LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the Liushuigou igneous complex, Tianshui area, West Qinling Mountains

The Liushuigou igneous complex distributed in the northern margin of the West Qinling Mountains consists of metagabbro, metagabbro-diorite and metadiorite. In order to determine the structural properties and discuss the process of tectonic evolution, the authors carried out the LA-ICP-MS zricon U-Pb dating as well as major and trace elements analysis. The results show that the zircon is of magmatic origin, and the age of the igneous complex is 548.7±1.8Ma(MSWD=0.56). The Liushuigou igneous complex is characterized by a wide range of SiO2(52.47%-66.95%), low TiO2, low ALK, a wider range of Mg#, and a positive correlation between oxide and SiO2 in variation diagrams. The REE patterns are flat, the ratios of LREE/HREE range from 1.14 to 2.60, LREE and HREE show low fractionation, and δEu values are between 0.65 and 1.90, mostly assuming negative anomalies, In the trace element spidergrams, there exist the enrichment of such LILEs as Cs, Sr, Th and U and depletion of Rb and K and such HFSEs as Nb, P, Ti and Y, the ratios of Ti/Y, Th/Yb, Th/Nb and Hf/Th are 113-253, 0.23-2.85, 0.09-1.00 and 0.41-2.97, respectively, suggesting that the igneous complex was formed in an island-arc setting. The Liushuigou igneous complex might indicate that there existed a "Pan-African orogeny" tectonothermal event in the Central Orogenic System. The results obtained by the authors provide evidence for the tectonic evolution of the Central Orogenic System in Late Neoproterozic and Early Paleozoic period.