Metamorphic evolution of two types of garnet amphibolite from the Qingyuan terrane, North China Craton: Insights from phase equilibria modelling and zircon dating

Abstract

The Neoarchean and Paleoproterozoic tectonic evolution of the North China Craton (NCC) has been controversially argued in recent decades, and these arguments can be primely resolved with the aid of retrieving metamorphic P–T paths. Two types of garnet amphibolite were selected from the Qingyuan terrane, North Liaoning, for documenting their metamorphic evolution on the basis of petrographic observation, phase equilibria modelling and zircon dating. Sample 19LJ02 is a clinopyroxene-bearing garnet amphibolite from a Neoarchean supracrustal sequence. It records an anticlockwise P–T path including pre-peak heating with increasing pressure, peak and post-peak cooling during decompression stages. The peak P–T condition is constrained to be ~ 10 kbar/870–890 °C using the high Ti and anorthite (XAn) contents in the cores of amphibole and plagioclase, respectively. Zircon dating yields a late Neoarchean metamorphic age of ~ 2.50 Ga. This late Neoarchean metamorphism is inferred to represent an Archean unique vertical tectonic regime or sagduction, consistent with the development of dome–and–keel structures and the coincidence between the metamorphism of supracrustal rocks and the host TTG activity. Sample 17 MJ10 is a garnet amphibolite that may derive from a metabasic dyke with amphibole. It documents a clockwise P–T path including pre-peak prograde, peak, post-peak isothermal decompression and subsequent cooling stages. The peak P–T condition is constrained to be 9–10 kbar/720–740 °C using the high Ti content in amphibole and minimum XAn from the core of plagioclase. Zircon dating gives a metamorphic age of ~ 1.85 Ga. This late Paleoproterozoic metamorphism is considered to correlate with crustal-thickening orogeny developed along the northern margin of the NCC.