Two phases of late Paleoproterozoic metamorphism in the western margin of the North China Craton: Records from pelitic gneisses in the Bayanwulashan complex of the eastern Alxa Block

The recognition and characterization of polymetamorphism are critical for understanding the tectonic evolution and geodynamic processes of metamorphic basement rocks and orogenic belts. In this study, pelitic gneisses, leucosomes, and felsic gneisses from the Bayanwulashan complex in the eastern Alxa Block, North China Craton (NCC), were investigated on the basis of petrography, mineral chemistry, phase equilibrium modelling, geothermometry, and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb analysis. Detailed petrographic observations of the crn-bearing grt–sil–bt–pl gneiss sample ALS2270 and crn-bearing sil–grt–bt–pl gneiss sample ALS2274 (pelitic gneisses) revealed two distinct phases of metamorphism. The first-phase metamorphism (M1) can be subdivided into four stages (M1-1–M1-4); the second-phase metamorphism (M2) is marked by the growth of andalusite and non-directional micro-grained biotite (Bt-3) around staurolite, corundum, and sillimanite, as well as within fractures of garnet. Phase equilibrium modelling yielded P–T conditions of 740–830 °C/9.4–10.8 kbar, 730–860 °C/5.9–9.8 kbar, and 660–725 °C/5.3–7.8 kbar for the M1-2, M1-3 and M1-4 of the sample ALS2270, respectively. Similar metamorphic P–T conditions were also obtained for the sample ALS2274. Combing the temperatures by Ti–biotite thermometry for Bt-3 with the pressure range of the andalusite-bearing phase fields in the P–T pseudosection, metamorphic conditions of 492–556 °C/0–4.3 kbar, and 503–557 °C/0–4.3 kbar were obtained for the M2 of sample ALS2270 and sample ALS2274. As a whole, the M1 is characterized by high-pressure granulite-facies metamorphism with a clockwise P–T path involving near isothermal decompression followed by decompressional cooling, whereas the M2 is marked by low-pressure metamorphism with a high geothermal gradient. LA–ICP–MS zircon U–Pb dating of the pelitic gneisses, leucosomes, and felsic gneisses yielded two age groups: ca. 1.98–1.81 Ga and ca. 1.77–1.75 Ga, corresponding to the timing of two separate phases of metamorphism. The M1 is considered as a product of collisional orogeny involving crust thickening and subsequent exhumation, whereas the M2 is likely caused by rift-related activity along the northern margin of the NCC.