THE KYANITE-TYPE METAMORPHISM AND METAMORPHIC AGES IN WUTAI-HENGSHAN MTS., NORTH CHINA CRATON: INSIGHTS INTO THE REGIONAL PALEOPROTEROZOIC P-T-T HISTORY

Disputations exist on the age and tectonic processes involved in the amalgamation of the North China Craton (NCC) through the Trans-North China Orogen (TNCO), although a general consensus proposing the final collision at ~1.85 Ga. Garnet-mica schist and garnet-orthoamphibole rock (GOR) samples were collected from the Wutai Group in the TNCO for determination of the metamorphic P–T paths and ages for better understanding the tectonic evolution of the NCC. The garnet-mica schist contains garnet porphyroblasts together with biotite, muscovite, quartz and ilmenite with or without plagioclase, epidote, ortho-amphibole, staurolite and kyanite. The modeling results using THERMOCALC and the petrographic observations suggest that these rocks may have experienced four metamorphic stages: the first-stage is dominated by heating; the second-stage exhibits a steep P–T vector with increasing P–T conditions to the pressure peak at >9.0 kbar/615–645 °C; the third stage is the decompression with heating to the temperature peak at ~7.5 kbar/660 °C; the fourth stage is the further decompression after the temperature peak. The GOR shows a prominent porphyroblastic texture involving garnet, orthoamphibole, kyanite, sillimanite, chlorite, rutile and quartz, with or without talc, hornblende and staurolite. Generalized prograde paths are inferred from subtle growth zoning and inclusion assemblages in garnet. The peak mineral assemblages are predicted to be characteristic of presence of garnet, orthoamphibole (gedrite), chlorite, kyanite and rutile under P–T conditions of 10–12 kbar and 650–670 °C. The post-peak history is characterized by isothermal decompression, developing coronae and symplectite assemblages. Metamorphic zircons dated in both rocks yield older metamorphic ages of ~1.95 Ga, which are interpreted to represent the pre- or peak stages of the kyanite-type metamorphism for the dated zircon grains contain inclusions similar to the minerals in the host rocks and the REE patterns in zircon show metamorphic characteristics. An alternate tectonic evolution could be constructed as: a long-lived orogenic process may exist in the TNCO where the main collision stage happened at ~1.95 Ga, followed by an isothermal decompression and a prolonged nearly isobaric cooling process from 1.90–1.78 Ga.