Petrological, Geochemical Compositions And The Origin Of The Mesozoic Basalts At The Northeastern Tibetan Plateau

The western Qiling Mountain is an important syntaxis of northeastern Tibetan Plateau, which was formed among the Tethyan-Alpian-Himalayan, Pacific and Paleo-Asian tectonic regimes. The major orogens of China are converged at the western Qiling belt. As the response to the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau, the magmatic activities in the western Qiling area were very strong since the Mesozoic, which is best represented by the basaltic magmas in the Mesozoic and the kamafugite and carbonatite magmas in the Cenozoic. In the present thesis, we presented the petrological, 40Ar-39Ar geochronological and geochemical data of the Mesozoic Duofutun volcanic lavas. In combination with previous regional survey data, the nature of their mantle source has been discussed and their original settings have been discriminated, which provide the information of the evolution and deep processes of the northeastern Tibetan Plateau.Whole-rock compositions the Duofutun volcanic rocks suggest that they are sodic alkali rocks with Mg# of 42-51. Whole-rock 40Ar-39Ar isotopic dating on three basalt samples obtained three ages, 84.4Ma, 79.3Ma and 98.7Ma, which suggest that the Duofutun volcanic rocks were erupted at the Mesozoic. Major elements of clinopyroxene phenocrysts show that they belong to dioposide. Their compositional characteristics and variation trends suggest that the Mesozoic and Cenozoic volcanic rocks were originated from similar mantle sources. The Duofutun volcanic rocks are enriched in both light rare earth elements and incompatible elements, but obviously depleted in Pb. Analyses of cpx phenocrysts in the Duofutun rocks through laser ablation ICP-MS (LA-ICP-MS) supported that cpx is the major mineral phase controlling the bulk-rock trace elements. The Sr-Nd-Pb isotope compositions show that the Duofutun volcanic rocks are similar to within-plate magmas and bear the Dupal anomaly. Temperature estimated by using the plagioclase geothermometers gives 1249-1382℃. Therefore, we suggest that the eruption of the Duofutun sodic alkalic rocks was related to the eastward flow of asthenopheric mantle beneath the Tibetan Plateau.