LA-ICP-MS zircon U-Pb dating of the Chahantaolegai syenogranites in Xiangride area of East Kunlun and its geological significance

A suite of syenogranites is exposed in the Chahantaolegai area along the eastern section of East Kunlun Orogenic Belt. LA-ICP-MS zircon U-Pb weighted average ages is 239.9±0.7Ma, suggesting the formation of syenogranites in Middle Triassic. According to the geochemical characteristics, it is suggested that Chahantaolegai syenogranites belong to peraluminous high-potassic calc-alkaline series. They are characterized by high SiO₂, Al₂O₃, K₂O and low TiO₂ and Na₂O values. A/CNK=1.05~1.13 (average A/CNK=1.10), K₂O/Na₂O ratios are about 0.95~1.16, and Rittmann index σ=1.56~2.24; the content of corundum exceeds 1%, as shown by CIPW standard mineral calculation. The existence of rock-forming minerals such as muscovite suggests that the syenogranites are of S-type. The values of rare earth elements are 61.47×10^-6~145.23×10^-6. The chondrite-normalized REE patterns of the samples show enrichment of LREE((La/Yb)_N=15.83~27.11)and strong negative Eu anomalies(δEu=0.26~0.49). The primitive mantle-normalized trace element diagrams show enrichment of Rb, La and Th and depletion of Ba, Ta, Sr, P and Ti. Characteristics of trace and earth elements suggest that syenogranites were from the earth's crust. It is suggested that the syenogranites resulted from melting of the sedimentary rocks of the upper crust. Combined with regional studies, the authors hold that Chahantaolegai syenogranites are of the transitional type formed in the transformation from subduction to syn-collision, being a product of the late stage of the subduction of Buqinshan-A'nyemaqen Ocean on the southern margin of the East Kunlun in Middle Triassic.