Geochemistry and its continental dynamic implication of the Cenozoic sodic picritic basalt from the Liuping area, northeastern margin of the Tibetan Plateau

The Cenozoic picritic basalt from the Liuping area, northeastern margin of the Tibetan Plateau located at the convergent region between Tibetan Plateau, North China and South China plates. It erupted during the Miocene Period with ages in the range 23 similar to 7. 1Ma. The rock exhibits SiO2 between 41. 72% to 42. 82%, Na2O > K2O, K2O/Na2O being 0. 51 (mean value), belonging to an association of the typical mantle generation sodic-alkaline basalt. The trace and rare earth elements show geochemical characteristics of the within plate volcanism. The Th and Rb are enrichment and the K2O is obviously depletion (0. 48% similar to 0. 90%), indicating that this volcanic rock association is different from the potassic-ultra potassic-shoshonitic Cenozoic volcanic rocks widely distributed in the north margin of the Tibet Plateau. Isotopic compositions of this group rock have radiogenic Sr, Nd and Pb (Sr-87/Sr-86 = 0. 704158 similar to 0. 704668, Nd-143/Nd-144 = 0.512831 similar to 0. 513352, Pb-206/Pb-204 = 18. 729871 similar to 18. 779184, Pb-207/Pb-204 = 15. 591395 similar to 15. 602454, Pb-208/Pb-204 = 39. 097372 similar to 39. 181458, epsilon(Nd) = 4. 0 similar to 14. 2). The Sr-Nd-Pb isotopic compositions located between EMI, EMIT, BSE, PREMA and HIUM, suggest that the basalt should be derived from a mixed mantle reservoir. The western Qinling-Songpan tectonic region was controlled by Tibet, North China and Yangtze blocks since Cenozoic, therefore, the region was in the stage of the substance converge from the lower mantle to upper crust, producing a mixed mantle reservoir in the studied area. The Miocene picritic basalt from the Liuping area occurred in the specific tectonic background can be interpreted to reflect lateral asthenospheric mantle eastward flow of the Tibet Plateau and partial melting of a mixed- asthenospheric mantle reservoir in the western Qinling-Songpan tectonic node.