Variscan Granite Magmatism In Guaizihu Area Of Ejinaqi, Inner Mongolia

The granitoid rocks in Guaizihu, located in Northern Margin of the Alashan. Yagan fault zone in northern Guaizihu, Engeerwusu ophiolite zone in southern Guaizihu. The geotectonic position of Guaizihu area belongs to the northeast accretion zone of continental margins in Tarim plate. At present, the study of geochronological, causes and tectonic setting of this area is not enough. In this paper, we carried out geochemical study on the granites in this area. The background of this paper is the regional geological survey(1:200000) and the geological survey of northern margin of Alxa massif. We discussed petrogenesis and tectonic settings of Variscan granite in Guaizihu area. At last, we summarized the magma evolution.In the paper, we implemented zircon U- Pb age determination on the granitoids in this area. The results showed that the granitoids in this area were formed in early Devonian to middle Devonian(367Ma-399Ma), early Carboniferous epoch to middle Carboniferous epoch(315Ma-351Ma), and early Permian(< 278Ma). Besides, after early Permian, magmatic activity was still continued.Devonian granites, from beginning to end, were granodiorite, adamellite, and granodiorite, respectively, which have the characteristics of high silicon, high potassium calc alkaline- calc alkaline, peraluminous. The dark-colored mineral in these granites is mainly biotite, and some has little muscovite, a small amount of hornblende and very little garnet. Carboniferous granites, from beginning to end, were syenite granite and quartz diorite respectively, which have the characteristics of high silicon, metaluminous high potassium calc alkaline – low potassium series. The dark-colored mineral in these granites is mainly biotite and hornblende exists in these granites. Permian granites, from beginning to end, were biotite granite, monzonitic granite, and syenite granite respectively, which have the characteristics of high silicon, high potassium calc alkaline- calc alkaline, weakly peraluminous(A/KNC < 1.1). The dark-colored minerals in these granites are mainly biotite and very little muscovite. Granites in all the ages enriched in large ion lithophile elements, like Rb, Th, while are deficit in high field strength elements, like Nb, Ta, Ti. The rare earth element, Eu, was separately strongly negative anomaly, strong negative anomaly, medium weak negative anomalies from the Devonian to Permian.Devonian granites in the study area are mainly S-type, and the source rock was the partial melting sedimentary rocks of the upper crustal. These granites were formed in syn-colg tectonic setting. Carboniferous granites and Permian granites are mainly type I, which were the product of partial melting in lower crust or crust mantle transition zone. Carboniferous granites were formed in the tectonic setting that syn-colg is switching to post-colg, while Permian granites were formed in post-colg tectonic setting. From the west to the east, and from the early age to late age, the characteristic of the evolution is from type S to type I. During the forming age, the magmatic source have experienced the island arc evolutionary stage. Granites formed in the tectonic setting that syn-colg is switching to post-colg. Therefore, the paleo Asia ocean, which is represented by Engel Usu ophiolite belt, disappeared completely at the end of late Carboniferous, then entered post-colg stage in the early Permian, and the tectonic regime transformed from compression to tension.