Neoarchean granitoid gneisses in Eastern Hebei, North China Craton: Revisited

Neoarchean granitoid gneisses are widely distributed throughout Eastern Hebei, eastern North China Craton, which are dominated by deformed and metamorphosed tonalite-trondhjemite-granodiorite (TTG), diorite, and granite. This study presents the results of systematic zircon U-Pb geochronological, whole-rock geochemical and Sm-Nd isotopic analyses of the Neoarchean granitoid gneisses in Eastern Hebei. These data provide insights into the Archean-Paleoproterozoic multiple tectonothermal events as well as the petrogenesis of these gneisses. Zircon U-Pb isotopic dating indicate that the granitoid gneisses were contemporaneously emplaced between 2546 +/- 10 and 2510 +/- 10 Ma, reflecting a giant Neoarchean igneous event throughout Eastern Hebei. Subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2.48-2.45 Ga. The close spatial and temporal relationships in terms of magmatism and metamorphism at ca. 2.5 Ga suggest a uniform tectonothermal evolution of Eastern Hebei. The granitic gneisses have mainly originated from the partial melting of juvenile metamorphosed greywackes, with minor involvement of basalts. The large geochemical and isotopic variations within the dioritic and TTG gneisses both provide evidence for the mixing of mafic and felsic magmas, coupled with fractional crystallization. However, the chemical differences between the dioritic and TTG gneisses might result from individual mafic magma sources, viz. basaltic and high-Mg melts. The mafic magma may also form the basalt or komatiite within the greenstone belt or evolve via fractional crystallization prior to the magma mixing. Large-scale granitoid activities were possibly related to mafic magma underplating. The combined geochronological, geochemical, and geological data support an Archean proto-mantle plume model for a geodynamic interpretation of the eastern North China Craton during the Neoarchean.