Neoarchean magmatism and Palaeoproterozoic metamorphism along the margin of the Qianhuai microblock in the North China Craton

The tectonic framework of the North China Craton (NCC) involved the building of microblocks and their incorporation into larger crustal blocks during the Late Archean followed by final cratonization through collisional assembly of the crustal blocks during the Late Palaeoproterozoic. Here we investigate a newly identified gabbro-diorite-anorthosite suite, metamorphosed under granulite facies, along the margin of the Qianhuai microblock based on field, petrological, geochemical and zircon U-Pb and Lu-Hf studies, with a view to understand the petrogenetic and metamorphic history. Mineral phase equilibria modelling of metagabbros from the suite reveals relatively high temperatures and pressures of 800-820 degrees C and 13-14 kbar, with a typical isobaric cooling pressure-temperature path characterized by initial heating followed by moderate to high-pressure metamorphism and cooling, consistent with a thickened arc setting. The gabbro-diorite-anorthosite association is considered to represent the fractionated sequence from a common mafic magma. Magmatic zircon grains from the metagabbro yield upper intercept ages of 2522 and 2540 Ma, corresponding to the timing of formation of the mafic protolith. The lower intercept ages of 1785 and 1833 Ma mark the timing of metamorphism associated with the collisional event between the Western and Eastern blocks of the NCC. Zircon epsilon(Hf)(t) values are mostly positive for the magmatic domains suggesting that the melts were derived from depleted mantle sources. The Hf model ages indicate that the magma source involved Neoarchean-Mesoarchean juvenile components and the continental crust was reworked during metamorphism in the Late Palaeoproterozoic. The lithological association and geochemical as well as isotopic features are consistent with a convergent margin setting along a continental arc, where basaltic melts were underplated beneath the overriding arc, accompanied by lower crustal melting. Our study confirms subduction-related crust building events during the late Neoarchean followed by Late Palaeoproterozoic high-grade metamorphism associated with the final cratonization of the NCC.