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

Abstract

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.