P-T path of staurolite-bearing metapelitic schist from the Taihua Complex, southern Trans-North China Orogen, and tectonic implications

Abstract

The Taihua Complex, located on the southern margin of the Trans-North China Orogen (TNCO), North China Craton, is a critical region for decoding tectonic evolution of the TNCO. In this study, an integrated investigation of petrology, phase equilibrium modelling and LA-ICP-MS zircon U-Pb dating is undertaken for the staurolite-bearing metapelitic schist in the Lushan Taihua Complex. Petrographic observations reveal a four-stage metamorphic evolution, including: (1) A pre-peak (M1) assemblage consisting of the garnet core and its mineral inclusions of staurolite, plagioclase, biotite, quartz, ilmenite and rutile; (2) The pressure peak (M2) assemblage represented by the inclusions of plagioclase, biotite, quartz, ilmenite and rutile within garnet rims, and the presence of kyanite pseudomorphs after sillimanite, which is confirmed by the Raman spectra; (3) The decompression stage (M3) characterized by the transformation from kyanite to sillimanite as well as partial replacement of rutile by ilmenite; and (4) the final stage (M4) marked by the fine-grained assemblage of biotite + plagioclase + quartz ± sillimanite, locally surrounding the garnet porphyroblasts. Phase equilibrium modelling results suggest that the metapelitic schist experienced the high-amphibolite facies metamorphism, with P-T conditions approaching 654 ∼ 661℃/8.7 ∼ 9.1kbar (M1) and 751 ∼ 766 ℃/10.7 ∼ 11.1 kbar (M2), followed by a decompression with minor heating to 778 ∼ 795 ℃/7.2 ∼ 7.9 kbar (M3), and finally, the rocks underwent a cooling process until 699 ∼ 702 ℃/7.1 ∼ 7.2 kbar (M4). Thus, a clockwise P-T path is retrieved. Zircon U-Pb dating yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1883 ± 9 Ma, interpreted as the time of the post-peak cooling. Therefore, it can be inferred that the Taihua Complex has been involved in the collision between the Western and Eastern Blocks of the NCC in the late Paleoproterozoic. Based on previously published data, we propose that the Taihua Complex records the pressure peak metamorphic ages of ca. 1.95 Ga and cooling ages of ca. 1.93(1.92) ∼ 1.80 Ga. The main crustal thickening process possibly occurs at ca. 1.95 Ga, followed by uplifting, and final cooling from ca. 1.93(1.92) − 1.80 Ga.