A mechanism for transforming buoyant North Chinese cratonic lithosphere to a denser equivalent for delamination

Abstract

Two models have been proposed to explain lithospheric thinning of North Chinese cratonic lithosphere: (1) thermal erosion or/and chemical metasomatism, causing the lower part of the lithospheric mantle to be transformed into asthenosphere, a mechanism that implies thinning of relatively buoyant lithosphere; (2) the delamination of lithospheric mantle, in whole or part, along with the lowermost crust, as an effect of their increased densities relative to the underlying asthenosphere. This paper explores possible mechanisms whereby buoyant cratonic lithosphere might be transformed into a denser equivalent susceptible to delamination by the convecting asthenosphere. The Yanshan mobile belt in Eastern China developed in response to a combination of subduction and collision. Its apparent 'counterclockwise' P-T-t metamorphic evolution suggests that underplated basaltic magma may have heated and, in turn, weakened the cool, rigid crust, allowing for compressional deformation and crustal thickening. Based on three independent lines of evidence (compressional deformation, the record of igneous activity, and lower crustal xenoliths) the thickness of continental crust is estimated to be about 50-65 km. Along with petrological and geochemical studies, thermal modelling shows that large-scale input of asthenospheric basaltic magma leads to granitoid partial melts in the lower crust, and the dominance of high-pressure eclogitic products following orogenic thickening may be necessary for eventual delamination to occur. © The Geological Society of London 2007.