Geochemistry of Neoarchean (ca.2.55-2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt,central orogenic belt,North China craton:Implications for geodynamic setting and continental growth

摘要

Geological investigation of the Neoarchean (2.55-2.50 Ga) Wutaishan greenstone belt in the central orogenic belt of the North China craton has provided new information on the geodynamic origin of this belt and its mineral deposits.Structural,geochronological,and geochemical characteristics of the Wutaishan greenstone belt suggest that it formed in a forearc tectonic environment at ca.2.55 Ga and accreted to the Eastern continental block at ca.2.50 Ga.A ridge subduction model is proposed to explain several unique geological features of the Wutaishan greenstone belt,such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns,formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF),extrusion of mafic to felsic volcanic rocks,and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG).Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy,resulting in its accretion to the continental crust.We propose that ridge subduction also played an important role in the growth of Archean continental crust.In this model,the origin of Archean TTG is genetically linked to eclo-gites through partial melting of accreted and/ or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction.TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs;subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust.