Characteristics And Genetic Significance Of Major Minerals In Eclogites From Dabie Shan Area

Dabie shan is located in the collision zone between the Yangtze and Sino-Koreancraton. It is famous for coesite-bearing and diamond-bearing eclogites. Eclogites areusually planar distribution in South Dabie, but only scattered in North Dabie.Coesiteand diamond have been confirmed in South Dabie, yet not reported in North Dabie.In this paper, modern technical means like optical microscopy（OM）、Electronprobe micro analysis（EPMA）and microscopic laser Raman spectrum have been usedto study mineral composition、chemical composition and structure characteristics ofmajor minerals in Dabie eclogites. Genetic significance and Exhumation pattern ofDabie eclogites have been discussed in paper.Analysis in this paper show that the garnet in North Dabie is pyrope-almandine and pyrope content in North Dabie is higher than in South Dabie.Alteration zone of brown pargasite is obvious in North Dabie garnet. Thephenomenon of mineral exsolution in omphacite is clear, oligoclase exsolution can beseen. The mineral assemblages of North Dabie eclogite are garnet+omphacite+pargasite+diopside+oligoclase+adesine+quartz, show the feature of high-pressuregranulite facies metamorphism. The garnet in South Dabie is grossular-almandine, thetypical inclusion in garnet is coesite. The mineral assemblages of South Dabieeclogite are garnet+omphacite+coesite+rutile+quartz, show the feature ofeclogite-facies peak metamorphism.Eclogites have different exhumation process in Dabie Shan area. Exhumationspeed of South Dabie eclogite is faster than North Dabie eclogite. After the rapidtectonic uplift, South Dabie eclogite arrival the level of the middle-upper crust. Forthe slow uplift, North Dabie eclogite only reach the level of lower crust. This is agood explanation for the phenomenon that the coesite only exist in South Dabieeclogite but rarely be seen in North Dabie eclogite.The temperature of North Dabie eclogite is680℃by calculating in this paper,belonging to mesophilic. It must be to pay attention to the impact of the applicablecomponents ranges in using of garnet-clinopyroxene geothermometer. The value ofXCaand Mg#in garnet obviously control the result of temperature calculating. Theincreasing of Mg（#Mg/Mg+Fe）in garnet correspond to the rising of temperature. Thevalue of KDdirectly affects the result calculated by formula of temperature andpressure, the value of KDis higher, the temperature calculated is lower.The value of Mg#in North Dabie eclogite is much higher than in South Dabieeclogite. The content of Si in pargasite in North Dabie eclogite is lower than6.0, buthigher than6.0in South Dabie eclogite. These show the feature that the temperatureof North Dabie eclogite is higher than South Dabie eclogite. According to the data ofthis paper and previous studies, the paper summarizes the good positive relationshipbetween the pressure of eclogite and the content of MgO in garnet in eclogite. Thecontent of MgO in North Dabie eclogite is rather higher than South Dabie eclogite.Thus this paper infers that the pressure of North Dabie eclogite is higher than SouthDabie eclogite.