Petrogenesis of the Neoarchean granitoid gneisses in northern Hebei Province.
冀北太古代花岗质片麻岩的成因

The northern Hebei Province is one major exposure region of Early Precambrian metamorphic basement of the North China Craton. The Neoarchean granitiod gneisses that distribute at the Chengde-Luanping and Chicheng-Zhangjiakou areas are mainly composed of tonalitic, trondhjemitic, granodioritic and monzogranitic gneisses (TTGM). LA-ICP-MS zircon U-Pb and Lu-Hf isotopic analyses for the monzogranitic gneiss yield a discordant upper intercept age of 2509±10Ma, reflecting its crystallization age. Combined whole-rock geochemistry and Sm-Nd isotopes with zircon Lu-Hf isotopes, three petrochemical series are recognized: (1) low-K calc-alkaline series produced by low degrees of partial melting of tholeiitic basalts; (2) medium-K calc-alkaline series derived mainly from partial melting of basalts and greywackes, with local melting of tonalitic gneisses; (3) high-K calc-alkaline series produced by partial melting of greywackes consisting chiefly of high-K intermediate to acid volcanic rocks.Integrated with recently Early Precambrian study in the northern Hebei Province, the petrogenesis of these granitic gneisses reveals an important Archean crust growth at ca.2.7Ga in the northern Hebei Province. Large scale of volcanic eruptions occurred at the Early Noearchean, which constructed the protoliths of these granitic gneisses, and subsequently partial melting of these volcanic and related volcanic sedimentary rocks produced the magmas of TTGM, followed by regional metamorphism and deformation. The geodynamic processes producing these Neoarchen granitic gneisses suggest that their tectonic background are analogous to the west coast of the South America, implying a transitional geodynamic regime from convergent compression to back-arc extension in the northern Hebei Province.