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

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.