Geochronology and Geochemical Characteristics of the Protolith Rock of Younusisayi High Pressure Granitic Gneiss: a Further Study on the Properties of Continental Crust Subduction Plate in South Altyn Tagh

The major elements of granite gneiss exhibit the following geochemical characteristics:SiO₂=71.01%~75.07%, Al₂O₃=12.31%~14.21%, Na₂O+K₂O=7.44%~8.37%, MgO=0.26%~0.60%, ^TFeO=1.75%~2.62%, and CaO=0.95%~1.70%, which reveals a typical calc-alkali peraluminous granitic rock series. The granitic gneisses are simultaneously characterized by relative enrichment in Rb, Th, K, La and depletion in Ba, Ta, Nb, Sr, Ti, their ∑_REE values range from 131.15×10^-6 to 234.34×10^-6, ∑_LREE/∑_HREE ratios are 5.07~7.67, (La/Yb)_N ratios vary from 4.82 to 9.58, and δ_Eu value are 0.42~0.63, showing the element distribution of collision-induced granite. Meanwhile, the geochemical results also show that the source rock of the gneiss may be mixed sand rock deposited by continental crust with Sr/Ba=0.11~0.15 and CaO/Na₂O=0.44~0.75>0.3. LA-ICP-MS zircon U-Pb isotopic dating has acquired a Neoproterozoic age of 899.7±4.0Ma. Based on geochemistry and chronology research, it is rationally suggested that Younusisayi granitic was formed in a syn-collision tectonic setting as the result of the partial melting of upper crust sandy rocks, as the response of Rodinia supercontinent aggregation event in the south. Meanwhile, this granite gneiss further indicated that the formation time of whole Altyn complex rocks should be Early Neoproterozoic. Moreover, a metamorphic age of~500Ma has been obtained, which is much younger than the protolith age, and indicates that the protolith rock of this granitic gneiss was intruded and located in the continental crust for about~400Ma before subduction and high-pressure metamorphism. Thus, the granitic gneiss is obviously the product of continental subduction, which produces new evidence for further defining the subduction-deep subduction of the continental crust during Paleozoic of south Altyn Tagh.