Metamorphic records in the Lüliang metapelites of the Jiehekou Group: Implications for the tectonic evolution of the Trans-North China Orogen, North China Craton

Abstract

To further understand the metamorphic evolution of the Trans-North China Orogen (TNCO), a combined study of petrography, pseudosection modeling and conventional thermobarometry was completed on garnet-sillimanite-bearing metapelitic rocks of the Jiehekou Group from the southern Lüliang metamorphic complex, in the central segment of the TNCO. The Lüliang metapelitic rocks preserve peak assemblages of garnet, biotite, sillimanite, plagioclase and quartz, with or without K-feldspar, that reflect c. 750 °C and 7.0 kbar. Garnet has mostly homogeneous grain cores with sparse mineral inclusions, enclosed successively by: (1) rims of higher spessartine and lower pyrope content and (2) symplectitic intergrowths of cordierite, biotite and plagioclase that present a spectacular “white-eye socket’ reaction microstructure. Quantitative phase equilibria modeling and traditional thermobarometric calculation results are consistent with this retrograde microstructure having developed during decompression to T ≈ 650 °C and P < 4.5 kbar. Microstructure and composition characteristics of biotite, plagioclase and quartz inclusions in garnet cores likely reflect the prograde segment of a clockwise P–T path for the Lüliang metapelitic rocks. The inferred metamorphic evolution is similar to that of rocks in much of the TNCO, and regional metamorphism in the complex is likely related to the collision of the Eastern and Western Blocks of the North China Craton (NCC) along the TNCO. Previous isotopic geochronological study on metapelitic and mafic samples in the study area indicate that crustal thickening in the Lüliang metamorphic complex probably occurred from c. 19.6 Ga, followed by uplift after c. 1.89 Ga.