LA-ICP-MS zircon U-Pb dating and geochemistry of the Taibai intrusion in Tianshui areas of western Qinling Mountains and their geological significance

An EW-trending Indosinian granitoide belt extends near northern Mianlue suture zone on the south margin of Qinling orogenic belt. The Taibai intrusion is a part of the Indosinian granitoids located in the Huixian County of Gansu Province. LA-ICP-MS zircon U-Pb dating of the Taibai intrusion indicates that the emplacement age of the Taibai intrusion is 221.8Ma±1.5Ma, suggesting Late Indosinian. The Taibai intrusion is mainly composed of gray-pale red, medium to fine grained monzonitic granite and granodiorite. According to the geochemical characteristics, it is suggested that Taibai intrusion belongs to the metaluminous to weakly peraluminous high-potassic calc-alkaline series and is characterized by strong depletion of high field strength elements (HFSE) Nb, Ti, P Y, Yb and large ion elements Ba, Sr with obvious negative Eu anomalies. The total amount of rare earth elements ΣREE=79.0×10-6~194×10-6，the rare earth elements (REE) exhibit right-oblique patterns with strong enrichment of light rare earth elements (LREE) and the relative depletion of heavy rare earth elements (HREE). The rocks of the Taibai intrusion was formed by rising and emplacement of partially melted granitic magma resulting from partial melting of the shell material, belonging mainly to intermediate-basic rocks. The Taibai intrusion, formed at the transitional stage from syn-collision (compression environment) to post-collision (extentional environment), is of late-orogenic granitoids and also a product formed by partial melting of thickened lower crust resulting from the collision between the North China plate and the Yangtze plate.