Rheology of the lower crust beneath the northern part of North China: Inferences from lower crustal xenoliths from Hannuoba basalts, Hebei Province, China

摘要

Lower crustal xenoliths brought up rapidly by basaltic magma onto the earth surface may provide direct information on the lower crust. The main purpose of this research is to gain an insight into the rheology of the lower crust through the detailed study of lower crustal xenoliths collected from the Hannuoba basalt, North China. The lower crustal xenoliths in this area consist mainly of two pyroxene granulite, garnet granulite, and light-colored granulite, with a few exception of felsic granulite. The equilibration temperature and pressure of these xenoliths are estimated by using geothermometers and geobarometers suitable for lower crustal xenoliths. The obtained results show that the equilibration temperature of these xenoliths is within the range of 785—900℃, and the equilibrium pressure is within the range of 0.8—1.2 GPa, corresponding to a depth range of 28—42 km. These results have been used to modify the previously constructed lower crust-upper mantle geotherm for the studied area. The differential stress during the deformation process of the lower crustal xenoliths is estimated by using recrystallized grain-size paleo-piezometer to be in the range of 14—20 MPa. Comparing the available steady state flow laws for lower crustal rocks, it is confirmed that the flow law proposed by Wilks et al. in 1990 is applicable to the lower crustal xenoliths studied in this paper. The strain rate of the lower crust estimated by using this flow law is within the range of 10~(-13)—10~(-11) s~(-1), higher than the strain rate of the upper mantle estimated previously for the studied area (10~(-17)—10~(-13) s~(-1));the equivalent viscosity is estimated to be within the range of 10~(17)—10~(19)Pa·s, lower than that of the upper mantle (10~(19)—10~(21) Pa·s). The constructed rheological profiles of the lower crust indicate that the differential stress shows no significant linear relation with depth, while the strain rate increases with depth and equivalent viscosity decrease with depth. The results support the viewpoint of weak lower continental crust.