Geochemical Characteristics And Tectonic Significance Of Syenite In Hongshiquan,Gansu

The North Qilian orogenic belt is located between the Alxa block and the central Qilian block,and a large number of Early Paleozoic plutons of different rock assemblages are exposed in the area,among which alkaline rocks are important for enriching and reducing the tectonic evolution process of the orogenic belt.restrictive meaning.In this paper,the Hongshiquan syenite in the North Qilian orogenic belt is selected as the research object,and the comprehensive research and analysis of petrography,chronology and element geochemistry of the syenite is carried out,and the diagenetic age and formation environment of the Hongshiquan syenite are clarified.The study of even-alkaline provides some new data,and provides a reliable basis and constraints for the early Paleozoic tectonic evolution of the North Qilian.Hongshiquan syenite is fleshy red and is mainly composed of potassium feldspar and albite.The LA-ICP-MS zircon chronology results show that the Hongshiquan syenite was formed in an age of 420.2±3.3Ma(MSWD=2.1),which was formed from the Late Ordovician to the Early Silurian.The test results of geochemical elements show that the Hongshiquan syenite belongs to the series of quasi-aluminous high-potassium alkaline syenite and A-type granite.The mean value of the Rittman index is 8.26,the mean value of AR is 4.3,and it is basic.The average A/CNK value is 0.88,which is quasi-aluminum.In terms of trace and rare earth elements,the Hongshiquan syenite is relatively enriched in large ion lithophile elements such as Rb and Th,and relatively depleted in high field strength elements such as Nb,Ta,P and Ti.The total amount of rare earth elements in the rock samples is high and light.The rare earth elements are relatively enriched,and the heavy rare earth elements are relatively depleted.The average value of δEu is 0.66,showing a moderate negative anomaly.From the analysis of syenite apatite minerals,it is concluded that the average oxygen escape of Hongshiquan syenite is-10.60.According to the zirconium saturation thermometer,its diagenetic temperature(T)is between 842°C and963°C.Geochemical evidence that the Hongshiquan syenite was formed by magmatic mixing.Combining with the tectonic setting of the North Qilian,it is concluded that the time limit of the early Paleozoic tectonic environment in the North Qilian orogenic belt changed from compression to extension occurred before 420 Ma.In the Late Ordovician,the North Qilian Ocean Basin began to close due to the reduction of the oceanic crust.From the Late Ordovician to the Early Silurian,the long-term collisional movement led to the continuous thickening of the lithosphere.Delamination and subsidence occurred in the orogenic belt at the root of the Qilian lithosphere,and the upwelling of asthenosphere material heated the lower crust,causing it to partially melt to form crust-derived magma,and mixed with mantle-derived magma to finally form Hongshiquan syenite.