Processes and mechanism of Mesozoic lithospheric thinningin eastern North China Craton: Evidence from Mesozoicigneous rocks and deep-seated xenoliths

Abstract

Mesozoic magmatisms in the eastern North China Craton (NCC) can be subdivided into four periods on the basis of the last isotopic geochronological data, i.e., the late Triassic (205～225 Ma) alkaline magmatism, the middle-late Jurassic (155～160 Ma) granitic magmatism, the early Cretaceous (112～132 Ma) bi-modal magmatism, and the late Cretaceous (73～92 Ma) alkaline basaltic magmatism. The discovery of eclogite xenoliths and their geochronological data (220 Ma±) in Mesozoic intrusive in the Xu-Huai region imply that the thickening of continental crust happened in the eastern North China craton in the Early Mesozoic. The break-off of subducted slab, quick exhumation of high- and ultrahigh-pressure metamorphic (UHPM) rocks, and the late Triassic (205～225 Ma) alkaline magmatism suggest the beginning of Mesozoic lithospheric thinning in the NCC. The delamination could be a major mechanism to result in the thinning of lithospheric mantle in the Early Mesozoic. The middle-late Jurassic (155～160 Ma) granitic magmatism formed in post-orogenic extensional environment, implying the continuity and development of Mesozoic lithospheric thinning. The early Cretaceous (112～132 Ma) bi-modal magmatism implys the coming of the peak period of Mesozoic lithospheric thinning. The silica-rich melt metasomatism in the mantle-derived dunite xenoliths and high~(87)Sr/~(86)Sr ratios and low e(Nd,t) values of basalts indicate that the chemical erosion of the asthenosphere to the lithospheric mantle could be a major mechanism to result in the thinning of lithospheric mantle in the period. The late Cretaceous (73～92 Ma) alkaline basaltic magmatism and the existence of “oceanic-like” mantle xenoliths indicate the decreasing of isothermal plane and the accretion of lithospheric mantle.