Metamorphic evolution of Daqingshan supracrustal rocks and garnet granite from the North China Craton: Constraints from phase equilibria modelling, geochemistry, and SHRIMP U–Pb geochronology

Abstract

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.