Geochemistry of lower crustal xenoliths from Neogene Hannuoba Basalt, North China Craton: Implications for petrogenesis and lower crustal composition

摘要

Thirty granulite and pyroxenite xenoliths from the Neogene Hannuoba basalt of the North China craton have been analyzed for major and trace element compositions. The granulites range in composition from mafic to felsic with SiO_2 = 45.7 to 73.0% and also contain metasediments. The compositions of mafic and intermediate granulites can be explained by fractional crystallization of a magma chamber in the lower crust. The magmatic granulite xenoliths are interpreted as product of basaltic underplating and subsequent fractional crystallization at the base of the crust. Thermobarometric studies and correlation of calculated P-wave velocities with regional seismic refraction results suggest that the upper part of the lower crust, which accounts for two thirds of the entire lower crust in the North China craton at a 24- to 38-km depth, is dominated by intermediate and felsic compositions. Only the lowermost crust (38-42 km) has a mafic composition. This is also supported by the abundance of intermediate and felsic granulite xenoliths, which account for 45% of the granulite population collected. The calculated bulk lower crust in the Hannuoba area has an intermediate composition with SiO_2 = 58%. Because other parts of North China also show a similar velocity structure, with the high-velocity layer confined to the lowermost 3 to 5 km of crust, the results from the Hannuoba area are considered to be representative of the reactivated North China craton as a whole.