Ultrahigh temperature (UHT) mafic granulites in the East Hebei, North China Craton: Constraints from a comparison between temperatures derived from REE-based thermometers and major element-based thermometers

Abstract

The extrapolation of the newly developed REE-based thermometers of Liang et al. (2013) and Sun and Liang (2015) to the Neoarchean mafic granulites in the East Hebei of North China Craton shows that the REE-based thermometers yield temperatures that are 150–350°C for clinopyroxene–orthopyroxene pair and 100–150°C for garnet–clinopyroxene pair higher than those from the major element-based Fe-Mg thermometers, implying that the REE-based thermometers are more reliable to recover the peak metamorphic temperature of granulites. Detailed petrographic investigations suggest that three types of mafic granulites from east to west in the East Hebei can be recognized, as two-pyroxene granulites in the Taipingzhai zone, garnet two-pyroxene granulites in the Saheqiao zone and garnet clinopyroxene granulites in the Malanyu zone. P-T estimation using major element- and REE-based thermobarometers yield UHT conditions of 1025–1060°C/0.9GPa and 950–1070°C/1.0GPa for the clinopyroxene-orthopyroxene pair in granulites from the Taipingzhai and Saheqiao zones, and high-pressure granulite conditions of 800–875°C/1.0–1.4GPa for the garnet-clinopyroxene pair in granulites from the Malanyu zone. All the granulites show the P-T paths characterized by cooling from their peak conditions to late amphibolite-facies of 720–780°C. The peak P-T conditions of the two-pyroxene and garnet-bearing granulites indicate a paired type of metamorphism with contrasting geothermal gradients of ~40°C/km and ~20°C/km, being particular in the Neoarchean high-grade terranes.