Pressure-temperature-time 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 would place important constraints on the regional tectonic evolution. There exists a Barrovian metamorphic belt in the Lüliang Group, Trans-North China orogen of the North China craton. Representative rock samples from chlorite zone, biotite zone, garnet zone, and staurolite-kyanite zone have been collected to delineate the pressure-temperature-time (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 and 560 °C. Decompression-dominated P-T paths involving peak pressure (6.8−9.2 kbar, 515−565 °C) and peak temperature (4.6−6.5 kbar, 560−615 °C) 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 Trans-North China orogen (ca. 1.95 Ga) but is coeval with the subduction-collision orogeny (1.90−1.82 Ga) along the northern margin of the North China craton. The distribution of the Barrovian metamorphic belt is also parallel to the latter orogen (E-W trending). Consequently, combining these results with field observations and regional geological evolution, it is inferred that the genesis of Barrovian metamorphism in the Lüliang Group may be related to the stress propagated from the 1.90−1.82 Ga orogeny. The heat sources and formation mechanisms of Barrovian metamorphism globally may vary case by case.