Multi-phase metamorphism in the northern margin of the North China Craton: Records from metapelite in the Hongqiyingzi Complex

Abstract

It is controversial about the tempo–spatial distribution and tectonic attributes of the Paleoproterozoic orogenic belts in the North China Craton (NCC). The Hongqiyingzi Complex in the northern margin of the NCC, comprising orthogneiss, supracrustal sequence and an ophiolitic mélange, was investigated to have experienced multiple tectonic evolution from Paleoproterozoic to Mesozoic. However, the depositional age and metamorphic P–T path of the supracrustal sequence have not been well addressed. Garnet mica schists are investigated on the basis of petrography, mineral chemistry, phase equilibria modelling and zircon and monazite dating, which record an extreme case with four phases of metamorphism. The first-phase is characterized by high pressure (HP) granulite facies with the peak conditions of 10–11 kbar/>780 °C, which occurred at ~1.95 Ga. It is considered to correspond to a collisional orogeny with crust thickening widespread in the Paleoproterozoic orogenic belts of the NCC, followed by an exhumation with heating at ~1.92 Ga. The second-phase is marked by the overprinting of staurolite-bearing assemblages with the peak conditions of 6–7 kbar/610–630 °C, which occurred at ~1.85 Ga. It may correlate a separate orogeny occurred along the northern margin of the NCC, following a ~1.90 Ga oceanic subduction. Phase modelling suggests this overprinting over the first-phase HP granulites is mainly controlled by water infiltration. The third-phase andalusite-type and the fourth-phase greenschist facies metamorphism are confined to local equilibrium domains and their ages have not been well constrained. With a comparison of regional geology, the third-phase metamorphism is inferred to be related with the 320–290 Ma extension in the northern part of the NCC during Late Paleozoic, and the fourth-phase metamorphism is correlated with the 250–230 Ma collision occurred along the Solonker Suture Zone in the Central Asia Orogenic Belt during Early Mesozoic. Moreover, detrital magmatic zircons yield ages of 2.0–2.1 Ga, providing a definite upper limit on the deposition of the Hongqiyingzi supracrustal sequence.