Metamorphic P–T–t evolution of the Lüliang Group, North China Craton: Insights from phase equilibria modeling and geochronology

Unraveling the tectono-thermal history of medium- to low-grade belts will place important constraints on the regional tectonic evolution. There exists a Barrovian metamorphic belt in the Lüliang Group, Trans-North China Orogen (TNCO) of the North China Craton (NCC). Representative rock samples from chlorite zone, biotite zone, garnet zone and staurolite–kyanite zone have been collected to delineate the P–T–t evolution. A two-stage prograde P–T path characterized by heating first and then pressurizing is recovered from the garnet zone by phase equilibria modeling. The peak P–T conditions are constrained to be ~7.0 kbar/560 ℃. Decompression-dominated P–T paths involving Pmax (6.8–9.2 kbar/515–565 ℃) and Tmax (4.6–6.5 kbar/560–615 ℃) stages are obtained from the staurolite–kyanite zone. Metamorphic zircon from the staurolite–kyanite zone and garnet from the garnet zone yield U-Pb ages of 1850 ± 31 Ma and 1882 ± 67 Ma, respectively. Biotite from the biotite zone gives an ⁴⁰Ar/³⁹Ar age of 1762 ± 3 Ma. The geochronological results indicate that metamorphism of the Lüliang Group is younger than the formation age of the TNCO (~1.95 Ga), but is coeval to the subduction–collision orogeny (1.90–1.82 Ga) along the northern margin of the NCC. The distribution of the Barrovian metamorphic belt is also parallel to the latter orogen (E–W-trending). Consequently, combining with field observations and regional geological evolution, it is inferred that the genesis of Barrovian metamorphism accompanying with crustal thickening in the Lüliang Group may be related to the stress propagated from the 1.90–1.82 Ga orogeny along the craton’s northern margin.