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

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 Luliang metamorphic complex, in the central segment of the TNCO. The Luliang metapelitic rocks preserve peak assemblages of garnet, biotite, sillimanite, plagioclase and quartz, with or without K-feldspar, that reflect c. 750\u202f°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\u202f≈\u202f650\u202f°C and P