P-T-t evolution of Neoarchaean to Paleoproterozoic pelitic granulites from the Jidong terrane, eastern North China Craton

Four metamorphic mineral assemblages, formed at different metamorphic stages, are identified in pelitic granulites exposed in the Jidong terrane, east Hebei Province, North China Craton (NCC). Prograde mineral assemblages (M1) are represented by biotite, plagioclase, quartz, and ilmenite, preserved as mineral inclusions within garnet porphyroblasts. Peak mineral assemblages (M2), which are represented by the garnet cores, rutile, pseudomorphs of sillimanite after kyanite, and some matrix minerals, formed at P-T conditions of ∼12-13 kbar/820-850 °C. Post-peak mineral assemblages (M3), which consist of the inner rims of garnets and some matrix minerals, formed at P-T conditions of ∼11 kbar/850 °C. The final retrograde mineral assemblages (M4) are mainly represented by matrix biotite without rutile exsolution, and minor matrix plagioclase with relatively high anorthite content. High-resolution secondary ion mass spectrometry (SIMS) U−Pb dating of metamorphic zircons demonstrates that the pelitic granulites possibly reached the peak metamorphic stage at ∼2.5 Ga. Younger metamorphic ages (2.41-2.45 Ga) possibly represent the cooling ages of the granulites. Accordingly, the pelitic granulites exposed in the Jidong terrane record clockwise P-T-t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, likely caused by a subduction-collision event during the late Neoarchean to early Palaeoproterozoic in the eastern NCC.