Petrogenesis of coexisting high-silica aluminous and peralkaline rhyolites from Yunshan (Yongtai), southeastern China |
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| Institution: | 1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China |
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| Abstract: | The Late Cretaceous bimodal Yunshan (Yongtai) volcanics in Fujian province contain peralkaline rhyolites, the only presence of such rhyolites in southeastern China. Whole-rock and mineral chemical compositions are analyzed for the coexisting aluminous (metaluminous to weakly peraluminous)-peralkaline high-silica rhyolites from the Yunshan volcanics. They are sparsely porphyritic, and contain K-feldspar, ferromagnesian minerals, quartz, magnetite, and titanomagnetite phenocrysts, as well as accessory minerals such as fayalite, chevkinite, apatite and zircon. The mineral assemblage indicates an oxidizing pre-eruption condition. These rhyolites exhibit diagnostic geochemical features of A-type granites, such as elevated 104 * Ga/Al (mostly greater than 2.6) and FeOT/(FeOT + MgO) ratios, enrichment in high field strength elements (HFSE) such as Zr (>400 ppm) and Nb, and strong depletion in Al2O3 (<13 wt%), CaO, Ba and Sr. On the basis of their petrographic and geochemical characteristics, it is suggested that the rhyolite magmas are derived from partial melting of H2O-poor (meta) granitic igneous rocks in the deep crust, and cannot be fractionated from the coeval Yunshan mafic magmas. Geochemical variations of major and trace elements indicate the possible fractionation of K-feldspar, calcium-rich pyroxene, Fe–Ti oxides and minor chevkinite during the magma evolution. In peralkaline rhyolites, we found that the pre-existing Fe–Ti oxide and hedenbergite phenocrysts had been transformed into aegirine + oxide and aegirine + oxide + fluorite assemblages, respectively. These mineral assemblages are the products of the subsolidus reaction of pre-existing phenocrysts and extraneous Na–F-rich fluids. Such Na–F-rich fluids may be derived from the degassing of the subvolcanic rocks. The reactions indicate that the Yunshan peralkaline rhyolites could be generated through the reaction of highly fractionated aluminous silica magmas and Na–F-rich fluids. |
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