Geological,Geochemical Characteristics And Its Boundary Of Basalts In Weixin Area,yunnan

Permian basalts in Weixin area of Zhaotong City belong to the Caledonian fold belt in eastern Yunnan Province,South China Plate,which are part of Emeishan basalt.According to the mantle plume structure of Emeishan,Weixin area is located at the junction of the middle zone and the outer zone of the Emeishan mantle plume,which belongs to the eastern rock area of the Emeishan Large Igneous Province?around the Guizhou plateau rock area?.Based on 1:50000 geological survey,systematic studies on field research,lithofacies,petrogeochemistry and chronology of basalts in Weixin area are carried out.The conclusions are as follows:1.Weixin Permian basalt is distributed in thin layers and is in unconformable contact with underlying Maokou limestone.In the middle,a complete paleo-weathering crust is developed on the top of Maokou limestone,and Permian bauxite minerals are produced in the weathering crust.A complete paleo-weathering crust is developed on the top of Maokou limestone and Permian bauxite minerals are produced in the weathering crust.Weixin Permian basalt is in conformity with the overlying Longtan Formation.2.The distribution of Weixin Emeishan basalt is obviously influenced by tectonism,mainly by NE-trending Shikan syncline and NNE-trending Yile-Chamu anticline folds.The appearances of basalts are mainly isolated dot,sheet and strip in the two wings of fold and the thickest part is only more than 50 meters.3.In petrographic research,Weixin Permian basalt is mainly a set of porous basalt,almond basalt and dense massive basalt,with intergranular-intergranular concealedstructure,intergranularconcealedstructure,stomatalstructure,pore-almond structure,massive structure and rare phenocrysts.4.The LA-ICP-MS zircon U-Pb dating of five samples shows that the age of zircon is 257.4±1.8 Ma,which is consistent with the main eruption time of Emeishan basalt.5.Major element characteristics show that Weixin basalt owe high Fe,Mg?MgO:2.09%-5.64%;FeOT:11.37%-14.72%?and low potassium and sodium?K2O:0.29%-1.99%;Na2O:2.06%-4.45%?.It is the Na-alkaline?Na2O/K2O:1.79-10.90?a nd sub-alkaline tholeiite series and a typical high-titanium tholeiite.?TiO2:4.18%-4.86%;Ti/Y:618.83-670.70?6.The REE show that there is a right-leaning REE distribution pattern(?LREE:249.37×10-6-277.83×10^-6;?LREE/?HREE:6.64-8.62),and the negative Europium anomaly is not obvious??Eu 0.91-1.02?.7.Trace elements show that the ratio of incompatible elements?Zr/Nb:7.47-9.28;La/Nb:1.02-1.16;Ba/Nb:3.16-19.00;Ba/Th:18.24-124.94;Th/La:0.13-0.17;Th/Nb:0.14-0.17;Ba/La:3.10-17.00;Rb/Nb:0.05-1.00?,which indicates the basalts in Weixin area are continental basalts which are contaminated by crustal materials.8.Using trace elements to distinguish tectonic environment,Weixin basalt is a set of overflow basalts in intraplate.The results of chronology and geology indicate that Weixin basalt is a component of Emeishan basalt and a product of Emeishan mantle plume.9.The comparative study of the upper and lower strata in contact with basalts in Weixin area shows that the eruption process of Emeishan basalt can also reflect the significant crustal uplift in the region.The marine limestone of Maokou Formation is uplifted and formed paleo-weathering crust and the Emeishan overflow basalt is continental facies.After the eruption,the Longtan Formation deposit is the land-sea interaction facies,which shows the uplifting of the crust during the development of the Emeishan mantle plume.10.Emeishan basalt is thinned spatially from west to east and from south to north.The study area in Guizhou plateau is triangular?tongue-shaped?with thick east and thin west,located in east of Panxi and at the junction of Yunnan,Guizhou and Sichuan.According to the triangular distribution of Emeishan basalts east of Panxi,the basalts in the study area are located at the upper boundary of the triangle and the Emeishan basalts gradually thin out at the upper boundary of the triangle.