Comparative Study Of Early Mesozoic And Archaean Granulites In Chifeng-jianping Area

The granulites formed in lower crust,recorded the evolution of the lower crust and even crust-mantle boundary,and they have important significance in evolution history of north China craton. This paper first make thoroughly comparative study on two kinds of granulites which belong to different times and different causes. The respectively formed in the stage of formation and destruction of north China craton. By combining with evolution of north China craton,the comparative study can provide information for the north China craton's formation mechanism.By comparative study in petrography,geochemistry and chronology,this paper found that there are significantly differences between the two kinds of granulite in the aspect of the kind of original rock,metamorphic history and metamorphic environment:1. They have different original rock. The original rock of Mesozoic granulites is cumulate norite-gabbro,and Archaean granulite is basalt and TTGs.2. The have different metamorphic history. The Mesozoic granulites'original rocks were shortly formed magmatic rocks,and the beginning temperature of metamorphic was higher. After undergoing isobarically cooling process,the original rocks became middle-high pressure granulites,then was captured by diorite magma to upper-middle crust,and occurrence low pressure granulite faces matamorphism. The whole progress lasted for 10 Ma. The Archaean granulites'metamorphic history was more integrated. Their original rocks were Archaean terrane made of basalt or TTGs. At that time there was a large scale of magma underplating. Because of compressional collision the terrane undergone a slow progress of heating and supercharging,when the pressure was big enough,they became granulites. Then with the uplifting they undergo retrograde metamorphism.3. They have different metamorphic environment. The Mesozoic granulites'metamorphic happened at a higher temperature. The two-pyroxene thermometer shows that the highest temperature during the Mesozoic granulites'evolution was 1090℃,the highest temperature during the Archaean granulites'evolution was 935℃. Because the two kinds of granulites had difference in mineral composition,the participation level of fluid is different. During the Mesozoic granulites'evolution,the participation level of fluid is low. During the Archaean granulites'evolution,the participation level of fluid is higher. It's related with the metamorphic reaction of amphibole.4. They formatted in different tectonic settings. Early Mesozoic underplating of basalt magma provided original rocks and large thermal energy for the formation of the granulites. The Archaean granulites formed in the environment of inner continental initial rift,and with a large scale of volcanic sedimentation. Shortly after,because of continental collision and crust thickening,it happened granulite faces metamorphism. The difference between the extensional tectonic setting in the Mesozoic and the compressive orogeny tectonic setting in Archaean result in the differences of the two kinds of granulites.