The North China Craton(NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods:(1) Late Paleozoic to Early Jurassic(~170 Ma);(2) Middle Jurassic to Early Cretaceous(160-140 Ma);(3) Early Cretaceous to Cenozoic(140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period,the subduction and closure of the PaleoAsian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression(Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range pro vince by the Mesozoic magmatic plutons and NE-SW trending faults.With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle(SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upw
This paper studies magnetic properties and composition of granulite-facies rocks of both the Neogene and Archean continental lower crust in the Neogene xenolith-bearing Hannuoba (汉诺坝) alkaline basalt and the exposed lower crustal section in the Archean Huai'an (淮安) terrain (Wayaokou (瓦窑口)-Manjinggou (蔓菁沟) profile), the northern North China Craton. It provides a unique opportunity for a comparative study of magnetic properties and composition of both the Archean and Neogene continental lower crust. We measure magnetic parameters (susceptibility ? and magnetic hysteresis parameters, such as saturation magnetization Js, saturation isothermal remanent magnetization Jrs, and intrinsic coercivity Hc) of eleven Hannuoba lower crustal xenoliths and nine terrain granulites from the Archean Huai'an terrain. Results indicate that the average values of ?, Js and Jrs of Archean granulites are 4 122×10-6 SI, 523.1 A/m and 74.9 A/m, respectively, which are generally higher than those of granulite-facies xenoliths (1 657×10-6 SI, 163.9 A/m and 41.9 A/m, respectively). These two types of granulites contain ilmenite, (titano) magnetite, minor hematite and some "magnetic silicates" (clinopyroxene, plagioclase and biotite). The Mg-rich ilmenite in granulite-facies xenolith is relatively higher than that in terrain granulites. We observe a more evolved character as higher magnetic as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd and V/Nd ratios in terrain granulites. These differences in magnetic characteristics reflect their different origins and evolutions. The high magnetization of granulites in the Huai'an terrain represents magnetic properties of the Archean continental lower crust, and low magnetization of granulite-facies xenoliths represents magnetic properties of the Cenozoic lower crusts in the northern North China Craton.
