Discovery of granulite-facies metamorphic rocks in the Ji'an area, northeastern Jiao-Liao-Ji Belt, North China Craton: Metamorphic P-T evolution and geological implications

The Ji'an area in the Jiao-Liao-Ji Belt of the North China Craton is characterized by granulite-facies metamorphic rocks, including orthopyroxene-cordierite-garnet pelitic granulites and clinopyroxene-orthopyroxene mafic granulites. The pelitic granulites contain orthopyroxene, cordierite, garnet, biotite, and feldspar, whereas the mafic granulites contain coarse-grained orthopyroxene, clinopyroxene, amphibole, biotite, plagioclase, and quartz. Petrographic observations, traditional thermobarometry, and quantitative phase equilibria modeling of the pelitic granulites reveal a clockwise P-T path involving near-isothermal decompression and decompressional cooling that followed the peak granulite-facies metamorphism. The peak metamorphic assemblage is interpreted to be garnet + biotite + plagioclase + quartz + ilmenite + magnetite with P-T conditions of 860-870 degrees C and similar to 10 kbar. Peak metamorphism was followed by a post-peak near-isothermal decompression with the development of biotite + cordierite + garnet + orthopyroxene + plagioclase + quartz + magnetite at 850-890 degrees C and-7.5 kbar, resulting in the formation of matrix orthopyroxene and cordierite associated with biotite, due to decompressional dehydration melting reactions involving biotite. Thermobarometric data for the mafic granulites show high temperatures of 821-872 degrees C that are similar to the peak temperatures of the orthopyrox-ene-cordierite-garnet pelitic granulites. The results indicate that the granulite facies metamorphism of the rocks in the Ji'an area of the Jiao-Liao-Ji Belt was a response to continent-continent subduction and collision of the Longgang and Nangrim blocks of the North China Craton, followed by exhumation.