Metamorphic evolution of Archean ultrahigh-temperature mafic granulites from the western margin of Qian'an gneiss dome, eastern Hebei Province, North China Craton: Insights into the Archean tectonic regime

The supracrustal rocks of granulite facies occurs as enclaves in TTG gneisses in eastern Hebei terrane, North China Craton. Their peak P-T conditions, paths and tectonic regime remain controversial. Three samples of mafic two-pyroxene granulites from the western margin of Qian'an dome were chosen for detailed studies of petrography and P-T evolution from REE- and major element-based thermobarometers and pseudosection modelling. Their peak temperatures were constrained to be 1000-1100 degrees C using REE-in-clinopyroxene-orthopyroxene and REE-in-plagioclase-clinopyroxene thermometers. The major element-based clinopyroxene-orthopyroxene and amphibole-plagioclase thermobarometers yield cooling conditions of 740-830 degrees C/7-8 kbar. Pseudosection modelling suggests that the mafic granulites experienced a cooling evolution with decompression from UHT peak conditions at > 1000 degrees C and 10-11 kbar in garnet-stability fields to normal granulite conditions at 820-900 degrees C and 7-8 kbar constrained from the stabilities of final solidus assemblages, which are well consistent with the results of REE- and major element-based thermobarometers. Both the P-T conditions and the cooling paths in P-T pseudosections are constrained by (i) the measured plagioclase compositions, especially the spectacular zoning patterns with increasing X-An [= Ca/(Ca + Na + K)] (0.46 -> 0.52) from core to mantle followed by decreasing X-An (0.52 -> 0.46) outwards in the rims; (ii) the measured zoning with core-rim increasing CaM2 [Ca in the M2 site] in clinopyroxene; and (iii) measured Ti in amphibole or biotite that yields temperature constraints for the final assemblages. The P-T evolution for the pre-peak stages is inferred to be a compression process based on the presence of a rounded magnetite-ilmenite grain included in clinopyroxene. Thus, an anticlockwise P-T path is inferred. Zircon U-Pb age dating yields a meta- morphic age of similar to 2.55 Ga, defining the protoliths of the mafic granulites were formed between 2.57 and 2.55 Ga. A summary of available age data reveals that the TTG magma activity, the formation of supracrustal rocks and their metamorphism took place simultaneously in a time span between 2.48 and 2.56 Ga. Therefore, a sagduction regime is favored to interpret the metamorphic evolution of supracrustal granulites in the east Hebei terrane.