Ultrahigh-temperature metamorphism of the felsic granulites in the eastern North China Craton and their implications on the Neoarchean tectonic regime

Abstract

The studies of the meta-pelitic and mafic rocks of the Eastern Hebei in the North China Craton have proved the existence of Neoarchean ultrahigh temperature (UHT) metamorphism. However, whether the metamorphosed felsic rocks of the area experienced the similar UHT metamorphism remains poorly constrained. Based on our new geological mapping and petrological observations, we identified felsic granulites in the Taipingzhai area of the eastern North China Craton. We combined thermodynamic modelling results with the temperature calculations of the ternary feldspar thermometer to retrieve the metamorphic history. Sample JD29–1 achieved the peak UHT condition at 9.3 kbar/1025 °C that was followed by a retrograde metamorphism at P < 8.0 kbar and T < 830 °C, whereas Sample JD11–2 recorded the peak P–T conditions of 8.0–9.5 kbar and 825–865 °C followed by a retrogression stage below 7.5 kbar/765 °C. Orthopyroxene crystals from both samples are subhedral to anhedral in crystal shape, and contain commonly high-TiO2 biotite inclusions and enriched-HREE and depleted LREE and MREE patterns, indicating a typical metamorphic origin. UPb zircon dating results reveal the protoliths of the felsic granulites were emplaced at 2537–2503 Ma and metamorphosed at 2503–2464 Ma. This study indicates that the UHT metamorphism was also recorded in the felsic granulites from the Eastern Block of the North China Craton. A Neoarchean mantle plume triggered sagduction model represents the most favorable tectonic regime that can well explain the existing metamorphic data from all the felsic, mafic and pelitic rock assemblages.