Geochemical Characteristics and Geological Significance of Late Ordovician Granite in Yangkang Area of Qinghai Province

The granite rocks in Yangkang area of Qinghai province are fine grained quartz diorite and biotite granodiorite. The geochemical data shows that these rocks have SiO₂ content of 54.36%~70.16%, Al₂O₃ content of 13.08%~17.01%, K₂O content of 1.57%~3.91%, Na₂O content of 2.37%~3.75%, CaO content of 1.91%~8.09%, and their Ritterman indexes are 1.5~2.61, which belong to the peraluminum-peraluminous calcium alkaline granite. Their total amount of rare earth (∑REE) are 121.7×10^-6~458×10^-6, with LREE/HREE ratios of 6.81~14.72, which are expressed as light rare earth enrichment type. Their δEu values are 0.46~1.16, with obvious Eu loss and slightly δCe loss. The large ion lithophile elements (K, Rb, Ba,) and the light rare earth elements (La, Ce, Nd) are enriched, while the heavy rare earth elements (Yb, Lu) and the high field strength elements (Nb, Ta) are loss, with obvious loss of the Eu and Sr, P, Ti. The LA-ICP-MS zircon U-Pb ages of quartz diorite and granodiorite are (455.5±1.1)Ma and 444Ma respectively, which belong to late Ordovician. Through the analysis of tectonic environment, it is believed that the granite in Yangkang area was formed by the magmatic emplacement in the relaxation stage, which was transformed from the extrusion to the tensile environment, belonging to the syn-orogenic granite.