Geochemistry and zircon U-Pb chronology of charnockites in the Yinshan Block, North China Craton: Tectonic evolution involving Neoarchaean ridge subduction

Abstract

The Yinshan Block, part of the Neoarchaean basement of the Western Block of the North China Craton, is composed of granite-greenstone and granulite-charnockite complexes. We report research on a suite of charnockites from the granulite-charnockite complex and characterize their geochemistry, zircon U-Pb geochronology, and Hf isotopic composition. The charnockites can be divided into intermediate (SiO2 = 59-63 wt.%) and silicic (SiO 2 = 69-71 wt.%) groups. U-Pb zircon data yield protolith formation ages of 2524 ± 4 Ma, 2533 ± 15 Ma, followed by metamorphism at 2498 ± 3 Ma, 2490 ± 11 Ma, respectively, for these groups. Although the intermediate charnockites are characterized by higher Al 2O3, TiO2, Fe2O3 T, MnO, MgO, CaO, P2O5, K2O, Sr, and ∑REE content than the silicic charnockites, the ages and Hf isotopic composition of zircons and REE patterns of both intermediate and silicic charnockites are remarkably consistent, which indicates that they are genetically related. These charnockites are predominantly metaluminous to slightly peraluminous, calc-alkalic to calcic, and magnesian-characteristics generally related to a subduction setting. High-Sr + Ba granites with low K2O/Na 2O characteristics, shown by these charnockites, imply a mixture of mafic and felsic magmas generated from an enriched mantle + lower crust. High MgO, Ni, Cr and Mg#, low K2O/Na2O, and metaluminous to slightly peraluminous natures imply that the source rocks most likely were amphibolites. Coeval calc-alkaline magmatism and high-T granulite-facies metamorphism under low-H2O activity in the area lead us to propose a model involving mid-ocean ridge subduction within a Neoarchaean convergent margin. The arc-related rocks accreted along the continent margin, and became a barrier when the lithospheric mantle ascended through the slab window. Melt derived from the decompressing mantle mixed with melt derived from the overlying, juvenile lower crust melt, which was warmed and metamorphosed by the ascending lithospheric mantle. © 2013 Taylor & Francis.