Ultrahigh temperature metamorphism recorded in the Lüliang Complex, Trans-North China Orogen: P–T–t evolution and heating mechanism

• Published in: Precambrian research Vol. 383; p. 106900
• Main Authors: Lu, Chengsen, Qian, Jiahui, Yin, Changqing, Gao, Peng, Guo, Minjie, Zhang, Wanfeng
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2022
• Subjects: Heat source; Pressure–temperature–time path; Trans-North China Orogen; Ultrahigh temperature metamorphism; Pressure–temperature–time path; Heat source; Ultrahigh temperature metamorphism; Trans-North China Orogen
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2022.106900

•Mafic and pelitic granulites from the Lüliang Complex have achieved UHT conditions.•Mafic and pelitic granulites have experienced cooling from 1.92 to 1.80 Ga.•The Trans-North China Orogen experienced a long-lived tectono-thermal evolution.•Radioactive heating is an important heat source for UHT metamorphism. Identifying ultrahigh temperature (UHT) metamorphism from hot orogens and establishing its pressure–temperature–time (P–T–t) path will provide notable insights into tectonic evolution and heat source (mantle/mafic magma vs crustal radioactive elements) responsible for the extreme conditions. We report here a combined investigation of petrography, mineral chemistry, phase equilibria modelling, geothermobarometer calculation, geochronology and heat source for mafic and pelitic granulites from the Lüliang Complex, Trans-North China Orogen (TNCO), a representative Paleoproterozoic hot collisional orogen. The rocks record clockwise P–T paths, including granulite-facies peak and post-peak retrograde stages. Garnet + pyroxene + high-Ti hornblende-bearing assemblage in the mafic granulite and garnet + feldspar + sillimanite-bearing assemblage in the pelitic granulite constrain the thermal peaks at 8.2–10 kbar and 880–959 °C, suggesting UHT conditions. Zircon and monazite U–Pb dating, hornblende 40Ar/39Ar dating and Ti-in-zircon thermometer calculation indicate that the rocks may have experienced sluggish retrograde metamorphism from 1.92 to 1.80 Ga, with a cooling rate of 2.0–2.5 °C/Ma. The newly discovered UHT metamorphism, combined with previous geological data, validates a long-lived tectono-thermal evolution from 1.97 to 1.80 Ga in the TNCO. The calculated heat flow produced by the decay of radioactive elements in the thickened crust is 37–77 mW/m2 (with an average of 56 mW/m2), which is high enough to achieve UHT conditions. Therefore, we suggest that radioactive heating can be an important heating mechanism for UHT metamorphism in a long-lived hot orogen like the TNCO.