Neoarchean growth and Paleoproterozoic metamorphism of an Archean ophiolite mélange in the North China Craton

Abstract

The North China Craton (NCC) preserves important records of Neoarchean crustal growth and Paleoproterozoic metamorphism. Here we investigate a suite of metamorphic rocks belonging to the Hongqiyingzi Group from the northern part of the Trans-North China Orogen (TNCO). These rocks are dominantly composed of amphibole plagioclase gneisses with minor retrograded eclogites and serpentinited peridotites. We employ zircon U-Pb geochronology and Hf-O isotopes on the amphibole plagioclase gneisses to unravel the formation age and metamorphic history. The magmatic zircon cores yield upper intercept ages of 2664 Ma and 2518 Ma, suggesting that the protoliths of the Hongqiyingzi gneisses formed in the Neoarchean. The metamorphic rims with lower U and Th concentration yield upper intercept ages of 1840 Ma and 1847 Ma, constraining the timing of metamorphism as ca. 1.85 Ga. The εHf(t) values of the magmatic zircon grains are dominantly positive, whereas those of the metamorphic grains are negative. Both magmatic and metamorphic zircon grains display a restricted range of δ18O (6.11–7.44‰), slightly higher than the normal mantle value. These isotopic features indicate that the younger metamorphic event did not modify the Hf-O isotopic compositions. The P-T computations yield temperatures of 700–760 °C and pressures to be 5–7 Kbar, suggesting the rocks have been retrograded to amphibolites facies. The retrograded eclogites and serpentinized peridotites of the Hongqiyingzi Group, exposed as tectonic lenses within the amphibole plagioclase gneisses, are interpreted as parts of an ancient oceanic lithosphere, whereas the amphibole plagioclase gneisses represent meta-volcano-sedimentary units. We thus interpret that the Hongqiyingzi Group represents a tectonically dismembered Neoarchean ophiolitic mélange that formed on the continental margin and metamorphosed to eclogite facies during deep subduction in Paleoproterozoic and exhumed during Phanerozoic.