Late Neoarchean or late Paleoproterozoic high-pressure granulite facies metamorphism from the East Hebei terrane, North China Craton?

Abstract

It is debated whether there are Neoarchean high-pressure (HP) granulites in the East Hebei terrane, North China Craton. Three mafic granulite samples from the Saheqiao belt are investigated here for their petrography, P–T evolution and zircon dating. All the samples contain the typical HP granulite assemblages involving garnet, clinopyroxene, plagioclase and amphibole. These assemblages commonly overprint on an early-phase assemblage dominated by coarse-grained clinopyroxene. These HP granulites share clockwise P–T paths with the peak conditions constrained to be ~900 °C/12.5 kbar and ~880 °C/12.8 kbar for two samples, using the isopleths of the maximum Ti content from amphibole and the minimum anorthite from plagioclase in P–T psueudosections. The post-peak P–T paths are inferred to be dominant of decompression with cooling based on the core-rim zoning with increasing anorthite in plagioclase, plagioclase coronas around garnet and decreasing Ti from core to rim in amphibole. Zircon dating yields one metamorphic age of 2.48 Ga for sample J1850 and two metamorphic ages of 2.47 and 1.81 Ga for sample YC223. The older zircons show enriched HREE patterns, indicating no garnet involved, whereas the younger zircons have flat HREE patterns, reflecting the presence of garnet. Moreover, the older ages are consistent with the late Neoarchean granulites that have anticlockwise P–T paths and the younger ages correspond to the HP granulites from both metabasic dykes and supracrustal rocks constrained already. Thus, the studied HP granulites are considered to be formed in the late Paleoproterozoic related to a crust-thickening orogenic event.