Metamorphic Evolution And Genesis Of The Ji’an Group In The Tonghua Region, Jilin Province

The north Jiao-liao-Ji paleo-proterozoic mobile belt consists partly of the Ji’an group’s metamorphic complex rocks in Tonghua region of the Jilin province. Studies on metamorphic evolution and genesis of the Ji’an group could give key constraints on recongnizing geodynamic process of the the Jiao-liao-Ji belt of the North China Craton.Based on the detailed field work, the metamorphic complex rocks of the Ji’an group in the Tonghua region were investigated carefully on petrography, geochemistry, metamorphism and zircon geochronology. For the first time, basic granulite with typical mineral assemblage of orthopyroxene+clinopyroxene+amphibole+plagioclase, pelitic granulite with typical mineral assemblage of garnet+cordierite+sillimanite+perthite, and even a little ultrahigh temperature metamorphic rocks with mineral assemblage of hypersthene+cordierite+perthite+ spinel+quartz were discovered in the Ji’an group, indicating granulite facies metamorphism. According to mineral assemblage, three stage and six episodes of metamorphic recrystallization process were recognized in the Al-rich gneiss. Using the traditional mineral geothermo-barometry, the peak metamorphic PT conditions were calculated as 0.6~0.7 GPa at ca. 800 ~850 ?C. the P-T path is characterized as clock-wise type, with post-peak isothermal decompression and followed by cooling, which is consistent with a thinning process of thickened continental crust.According to zircon U-Pb dating results, protolith ages of the Ji’an group’s rocks concentrate in 2.1 Ga and a few older than 2.5G. Metamorphic zircons from these metamorphic rocks record two periods of metamorphic ages of 1.92~1.9 Ga and 1.90~1.85 Ga, which maybe identical to peak metamorphism and later cooling stage, respectively. Zircon U-Pb dating and Hf isotope analysis showed gneissic granite emplacement age of 2114 ± 17 Ma, and subjected to the transformation of post 1.92 ~ 1.85 Ga tectonic thermal event, εHf(t) values between-2 to +2, model ages 2.7 ~ 2.8Ga, shows it is mainly formed of Archean TTG remelting. Late porphyritic granite emplacement age of 1888 ± 8 Ma and 1878 ± 7 Ma. Porphyritic granite and Ji’an Group metamorphic rocks has a negative εHf(t) value and the model ages(2.5 ~ 2.6Ga) similar, with homologous features, show porphyritic granite mainly from remelting of supracrustal rocks.In a word, Ji’an group’s metamorphic rocks in Tonghua region probably formed in extensional back-arc basin, in the late Paleoproterozoic(1.92 ~ 1.85Ga),due to the collision of continental blocks, resulting in a closed basin and subjected to granulite facies metamorphism.