Discovery and determination of high-pressure garnet-amphibolite in Dahongshan Group in Gasa area, southwest margin of central Yangtze, Yunnan Province

In the course of field geological investigation, a new set of metamorphic rocks with strong metamorphism was discovered in the Hongshan Formation of Dahongshan Group in the shallow lava iron ore area of Jiesa, Xinping County, central Yunnan Province. Petrographic studies show that the metamorphic rocks are eclogite amphibolites with strong retrometamorphism and only partial subduction−reentrant mineralogical records. There is no mineral association in the early metamorphic and peak metamorphic stages of the rocks, and the mineral generations have the characteristics of clockwise metamorphic tracks. According to the results of electron probe test, the maximum metamorphic pressure recorded by GBP (Garnet−Biotite−Plagioclase) average temperature manometer is 1.52 GPa. The maximum metamorphic pressure of hornblende recorded by temperature manometer method is 0.66 GPa, which indicates that the eclogite in this study is of high pressure metamorphic grade. According to the diagenetic age, metamorphic age and geochemical characteristics of Dahongshan Group, it is believed that Dahongshan Group was formed in the cracking period of the Columbia supercontinent and is a rock component of the Late Paleoproterozoic ocean basin. The main−stage metamorphism was formed by the subduction−reentrant process of the Rodinia supercontinent, and the Dahongshan Group may be an incomplete set of ophiolitic melanomics, representing the remnants of the Late Paleoproterozoic ocean basin.