Petrological, Geochemical And Geochronological Characteristics On The Origin Of The Late Mesozoic Basalts At The Northeastern Tibetan Plateu

West Qinling orogenic belt located at the northeastern o f Tibetan Plateau is atransition belt among of the Paleo Asian tectonic domain, Tethyan tectonic domainand the Pacific tectonic domain, and is a key regions of dividing geographical andgeological structures between the eastern and western of China. A large amounts ofMesozoic volcanic rocks is widely distributed in West Qinling. However, somedebatable issues on the petrogenesis and geodynamic settings of the Mesozoicvolcanic rocks, because of owing to the basic geological research in the past,especially lacking the reliable geochronological and geochemical data of the volcanicrocks, have been continued for a long time, the understanding for the tectonic settingand evolutionary history of West Qinling has been affected by which. In this paper,we present petrographic, geochemical and isotopic chronological data of Mesozoicbasalts in Hongqiang area of Xiahe country in West Qinling, Gansu Province, anddiscuss preliminarily its petrogenesis and geodynamic setting. The Hongqiangvolcanic erupted at about105¹01Ma which is consistent with field observations,and indicates that the magma activity of the Hongqiang basalts occurred mainly in theEarly Cretaceous. The observation and research of petrography and geochemistry ofthe volcanic rocks shows that plagioclase phenocrysts and large amound ofmicro-crystallite or micro-phenocrysts of plagioclase appeared in the basalts and inthe groundmass of the basalts. The characteristics of higher Al₂O₃, Na₂O and lowerSiO₂, CaO of the basalts demonstrate the basalts are belong to a sodic-alkalinemagma series. On the geochemistry, the Hongqiang basalts are rich in REE andHREE（Nb, Ta, Zr, Hf, Ti）and slightly depleted in LILE（Rb, K）, and have positiveεNd（t） values（5.91⁷.48）, low initial87Sr/86Sr ratios（0.703269⁰.705505）and Pb isotopic ratios are²⁰⁶Pb/²⁰⁴Pb（t）=17.2485¹8.3193，²⁰⁷Pb/²⁰⁴Pb（t）=15.3752¹5.6257and²⁰⁸Pb/²⁰⁴Pb（t）=37.0982³8.4542respectively. All of these mentionedabove indicate the early Cretaceous basalts of Hongqiang have the geochemicalcharacteristics similar to continental OIB, and their soures may be a depletedasthenosphere mantle. Combined with a comprehensive analysis of the geotectonicsetting of the Hongqiang basalts and contrastive research between Hongqiang andDuofutun basalts Early Cretaceous, we suggested that the Early CretaceousHongqiang basalt may be a production of continental rift magmatism, its petrogenesis and dynamic background are strongly associated with the nature and evolution ofNorth-South Tectonic Belt since the Mesozoic. The differences of Early Cretaceoussodic-alkaline basalts on the petrography, whole rock geochemistry, series and typesof the volcanic rocks between Hongqiang and Duofutun in West Qinling may berelated to the evolution of the asthenospheric mantle source during different periodsand the genetic depth and the partial melting degree of magma.