Metamorphic Evolution of Basic Rock Series in Dongshengmiao Area, Inner Mongolia

The Langshan area is located in the western section of the northern margin of North China Craton. This area has experienced complex tectonic evolution, the different occurrence and different stages of basic metamorphic rocks in Langshan area provide important information for the inversion of their regional geological evolution history. The petrological and mineralogical characteristics of metamorphic rocks in various periods have been studied by the means of petrographic observation, chemical total analysis and electron probe. According to the age, occurrence of basic metamorphic rocks and their relationship with the metamorphism, deformation and migmatization of basic metamorphic rocks, these basic metamorphic rocks can be divided into four stages according to their sequence of formation. The petrological results show that the primary rocks except greenschist are basic volcanic rocks. In the first phase of the amphibolite enrichment, there is a significant depletion of Pb, K, Nb and Ta, with weak depletion of Hf, Zr and Ti. The difference between light and heavy rare earths is obvious, belonging to the island arc basalt. The second and third stages of amphibolite are characterized by intraplate and island arc transitions. In the fourth stage, the light rare earth elements of the amphibolite are relatively low, which is manifested by the emplacement environment in the intraplate tension crack environment. The greenschist is depleted in Sr, K, Rb, Ba, but enriched in Th, which is characterized by basalt in island arc or back-arc basin. The plagioclase and amphibole symbiotic minerals have been used to calculate the temperature and pressure of metamorphism experienced by amphibolite in various periods. The first stage of amphibolite calculation shows that the amphibolite belongs to the early high amphibolite facies-granulite facies;The second stage of basalt calculation shows that it represents the metamorphic process from the late amphibolite facies to the low amphibolite facies in the early stage;The third stage of amphibolite has been calculated to be a low amphibolite facies;The fourth stage of amphibolite belongs to the contact metamorphic green-curtain amphibolite facies.