Geochemistry And Geodynamic Implications Of The Cenozoic Basalts And Included Mantle Xenoliths From Yangyuan And Datong, Western North China Craton

Major, trace element composition and Sr-Nd-Pb isotopes of the Cenozoic basalts from Yangyuan (Hebei Province) and Datong (Shanxi Province), which are located to west of the Daxinanling-Taihanshan gravity lineament, as well as mineral chemistry, major and trace element composition and Sr-Nd isotopes of mantle xenoliths from Yangyuan are reported in this thesis. These data are used to characterize the nature and processes of the upper mantle beneath western North China Craton. The Early Tertiary alkali basalts from Yangyuan which contain abundant mantle xenoliths exhibit very homogeneous Sr-Nd isotopes with SNd = 5.5. Their Pb isotopes are similar to those of the depleted mantle (DM) and their trace element characteristics resemble those of oceanic island basalts (OIB). Therefore, they most likely originated from the asthenospheric mantle at a relatively great depth. The Quaternary basalts at Datong include both alkaline and tholeiitic types. Given their OIB-type trace element pattern and depleted Sr-Nd isotopic composition, the Datong alkali basalts are interpreted to be generated within the asthenospheric mantle but at a shallow depth compared to the Yangyuan alkali basalts. The Datong tholeiitic basalts exhibit low SNd values (1-4). Their Sr-Pb and Nd-Pb isotopic correlations are consistent with a mixing between the depleted mantle and EMI-like component. This may reflect the role of the asthenosphere-lithosphere interaction in the genesis of the Datong tholeiites, which were derived from a shallower depth than the alkali basalts. The distinct spatial distribution of the tholeiitic and alkali basalts at Datong may be related to the effect of local lithospheric extension on the passive upwelling of asthenosphere and variable extents of interaction between lithosphere and asthenosphere.The xenoliths in the Yangyuan, including Iherzolite and harzburgite, represent the direct samples from the upper mantle beneath western North China Craton. Thermobarometric calculation shows that the Iherzolites were equilibrated in a temperature ranging from 926 # -1050# . A similar range of equilibrium temperature (916 #-1040 #) is obtained for the harzburgites. Therefore, there is no significant difference in equilibrium temperature for different rock-types. The mineralchemistry amd elemental variations in whole rocks can be ascribed to results of partial melting of various degrees. According to the fractional melting model, the partial melting degree for the Iherzolites is less than 5 percent, and about 7 to 20 percent for harzburgites. The trace element compositions and Sr-Nd isotopes suggest that the upper mantle beneath the studied area were subjected to multiple episodes of metasomatism, probably by small melt fractions derived from the asthenospheric mantle. Some samples display EM-like Sr-Nd isotopic ratios, thus providing the evidence for the old, enriched mantle beneath western North China Craton.