Zircon U-Pb dating and trace elements geochemistry of the Gaotan gabbro from Ziyang County, Shaanxi Province

Abstract

Basic dike swarms are widely distributed in the Ziyang-Langao-Zhenping area of Ankang in Shaanxi Province and Zhuxi area in Hubei Province, intruding the Early Paleozoic strata which characterized by bimodal volcanic association. The gabbro dikes emplacement dated 410±5 Ma by the LA-ICP-MS zircon U-Pb dating. They show the patterns of enriched LREE with high fractionation of light and heavy REE, and enrichment of incompatible elements in the primitive mantle-normalized spider-diagrams with peaks of Ba, Nb and Ti and valleys of Pb and U. The ratios of Th/Nb is less than 0.11 and those of Nb/Zr more than 0.15, indicating that they were derived from enriched mantle source closely related to the plume. The ratios of Nb/U, Ce/Pb and concentration of U, Th, Pb and Ni in the gabbro dikes indicate that the pyroxenes and olivines were main fractional minerals in the fractional crystallization during the magma evolution, and they were not affected by contamination of continental crust from the mantle to crust. These helps to infer that the extensional tectonics developed constrainedly in the Early Paleozoic by the mantle plume activities that happened at the boundary of paleocontinent on the northern margin of Yangtze Plate, thus producing the large group of basic dike swarms in these areas. The deep thermodynamic condition is responsible for the crustal extension and end. Thus the age of 410±4.5 Ma not only represents the magma emplacement, but also the extension evolution history of this continent crust rift from beginning to end in Silurian.