A possible model for the lithospheric thinning of North China Craton: Evidence from the Yanshanian (Jura-Cretaceous) magmatism and tectonism

It is well known that the North China Craton was largely formed in the Archean, and was reactivated and transformed during the Jura-Cretaceous (Yanshanian) time into an orogenic belt, which is believed to be related to the lithospheric thinning. Recent debate is centered on the mechanisms and processes of the lithospheric thinning. There are two prevailing models for the lithosphere thinning: (1) thermal erosion or/and chemical metasomatism allowed the lower part of the lithospheric mantle to be transformed into asthenosphere, (2) delamination of the lithospheric mantle, and perhaps also the lowermost crust. In this paper, we attempt to explain how the buoyant cratonic lithosphere may be transformed into a denser one, allowing delamination to take place on the basis of field observation, tectonic analysis and petrologic data on igneous rocks formed during the Yanshanian. We recognize four episodes of contractional deformation that resulted in significant crustal shortening and vertical thickening. The counter-clockwise Pressure-Temperature-time path of the tectonomagmatic events suggests that the underplating basaltic magma may have heated and weakened the existing cold and strong crust. This crustal change in rheology may have facilitated the contractional deformation and crustal thickening. Petrologic data of the contemporary igneous rocks and the lower crustal xenoliths suggest that the crust had reached similar to 50-65 km in thickness. It suggests that input of large amount of asthenosphere-derived mafic magmas is required to cause crustal melting. Thus, a large amount of eclogite may be formed at the lowermost crust following the transient thickening events. The dense eclogite may trigger the lithosphere delamination. (C) 2006 Elsevier B.V. All rights reserved.