvolcanic rocks. Methods and results. The U-Pb isotopic system of zircon was studied by 5-collector mass-spectrometer of high precision and emission of the secondary ions SHRIMP-II (ASI, Australia) in the VSЕGEI Institute. U-Pb relations were investigated by a procedure developed by I.S. Williams. The U-Pb data obtained based on 13 zircon grains showed the age of 422.8 ± 3.7 Ma. Conclusions. The U-Pb dating of zircon obtained previously from the lens-shaped andesite bodies of the western side of the Safyanovskoe deposit gave the age of 422.8 Ma, which corresponds to the Przydoli series epoch of the Upper Silurian. We established that, among the volcanic rocks of the Saf’yanovskoe deposit, the effusive formations of the Upper Silurian are present.
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