P–T–t constraints of the Barrovian-type metamorphic series in the Khondalite belt of the North China Craton: Evidence from phase equilibria modeling and zircon U–Pb geochronology

Abstract

The Barrovian metamorphic series represents a typical medium P/T regional metamorphism developed in pelitic lithologies, with metamorphic grades progressing from greenschist facies to granulite facies. As the metamorphic conditions are not too high in some lower-grade zones, some geological records of the prograde metamorphic stage may be well preserved. This is significant to the study of orogenic evolution. A sequence of Barrovian metamorphic series rocks from the garnet zone to the kyanite zone outcrops in the eastern part of Daqingshan terrane. An integrated study of petrography, phase equilibria modeling and zircon SIMS U–Pb geochronology is used to establish the P–T conditions and timing of this Barrovian metamorphism. Based on the petrology and mineral chemistry, minerals in each sample from different Barrovian metamorphic zones are divided into prograde, peak and retrograde metamorphic stages. Pseudosections are calculated using THERMOCALC for three samples from each zone, and the peak metamorphic conditions are confined to be 585°C and 0.59GPa for the garnet zone, 590°C and 0.60GPa for the staurolite zone, 645°C and 0.71GPa for the kyanite zone. The P–T paths of the Barrovian metamorphic rocks recorded a crustal over-thickening event during prograde metamorphic stage, and isobaric heating after reaching Pmax. The retrograde phase is characterized by cooling with decompression. Zircons from two higher metamorphic zones are dated using a CAMECA instrument. According to the results, the metamorphic ages of the different metamorphic zones are coeval and are estimated to be 1896Ma. This is quite similar to the protolith age of the nearby arc-related Halaqin volcanic rocks. We suggest that the Daqingshan terrane experienced crustal thickening at 1896Ma, with an arc magmatism superposition simultaneously.