Metamorphic P–T paths and New Zircon U–Pb age data for garnet–mica schist from the Wutai Group, North China Craton

Abstract

The amalgamation of the North China Craton (NCC) is popularly viewed as a result of assembling between the Western and Eastern Blocks at ∼1.85Ga, forming the Trans-North China Orogen. Disputations still exist on the age and detailed tectonic processes involved in the assembling. Garnet–mica schist samples were collected from the lower part of Wutai Group in the central Trans-North China Orogen for determination of the metamorphic P–T path and age and for better understanding the tectonic evolution of the NCC. The garnet–mica schist generally contains mega-garnet porphyroblasts together with biotite, muscovite, quartz and ilmenite with or without plagioclase, epidote, ortho-amphibole, staurolite and kyanite. Garnet is mostly weakly zoned with core–rim increasing pyrope and decreasing grossular, but occasionally exhibits pronounced zoning involving different patterns in the core and mantle, suggesting two-stage growth. Pseudosections were calculated using THERMOCALC for three representative samples. The modeling results, together with the petrographic observations suggest that the garnet–mica schist may have experienced four metamorphic stages. The first-stage (S-I), which is revealed only from the infrequent garnet core zoning, is characterized by a flat P–T vector dominated by heating, closing to the low-pressure facies series. The second-stage (S-II) exhibits a steep P–T vector with increasing pressure and temperature to the pressure peak at >9.0kbar/615–645°C, being the medium-pressure facies series (kyanite type). The third stage (S-III) is the decompression with heating to the temperature peak at ∼7.5kbar/660°C. The fourth stage (S-IV) is the further decompression after the temperature peak, accompanied with considerable cooling. Metamorphic overgrowth rims of zircon and fully metamorphic crystals from two samples dated using SHRIMP and LA-ICP-MS methods yield new metamorphic ages of ∼1.95Ga, which were interpreted to represent the pre- or peak stages (i.e. S-II and S-III) 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. This suggests that there would be a collision or crust thickening event coupled with the kyanite type metamorphism occurring at ∼1.95Ga, which is about 100Ma older than the popularly viewed collision age (∼1.85Ga). A probable interpretation is that there may be a long-lived orogenic process where the older metamorphic ages of ∼1.95Ga represent the main collision stage, and the younger ages of ∼1.85Ga indicate uplifting and cooling of high-grade terranes. The first stage (S-I) metamorphism of low-pressure facies series could be an indicator of crust extension before the collision event.