Metamorphic P-T evolution and tectonic implications of pelitic granulites in the Ji'an area, northeastern Jiao-Liao-Ji Belt, North China Craton

摘要

The Ji'an area in the Jiao-Liao-Ji Belt of the North China Craton contains garnet-cordierite-sillimanite pelitic granulites that preserve representative metamorphic reaction textures. Petrographic observations and quantitative phase equilibria modeling indicate clockwise P-T paths with near-isothermal decompression and decompressional cooling following the peak metamorphism. The peak metamorphic assemblage was garnet + biotite + K-feldspar + plagioclase + sillimanite + quartz + ilmenite + magnetite with P-T conditions of 9-11 kbar and 830-880 degrees C. The ensuing post-peak near-isothermal decompressional assemblage was garnet + biotite + cordierite + K-feldspar + plagioclase +/- quartz +/- sillimanite +/- magnetite +/- ilmenite with P-T conditions of 4.5-7.5 kbar and 810-900 degrees C, and this assemblage was characterized by large amounts of matrix cordierite due to the biotite decompressional dehydration melting reaction of Bt + Sil + Qz +/- PI -> Crd + Kfs +/- Ilm + Melt. LA-ICP-MS U-Pb analyses of detrital and metamorphic zircons in the sillimanite-gamet-cordierite pelitic granulites yielded a protolith age of similar to 2000 Ma and a late Paleoproterozoic metamorphic age of 1890-1870 Ma. Moreover, LA-ICP-MS U-Pb dating of monazites revealed a similar metamorphic age of 1880-1840 Ma. It had been suggested in the earlier literature that the metapelitic rocks from the Ji'an area reached the amphibolite-facies with a counterclockwise P-T path. However, our results indicate a clockwise P-T path and a granulite-facies metamorphism. We find that granulite-facies metamorphism was widespread throughout the Jiao-Liao-Ji Belt, as at Ji'an, Southern Liaohe, Jiaobei, and Bengbu, and the clockwise P-T paths and similar metamorphic ages obtained for all those areas indicate that the granulite-facies metamorphism within the Jiao-Liao-Ji Belt was a response to Paleoproterozoic continent-continent subduction and the collision of the Longgang and Nangrim blocks of the North China Craton during the peak metamorphic stage, followed by post-collisional exhumation in the post-peak decompressional stage.