Metamorphic evolution and SIMS U-Pb geochronology of orthopyroxene-bearing high-P semipelitic granulite in the Fuping area, middle Trans-North China Orogen

The Fuping area lies in the middle part of the Trans-North China Orogen, which is a critical region for understanding the metamorphic-tectonic evolution of this orogenic belt in the Precambrian. The newly discovered orthopyroxene-bearing semipelitic granulite in this area is a coherent stratigraphic unit, among which three generations of metamorphic mineral assemblages (peak, post-peak and cooling retrograde) have been identified based on petrographic observation and mineral chemical analysis. The peak metamorphic mineral assemblage mainly consists of porphyroblastic garnet plus matrix minerals including biotite, plagioclase, orthopyroxene, quartz as well as accessory minerals including pyrite, monazite, zircon and apatite. Geothermobarometry calculations and phase equilibria modelling confirm that the representative samples record different P-T conditions and metamorphic stages due to retrogressive metamorphism, with the highest P-T conditions reaching high-P granulite facies. The retrieved clockwise metamorphic P-T paths pass from 12.5 to 13.5 kbar/855-880celcius (peak stage) through 8.5-11 kbar/850-886celcius (post-peak stage) and to a speculative cooling phase (retrograde stage), reflecting near isothermal decompression (ITD) followed by near isobaric cooling (IBC). This P-T path is interpreted to reflect a subduction and/or collision event followed by a rapid exhumation. Secondary ion mass spectrometry (SIMS) U-Pb dating on metamorphic monazite and zircon yielded metamorphic ages ranging from 1,825 to 1,815 Ma, possibly constraining a retrograde metamorphic age. Therefore, the semipelitic granulite in the Fuping area records the subduction/collision event between the Eastern and Western Blocks of the North China Craton during the Late Palaeoproterozoic.