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摘要
For a long time, UHP metamorphism in the Dabie orogenic belt has been a research hotspot, while the research on HT or UHT in this orogenic belt is relatively limited, and mainly concentrated on the characteristic rocks remaining in the North Dabie metamorphic belt eclogite and metabasic rocks. Thus the metamorphic properties and time limit of felsic and argillaceous gneiss are rarely lack of attention. It restricts people's full understanding of the formation and evolution process of the orogenic belt. In this study, we took corundum gneiss, pyroxene-bearing granodiorite gneiss and biotite plagioclase gneiss widely exposed in Gongjialing area of North Dabie as the research object, carried out detailed field investigation, petrography, mineral chemical analysis, thermodynamic evaluation and chronology research. The research shows that these gneisses have undergone three metamorphism stages of granulite facies, amphibolite facies and greenschist facies and the P-T conditions are ①T = 724 ℃～762 ℃ and P = 0.3～0.4 GPa; ② T = 586 ℃～595 ℃ and P = 0.3～0.4 GPa; ③ T = 382 ℃～395 ℃ and P = 0.1～0.3 GPa respectively, which showing a characteristic of near isobaric cooling. Among them, the P-T-① basically reached the category of HT metamorphism, which is the main metamorphic condition of gneiss records widely developed in North Dabie metamorphism belt. Zircon U-Pb dating shows that corundum gneiss have Group-玉and Group-Ⅱ ages of 709±19 Ma～677±19 Ma and 207±2 Ma, respectively. However, pyroxene-bearing granodiorite gneiss records zircon ages of Group-玉, Group-Ⅱ and Group-Ⅲ, which are 602±23 Ma～237±11 Ma, 217±4 Ma and 192±5 Ma, respectively. Combined with the CL diagram of zircons and Th/U ratio analysis, The Th/U ratio of zircons of Group-I group of the two gneisses is more than 0.1（most of them more than 0.4）, which are magmatic inherited zircons, reflecting early multiple magmatic events. The Th/U ratio of zircons in Group-Ⅱ group is less than 0.1, which show metamorphic recrystallization characteristics, and their 217～207 Ma ages correspond to the main metamorphic time limit. The Th/U ratio of zircons in Group-Ⅲ group is less than 0.1, showing the characteristics of metamorphic-accretionary zircons, and its 192 Ma corresponds to amphibolite facies metamorphism. This study shows that the main body of the North Dabie metamorphic belt has experienced shallow high-temperature metamorphism and lacks the record of high-temperature and ultra-high-pressure metamorphism. ? 2023 Science Press.
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