Genesis and emplacement of syntectonic granitic plutons during Late Mesozoic extension in eastern North China

Much is known about the origin, migration, and emplacement of granitic magma with respect to the rheology of the continental lithosphere. However, magma sources, emplacement mechanisms, and the relationship between stress fields and melt flow in intraplate extensional settings are still disputed. During the Late Mesozoic, tectonic regime of the eastern North China Plate became gradually into regional extension, and a series of ductile shear zones, metamorphic core complexes, and detachment faults developed in the middle-shallow crust, accompanied by syntectonic granitic plutons. Thus, eastern North China is a favorable region for the study of syn-extensional formation of granites. Based on previous published data, we present a geochemical and geochronological investigation of typical syntectonic granites in this region. Results indicate that the syntectonic plutons had multiple magma sources, whereby magma of the earlier intermediate intrusive sheets was derived from the partial melting of juvenile lower crust or crust-mantle mixing, and later felsic sheets were derived from the partial melting of ancient lower crust. These findings reveal a deep-shallow evolution of the magma source for the syn-extensional plutons, and reveal that fluid and heat from the mantle triggered partial melting of the crust under regional extension. Three emplacement mechanisms for these syn-extensioanl plutons are inferred : (1) the plutons were emplaced in a horizontal ductile shear zone in the middle crust as tabular sills or batholiths; ( 2) the magma formed batholiths with the long-axis parallel to detachment faults by subvertical migration into the core of metamorphic core complexes or the footwall of detachment faults; and (3) the magma was located in reactivated pre-existing faults and, by injection under pressure into wallrock, emplacement space was produced by ballooning of magma and synchronous deformation of surrounding rocks, producing an emplacement mode similar to diapirism. Results also suggest that shear stress and buoyancy were important mechanical parameters affecting the orientation of magma ascent and migration. During the process of magma ascent from the source, the flow direction of the melt was controlled mainly by buoyancy; shear stress played a controlling role during magma intrusion into ductile shear zones; and buoyancy was a primary control during magma migration into the brittle-plastic transition zone in the shallow crust.