Geochemical Characterisitics And Tectonic Environment Of The Chayong Ophiolite In Zhiduo County,Qinghai Province

Based on the field geological investigation and interior integrated studies on the Chayong ophiolitic melange, which is situated in Zhiduo County, Qinhai province, this article makes a comprehensive research on its material composition, rock association, geochemical characteristics, tectonic setting and formation age from geology, petrology, isotopic chronology, geotectonics and other aspects. Compared with Litang ophiolite suture zone,the tectionic evolution at Ganzi-Litang has been discussed, and some understanding and conclusions in this work are as follows:ChaYong ophiolitic melange is mainly a set of “overall disorder, partial order” formation. In this work, ophiolite sequence is rearranged to restored ophiolite sequence synthesis section, which consists of olive pyroxenite, gabbro, basic dyke, basic lava and deep-sea siliceous rocks from below to above. The ophiolite, in combined with basic volcanic construction, deep clastic rock formation, exotic limestone, carbonate block form the Chayong ophiolitic melange belt, which stricks north west to south east, pinches west gradually, and eastwards into GanZi- LiTang region.The analysis of rock geochemistry shows that the set of ophiolitic melange originated from alkaline – sub-alkaline magma. The cumulate and basic lava present that Al, Ti, P grow, and Mg reduces with the increase of SiO2, which show that cumulate has the characteristics of higer Mg and lower Al, Ti, P, and basic dyde and basic lava process the higher Al, Ti, P; Chondrite-normalized REE patterns wholely incline to right gently, and the curve trends to dull from cumulate to basic lava, which is similar to MORB; Spider diagram of trace element shows the enrichment of Rb, Ba, Th, and depletion of Yb, Y, Zr. All above evidences suggest that Chayong ophiolite originated in ocean basin environment.The La-ICP-MS zircon U-Pb dating of pyroxenite yields an age of 220.7- 228.0 Ma, and most datas distribute around the concordia line, and the weighted average age is 223.5 ±5 Ma, which indicates that the Chayong ophiolite forms at the Middle Upper Triassic.According to the reseach conclusion of the characteristics of Chayong ophiolite geochemistry, tectonic settings and area geology, the tectonic evolution stages in this research area are drawn up as follows:The main event in Triassic is Ganzi- Litang ocean- land conversion. At the early-middle stage, ocean basin expanded and formed the oceanic crustal. Pyroxenite Zircon U-Pb age in the area is middle Upper Triassic, which indicates that the peak ocean basin expansion was middle Upper Triassic, and the ocean basin closed in later upper Triassic. In the Late upper Triassic, Tongtian River complex ophiolite belt was formed, and Yanshanian intermediate-acid magmatism intruded on the edge close to one side of Balyanlkalla two-way foreland basin. At this point, the end of large-scale ocean-land conversion tectonic evolution in this orogenic belt was over, and got into the period of Jurassic intracontinental orogenic evolution.