P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: Insights from phase equilibria and geochronology

Abstract

The Hengshan complex has drawn attention for the occurrence of typical high-pressure mafic granulites, representing the Paleoproterozoic collisional orogeny in the North China Craton. Mafic granulites occur as boudins of various scales in tonalite–trondhjemite–granodiorite (TTG) gneisses. They are composed of garnet, clinopyroxene, plagioclase, orthopyroxene, hornblende, biotite, quartz, rutile and ilmenite. Garnet porphyroblasts are usually surrounded by plagioclase coronas, and early jadeite-rich clinopyroxene is totally decomposed to the symplectite of clinopyroxene and plagioclase. Two high-pressure granulites and one two-pyroxene granulite were studied by the pseudosection approach, and clockwise P–T paths with four metamorphic stages were recognized. The pre-peak prograde stage was identified from garnet core to mantle compositions, with increasing P–T from 630 to 710 °C at 11–13 kbar. The peak pressure stage hasn’t been well preserved but its mineral assemblages inferred from petrography to be garnet + jadeite-rich clinopyroxene + quartz + rutile ± plagioclase, with P–T condition of 760–820 °C at ∼15 kbar. The post-peak decompression with slight heating is characterized by symplectite, plagioclase coronas, and the formation of orthopyroxene. Locally, two-pyroxene granulite assemblages develop, defining P–T condition of 6–8 kbar and 840–860 °C for the temperature peak (Tmax). The post-Tmax cooling is represented by the occurrence of amphibolite facies assemblages especially in boudin margins with P–T condition of 6–8 kbar and 720–760 °C, suggesting an isobaric cooling process with fluid infiltration. U–Pb dating of metamorphic zircons record ages mainly ranging from 1897 to 1830 Ma with weighted mean ages of 1862 ± 11 Ma for the high-pressure granulite and 1837 ± 7 Ma for the two-pyroxene granulite. These metamorphic ages were interpreted to record the post-peak cooling stage especially the time close to solidus, which is also supported by the result of Ti-in-zircon thermometer. A summary of P–T–t evolution for metamorphic terrains corresponding to different crust levels in Hengshan–Wutai area indicates that the main episode of crust thickening-dominated collisional orogeny is ∼1.95 Ga or earlier and finished at ∼1.92 Ga. This was followed by post-orogenic cooling-uplifting or the overprinting of another thermal-tectonic event during the period of 1.92–1.80 Ga.