Geochemical Characteristics And Genesis Of The Shizhisi Volcanic Rocks In The Western Qinling

The Western Qinling Mountains are located in the Northeastern margin of Qinghai-Tibetan Plateau,a special area where the paleo-Asian tectonic domain,the Tethys tectonic domain and the coastal Pacific tectonic domain converged.Since the Cenozoic,the Collision between the Indian plate and the Eurasian plate led to the uplift of the Tibetan Plateau,Cenozoic volcanic rocks developed in the West Qinling region at the Northeastern margin of Qinghai-Tibetan Plateau,and recorded the deep dynamic information of the plateau expansion and growth and the tectonic deformation in the Northeastern margin,which is of great significance for the study of the expansion and growth of the Northeastern margin of Qinghai-Tibetan Plateau and the crustal uplift and deformation in the Northeastern margin of Qinghai-Tibetan Plateau.This paper takes the newly discovered Shizhisi volcanic rocks in Lintao County,Dingxi City,Gansu Province as the object of study,and carries out field geological investigations,petrographic and main trace element geochemical studies to investigate the petrogenesis,and to define the age of formation of the rocks in relation to the regional stratigraphy and relevant geological data already obtained by previous authors,and to compare them with contemporaneous alkaline basalts from other areas of the West Qinling Mountains,providing new evidence and geochemical constraints to reveal the deep material composition of the West Qinling Mountains and the uplift of the Qinghai-Tibetan Plateau.The Shizhisi volcanic rocks are mainly composed of pyroxene basalt,inanite and volcanic clastic rock types.According to the contact relationship between the volcanic rocks and the surrounding strata and the previous geological survey data,the formation age of the volcanic rocks is determined as Cenozoic.The Shizhisi basalt is grey-black on the fresh surface,porphyritic and massive,with porphyritic crystals consisting mainly of pyroxene and plagioclase,and a matrix of pyroxene and plagioclase microcrystals and melanocratic minerals.The basalts show moderate Si O₂ content,low K₂O content,high Na₂O content and K₂O+Na₂O content,that belong to mantle-derived sodium-alkaline basalts.Trace and rare earth elements are similar to OIB basalts,with significant enrichment in Ba,Th,Sr and especially Nb,Ta and Pb,and a deficient in Rb,with no negative anomalies in Eu.Geochemical signatures indicate that the Shizhisi basalts were not affected by significant crustal mixing and segregation crystallisation during their formation,but were controlled by partial melting.The results of the main trace element analysis suggest that the Shizhisi basalt may have come from an enriched lithospheric mantle that was metasomatized by the athenosphere and formed by the partial melting of a hornblende-bearing pyroxenite mantle.Compared with the Cenozoic basalts in other areas of the West Qinling Mountains,the Shizhisi basalts differ in petrographic and geochemical characteristics from the Cenozoic basalts in other parts of the West Qinling Mountains,and in their mantle source areas,indicating the complexity of the mantle source areas of the Cenozoic basalts in the West Qinling Mountains.According to the petrological and geochemical characteristics of the Shizhisi volcanic rock and its age of formation,combined with the previous research results and a large number of geophysical data,we believe that the formation of the Shizhisi volcanic rock may be related to the collision between the Indian plate and the European plate since Cenozoic.