Lower crustal processes leading to Mesozoic lithospheric thinning beneath eastern North China: Underplating, replacement and delamination
Our recent studies show that the lower crust and lithospheric mantle in the eastern North China Craton (NCC) was thinned in the Mesozoic. The lower crust played a key role in the tectonic mechanism that led to lithospheric thinning. Geochemical data from volcanic rocks and xenoliths support the idea that delamination of the lower crust and lithospheric mantle took place only along the northern and perhaps the southern margins of the NCC, and this was linked to post-collisional orogenic processes. However, it is difficult for the delamination process to act on the whole eastern NCC and remove 80-120 km of the lithosphere. Lower crustal xenoliths in Mesozoic and Cenozoic basalts within the NCC are of two kinds. One is granulite-facies metamorphosed two pyroxene gabbros and eclogite-facies metamorphosed garnet pyroxenites formed at 140-120 Ma. The other is Precambrian granulites that were strongly overprinted by Mesozoic metamorphism. Large-scale lower crustal replacement took place in the Mesozoic beneath the NCC. In other words, some or perhaps most part of the present lower crust of the NCC is not Precambrian. The present lowermost crust is mostly composed of Mesozoic meta-gabbros and pyroxenites. Abundant late Jurassic, high-Sr granites in the NCC, which are compositionally similar to a dakite, probably formed by partial melting of the lowermost crust, leaving a residue of eclogite or garnet amphibolite. A hot upwelling mantle was necessary to partially melt the pre-existing lowermost crust and underplate magma in order to form the present lower crust. Although eclogites could sink into the mantle, it is hard to imagine that there was a thick enough eclogite layer that could drag 80-120 km of the lithosphere into the asthenosphere beneath the NCC. Magmatic underplating seems to be a better alternative mechanism. Thus, the formation of the present lower crust through the underplating and replacement of the pre-existing one was closely related to lithospheric thinning, and represented a continuation of the same geodynamic process.