Ultrahigh-temperature mafic granulite in the Huai'an Complex, North China Craton: Evidence from phase equilibria modelling and amphibole thermometers

The Huai'an Complex in the north-western Trans-North China Orogen, North China Craton is recognized to have undergone high-pressure metamorphism with controversial ages. Two mafic granulite samples (17HT13 and 16HT22) from Huangtuyao of the Huai'an Complex are documented for metamorphism based on petrography, mineral chemistry, phase equilibria modelling and zircon dating. The rocks comprise garnet, clinopyroxene, orthopyroxene, amphibole, plagioclase and ilmenite. They show clockwise P-T paths involving the pre-T-max decompressional heating, the T-max and the post-T-max cooling stages. The pre-T-max decompressional heating is revealed by the core-rim/mantle anorthite-ascending zoning in coarse-grained plagioclase. The T-max stage is constrained to 8.5-10.5 kbar/980-1010 degrees C (17HT13) and 7.5-9.5 kbar/1000 -1030 degrees C (16HT22) by the high Ti content of amphibole, and the anorthite content of coarse-grained plagioclase rims/mantles and fine-grained plagioclase cores. These temperatures are slightly higher than the results of amphibole thermometers (similar to 940-980 degrees C). The post-T-max cooling is defined by the later growth of amphibole and some conventional geothermobarometers. Because it is dominated by temperature with ilmenite present, the Ti content of amphibole is proposed as a reliable thermometer, and the Ti-rich amphibole with Ti > 0.28 p.f.u. or TiO2 > 2.4-2.6 wt% is indicative of ultrahigh-temperature (>900 degrees C, UHT) conditions. The anorthite content of plagioclase is also significant in confining peak temperatures for UHT granulite but requires defining proper water content. Zircon dating for 17HT13 yields an upper intercept age of 1910 +/- 35 Ma, representing the cooling of UHT granulite. We conclude that the Huai'an Complex may have at least locally undergone UHT metamorphism at similar to 1.92-1.91 Ga after high-pressure granulite metamorphism at similar to 1.95 Ga, similar to the Jining Complex. Therefore, the Huai'an and Jining Complexes share the same tectonic evolution involving similar to 1.95 Ga crustal thickening, similar to 1.92-1.91 Ga extension with regional or local UHT metamorphism, and subsequent cooling and uplifting. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.