Geochemical Characteristics And Tectonic Significance Of Diorite In Qingshanbao,Gansu

The North Qilian orogenic belt is located between the Alxa block and the Middle Qilian block.A large number of Early Paleozoic intrusive rocks with different rock assemblages are exposed in the area,and the intermediate acid rocks are the dominant ones,among which the intermediate rocks have important restraining significance for enriching and reducing the tectonic evolution process of the orogenic belt.In this paper,the Qingshanbao diorite in the North Qilian orogenic belt is selected as the research object,and the comprehensive analysis of petrography,chronology and element geochemistry of diorite is carried out to clarify the diagenetic age and formation environment of diorite in Qingshanbao.It provides certain new data for the study of intermediate rocks in the North Qilian,and provides a reliable basis and restriction for the Early Paleozoic tectonic evolution of the North Qilian.Qingshanbao diorite is grayish green,mainly composed of plagioclase and amphibole.LA-ICP-MS zircon geochronology results show that the U-Pb ages of two groups of magmatic zircons of Qingshanbao diorite are 426.1�5Ma(n=13,MSWD=2.2)and426.9�4Ma(n=7,MSWD=0.32),indicating that the diorite was formed in Silurian.The results of major element tests show that the Qingshanbao diorite is rich in aluminum and iron,poor in titanium and phosphorus.Its Ritman index(?)is between 1.47 and 2.35,and A/CNK is 0.83 to 0.95,belonging to calc-alkaline quasi-aluminum rock.The results of trace and rare earth elements analysis show that the Qingshanbao diorite is enriched in high field strength elements(Th,U)and light rare earth elements(La),while depleted in large ion lithophile elements(Ba,P)and high field strength elements(Ta,Ti,Nb).The rare earth distribution curve showed a right-leaning trend,and showed weak Eu anomalies(?Eu=0.70?0.86,mean value 0.77)and weak Ce anomalies(?Ce=0.99?1.13,mean value1.03).The composition of Qingshanbao diorite hornblende indicates that the rock was formed under the condition of high oxygen escape.Its diagenetic temperature is about751?835 ?,and the pressure is about 69?146 MPa.Thefore,it is estimated that the emplacement depth is about 2.6?5.5 km.The comprehensive study of mineralogy and geochemistry shows that Qingshanbao diorite may originate from partial melting of the earth's crust,and the process of mixing of melt and mantle-derived material occurred during the formation process.It is the product of magma mixing and may be formed in postcollisional extensional tectonic environment.The comprehensive study shows that the transition of the North Qilian orogenic belt from compressional environment to extensional environment occurred before 426 Ma.In the Ordovician compression environment,the study area experienced crustal thickening,followed by lithospheric delamination and subsidence,and formed adakitic rocks.During Silurian,the region entered an extensional environment.With the subsidence of the lower crust,the asthenosphere kept upwending and heating the lower crust continuously,resulting in partial melting of the lower crust to form silica-alumina magma,which mixed with mantle-derived ferro-magnesia magma,and finally formed A-type granite and diorite dike.