UHT Metamorphism Peaking Above 1100 °C with Slow Cooling: Insights from Pelitic Granulites in the Jining Complex, North China Craton

Abstract

The peak temperature and duration of ultrahigh-temperature (UHT) metamorphism are critical to identify and understand its tectonic environment. The UHT metamorphism of the Jining complex in the Khondalite Belt, North China Craton is controversial on the peak temperature, time and tectonic setting. A representative sapphirine-bearing granulite sample is selected from the classic Tianpishan outcrop for addressing the metamorphic evolution and timing. The rock is markedly heterogeneous on centimetre scale and can be divided into melanocratic domains rich in sillimanite (MD-s) or rich in orthopyroxene (MD-o), and leucocratic domains (LD). On the basis of detailed petrographic analyses and phase equilibria modelling using THERMOCALC, all three types of domains record peak temperatures of 1120-1140 °C and a series of post-peak cooling stages at 0.8-0.9 GPa to the fluid-absent solidus (∼890 °C), followed by sub-solidus decompression. The peak temperature for MD-s is constrained by the coexistence of sillimanite-I + sapphirine + spinel + quartz, where sillimanite-I contains densely exsolved aciculae of hematite, yielding reintegrated Fe2O3 contents up to 2.1-2.3 wt %. The post-peak cooling evolution involves the sequential appearance of K-feldspar, sillimanite-II + garnet, orthopyroxene and biotite, where sillimanite-II is exsolution-free and contains variable Fe2O3 contents of 1.3-1.8 wt %. The peak temperature for MD-o is constrained by the sapphirine + orthopyroxene assemblage, where orthopyroxene has a maximum AlIV of 0.22 (Al2O3 = 9.5 wt %) in the core. The cooling evolution involves the sequential appearance of K-feldspar, garnet and biotite, and the decreasing AlIV (0.22→0.17) from core to rim in orthopyroxene. The peak temperature for LD is constrained by the inferred K-feldspar-absent assemblage and the maximum anorthite content of 0.11 in K-feldspar. The cooling evolution involves the crystallization of segregated melts, exsolution of supra-solvus ternary feldspars and growth of biotite. The Al in orthopyroxene, Fe2O3 in sillimanite and anorthite in K-feldspar are good indicators for constraining extreme UHT conditions although they depend differently on bulk-rock compositions. In-situ SHRIMP U-Pb dating of metamorphic zircon indicates that the UHT metamorphism may have occurred at >1.94 Ga and the cooling under UHT conditions lasted over 40 Ma. The extreme UHT metamorphism in the Jining complex is interpreted to be triggered by the advective heating of intraplate hyperthermal mafic magmas together with a plume-related hot mantle upwelling, following an orogenic crustal thickening event. Show more