Metamorphic P–T path of mafic granulites from Eastern Hebei : Implications for the Neoarchean tectonics of the Eastern Block, North China Craton

This study documents the metamorphic evolution of mafic granulites from the Eastern Hebei Complex in the Eastern Block of the North China Craton. Mafic granulites from Eastern Hebei occur as boudins or enclaves within Neoarchean high-grade TTG gneisses. Petrographic observations reveal three characteristic metamorphic mineral assemblages in the mafic granulites: the pre-peak hornblende + plagioclase + ilmenite + quartz + sphene assemblage (M₁) existing as mineral inclusions within coarse-grained peak assemblage (M₂) represented by garnet + clinopyroxene + orthopyroxene + plagioclase + hornblende + ilmenite + quartz, and post-peak assemblage (M₃) marked by garnet + quartz ± ilmenite symplectites surrounding the peak pyroxene and plagioclase. Based on pseudosection modeling calculated in the NCFMASHTO model system using the program THERMOCALC, P–T conditions of the pre-peak (M₁), peak (M₂) and post-peak (M₃) assemblages are constrained at 600–715 °C/6.0 kbar or below, 860–900 °C/9.6–10.3 kbar, and 790–810 °C/9.6–10.4 kbar, respectively. These P–T estimates, combined with their mineral compositions and reaction relations, define an anticlockwise P–T path incorporating isobaric cooling subsequent to the peak medium-pressure granulite-facies metamorphism for the mafic granulites from Eastern Hebei. Such an anticlockwise P–T path suggests that the end-Neoarchean metamorphism of the Eastern Hebei Complex correlated closely with underplating and intrusion of voluminous mantle-derived magmas. In conjunction with other geological considerations, a mantle-plume model is favored to interpret the Neoarchean tectonothermal evolution of the Eastern Hebei Complex and other metamorphic complexes in the Eastern Block. The prograde amphibolite-facies metamorphism (M₁) was initiated due to the upwelling of the relatively cooler mantle plume head, followed by the peak medium-pressure granulite-facies metamorphism (M₂) as triggered by the uprising hotter plume “tail”, and finally when plume activity ceased, the heated metamorphic crust experienced nearly isobaric cooling (M₃).