Archean UHT metamorphism with counterclockwise P–T path: Insights from pelitic granulites from East Hebei, North China Craton

Abstract

Archean supracrustal rocks that occur as rafts within tonalite–trondhjemite–granodiorite (TTG) gneisses can reach ultra-high temperature (UHT) conditions. Their metamorphic P–T paths and geodynamic regimes are debated. Herein, we present detailed analyses of UHT pelitic migmatites (JD1730 and JD1733) from the East Hebei, North China Craton, using petrology, phase equilibria modelling, and zircon dating, to recover the metamorphic history. Sample JD1730 is a sillimanite–orthopyroxene–cordierite migmatite that contains the sillimanite + orthopyroxene (+cordierite) symplectites, which are interpreted to be the pseudomorphs after sapphirine. JD1733 is a corundum-bearing garnet–cordierite–sillimanite migmatite that contains the corundum + spinel ± ilmenite/rutile aggregates, representing a stable mineral association of low pressures in local Si-unsaturated domains. The two rocks record consistent counterclockwise P–T paths involving pre-peak up-P processes to the UHT peak conditions followed by cooling. The pre-peak up-P processes occurred at 5–7 kbar and > 970 °C constrained from the textural relations involving (i) spinel enveloped by sillimanite and cordierite and mantled by the Sil + Opx symplectites in JD1730, and (ii) the inclusion variation in garnet of JD1733 with the Spl + Crn (±Ilm) aggregates embayed by sillimanite in the core and mantle, and worm-like quartz in the rim. The UHT peak conditions can be constrained to be 7–8 kbar, 980–1060 °C based on the stability of the inferred peak mineral assemblages in P–T pseudosections added with the plots of the re-integrated anorthite contents from perthite. The post-peak cooling processes to 5–8 kbar / 820–850 °C are marked by the growth of biotite in leucosomes of both samples. Zircon dating and geothermometry results suggest that the UHT metamorphism might have persisted for more than 20 Myr from ∼ 2.52 Ga to ∼ 2.50 Ga and the post-UHT cooling continued another 20 Myr or even 40 Myr. Considering the geological features in the terrane, a vertical sagduction regime was preferred for the Neoarchean tectonic evolution of the East Hebei terrane.