MESOZOIC REPLACEMENT OF BOTTOM CRUST IN NORTH CHINA CRATON: ANOROGENIC MANTLE-CRUST INTERACTION

The granulite terrain and granulite xenoliths in Cenozoic basalt in northern North China represent Precambrian lower crust and present lower crust of the North China craton, respectively. (1) The high-pressure mafic granulites and other types of early Precambrian metamorphic rocks in the border area of JJM along Huai'an – Fengzhen – Shangyi constitute an exposed cross section of Precambrian lower crust, with continuous decreasing metamorphic grade from high-pressure mafic granulite to mid-low-pressure granulite and to amphibolite. Their 2.6-2.7 Ga, ~2.5 Ga and ~1.8 Ga isotopic data have been suggested to forming age and two metamorphic ages. (2) The granulite xenoliths in Hannoba Cenozoic basalts in Hebei province consist mainly of mafic granulites and also small amount of tonalitic granulites. Most of the rocks have remarkable cumulate structure and zoning structure, including deformed zoning and mineral zoning. The zircon ages of granulite xenoliths are in range between 140.2+/-0.5 Ma and 120.9+/-0.6 Ma. (3) Geophysical profiles show that the Precambrian lowermost crust of the North China craton was mostly or wholly replaced by the present lower crust. This replacement of lower crust is suggested to link with the Mesozoic tectonic inversion of the North China craton. The Mesozoic tectonic inversion is featured by that a new extensional tectonic system replaced the old compressive tectonic system, and a super-strong lithosphere thinning and mantle upwelling occurred in East China in Mesozoic to make the lithosphere of East China lose about 120 km continental root. (4) Delamination that is related to Dabie-Sulu orogenic process is important to the replacement of bottom crust in parts of the craton, which took place mainly in the southern margin of the North China craton. However it is still difficult to explain mechanism of the replacement of bottom crust in the whole craton. A considerable factor is there was a Middle-late Mesozoic mantle plum, which was produced by joint affection from other neighboring continental blocks.