| The western Yunnan Province preserves a complete record of paleo-Tethys sedimentary evolution in China and even in the world.With the paleo-tethyan major oceanic basin(Changning-Menglian Ocean Basin)dive eastward,there had formed a series of continental marginal volcanic belts on the west side of the Lancang River,the main arc-forming period was in the Permian,and there are only a few reports on late Paleozoic magmatism records in this area.It is because of the lack of high-precision isotopic geochronology that has led to the unclear of late Paleozoic Paleo-Tethys evolution.In this paper,we identified a set of Permian metamorphic volcanic rock series in the area of Fengqing County-Yun County.The source rocks are medium-acid lava,pyroclastic rocks,and a few basic volcanic rocks.This paper takes rhyolite as the research object,through field and mirror observation,zircon U-Pb dating and whole rock geochemical analysis,disscussed the chemical characteristics,rock genesis and geological significance of rhyolite.Providing important information for the evolution of the Late Paleozoic Paleo-Tethys Ocean in the area.The rhyolite zircon dating results show that the rhyolite zircon U-Pb age is 270±1.6Ma and the formation age is early Middle Permian,the zircon cathodoluminescence(CL)map shows the characteristics of the magmatic oscillation ring zone,the Th/U ratios are between 0.36 and 0.90,which indicate the zircons are magmatic origin zircons.The major elements show the characteristic of rich-silicon,low-magnesium,low-calcium,low in titanium and phosphorus.The spider diagram of trace element ratio shows enrichment of large ion lithophile elements,relative defect of Nb,Ta,Zr,and strong depletion of Sr.The distribution mode of REE shows a right-angled “seagull pattern”,it reveals that LREE is enricher and have distincter fractionation than HREE,the rare earth element Eu has a strong negative anomaly.The characteristics of the main and trace elements of rhyolite indicate that the rhyolite magma in this area is partially melted from the upper crust and may be mixed with a small amount of mantle-derived magma.The negative anomalies of Sr,Ba,Eu,P,and Ti indicate that rhyolite has undergone certain amount of plagioclase,apatite,titanomagnetite segregation,or magmatic source have residual phases of plagioclase,apatite,hornblende during the formation process,and the characteristics of rhyolite crystal porridge indicate that the degree of crystallization differentiation is limited.The ratios of 10000Ga/Al of trace elements in rhyolite are between 2.82 and 3.73,they locate at A type granite area in granite discrimination diagram,the rhyolites belong to A-type rhyolites,and the formation environment is related to the stretching effect of the region.In the granite classification diagram established by Eby,rhyolites samples locate at the transitional area of A1-A2,mainly focus on A2 area,combine with the regional geologic background: The paleo-tethyan major oceanic basin(Changning-Menglian Ocean Basin)began to subduct eastwards in the Late Carboniferous,formed an island-arc environment related to subduction in the Fengqing-Yunxian area of the eastern margin of the Changning-Menglian ocean basin.Around 270 Ma,the oceanic crust subduction and the reverse advection at the bottom of the lithosphere which formed by rising heat flow prompted the cracking of early island arcs,formed a relatively small inter-arc basin between the island arcs.The high-temperature mantle heat flow risesd along the inter-arc basin to the bottom of the upper crust.The upwelling of the mantle heat flow provided heat for the upper crust,it caused the upper crust partially melted into felsic magma chamber.The magma erupted to the surface along the inter-arc basin and formed Permian rhyolite.With the continuous subduction of Changning-Menglian Ocean Basin,the inter-arc basin gradually closed,the next collision orogenesis led to the deformation and metamorphism of rhyolite,finallay formed the epi-metamorphic rhyolite and outcropping in Fengqing County-Yun County now. |
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