P-T-t path and tectonic significance of pelitic migmatites from the Lüliang Complex in Xiyupi area of Trans-North China Orogen, North China Craton

In order to reveal the metamorphic evolution of the Lüliang Complex, a combined study of petrography, phase equilibria modeling and zircon U-Pb dating is carried out on two mesosome of pelitic migmatites from the Jiehekou Group in the Lüliang Complex. Sample (15XYP-02) preserved the thermal peak assemblage of garnet, biotite, K-feldspar, sillimanite, quartz and ilmenite, without plagioclase. It recorded two near-isothermal decompressional indicators that rutile was moated by ilmenite and garnet was replaced by cordierite. Small plagioclase occurred in the rim of K-feldspar and garnet. Four stages (M1-M4) of metamorphic evolution have been identified based on the mineral assemblages and P-T pseudosection. It is interpreted to record pre-Tmax decompression from rutile-stability field (M1, represented by rutile relicts in matrix and biotite + sillimanite + quartz inclusions in garnet porphyroblasts) at ∼8.9 kbar to the Tmax stage (M2, characterized by garnet rim with ilmenite and biotite + K-feldspar + sillimanite + quartz in matrix) of 815-838 °C. The following suprasolidus post-Tmax cooling comprises M3 stage (represented by appearance of cordierite in the assemblage of garnet + biotite + K-feldspar + sillimanite + ilmenite + quartz) of 780-838 °C/5.0-6.3 kbar and M4 stage (defined by the final mineral assemblage of garnet + biotite + K-feldspar + plagioclase + sillimanite + cordierite + ilmenite + quartz) of ∼775 °C/∼5.0 kbar. Another sample (15XYP-21) preserved the thermal peak assemblage of garnet, biotite, plagioclase, K-feldspar, sillimanite, quartz. It also records pre-Tmax decompression from rutile-stability field at 9.0 kbar to the Tmax stage of 796-821 °C/6.4-7.1 kbar, followed by suprasolidus post-Tmax cooling stage of 778-803 °C/5.0-5.7 kbar. The clockwise P-T paths are consistent with those of orogenic belt. Furthermore, the zircon U-Pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1.95 Ga and 1.91 Ga, respectively, in which, the former represents the early stage of cooling after Tmax, nearly close to the time of pressure peak metamorphism. Combined with those granulites in the Trans-North China Orogen, a common long-lived continent-continent collision processes was proposed, which may culminated at ∼1.95 Ga and followed by a prolonged cooling process.