Geochemical Characteristics And Tectonic Significance Of The Shibandun Granotoids In The Central Qilian

The Central Qilian orogenic belt is a part of the Qilian orogenic belt,which is sandwiched between the southern Qilian fold belt and the North Qilian fold belt and is an intermediate block of the basement of the upper Cambrian strata.The Paleozoic magmatic activity in Shibandun region located in the Western of Qilian block is relatively strong,and there are widely distributed in different types of mafic ultramafic rock and acid rock class,but compared with the North Qilian orogenic belt,the overall research degree is low.In this article,on the basis of field geological survey and literature reading,through the research on granite in petrology,rock geochemistry,LA-ICP-MS zircon U-Pb dating of the Shibandun region located in of Qilian block,probes into the rock forming age,petrogenesis and tectonic environment.The main conclusions are listed below:1.The LA-ICP-MS zircon age of the diorite of North Zone is 451-484Ma,the weighted average age is 469.3±2.8Ma(MSWD=1.4),the LA-ICP-MS zircon age of syenite diorite is 452-477Ma,the weighted average age is 461.2±3.3Ma(MSWD=1.4),the LA-ICP-MS zircon age of granodiorite is 460-478Ma,the weighted average age is 470±2.5Ma(MSWD=0.91);the LA-ICP-MS zircon age of granite diorite of South belt is 458-484Ma,the weighted average age is 470.9±2.8Ma(MSWD=0.57),the LA-ICP-MS zircon age of granite is 470-476Ma,the weighted average age is 472.3±4.2Ma(MSWD=0.22);the age mentioned above can be on behalf of magmatic crystallization age that is late Early Ordovician to the middle Ordovician.2.Rocks of two belts,show the respective geochemical characteristics.Rocks of North zone that SiO2 content is 53.2-66.1%,high Al₂O₃(Al2O3=15.58-17.4wt%),TiO₂(TiO₂=0.8-2wt%)MgO(MgO=1.09-3.32wt%),TFe₂O₃(TFe₂O₃=5.14-12.09wt%)Cao(CaO=2.91-6.35wt%),K₂O(K₂O=1.83-4.1wt%),(Na₂O Na₂O=3.91-4.31wt%),A/CNK<1.1.Enrichment of LILE Rb,K and Pb,loss of HFSE Nb,Ta,Ce,Hf,Ti and Sr,Rb/Sr ratio is higher,0.26-1.01,the total REE is higher,LREE enrichment,HREE nearly flat type distribution.(La/Yb)_N=3.56-6.47,moderate negative Eu anomaly,the?Eu=0.6,the above features show that the of granite the North belt with the characteristics of volcanic arc granites,may be the product of the asaltic magma produced by mantle partial melting,in the process of rising strongly contaminated by crustal materials.Rocks of South zone that SiO2 content is higher(67.13%-70.73%),high Al2O3(Al2O3=15.86-16.42 wt%),MgO(MgO=1.17-2.34 wt%),Mg^#(Mg^#=50.11-57.77),TFe₂O₃(TFe₂O₃=2.31-3.39 wt%),CaO(CaO=2.97-4.61 wt%),rich Na and poor K(Na₂O=4.01-4.67 wt%,K₂O=1.53-1.85 wt%),A/CNK<1.1,the rocks show the obvious characteristics of high Sr low Yb,Y,enrichment of LIL Rb,Sr,K,etc.,loss of high HFS Nb,Ta,P,Ti,etc.,(La/Yb)_N=5.7-34.19,the total REE is lower,LREE enrichment,HREE loss,light and heavy REE obvious fractionation,slight negative Eu anomaly to the positive anomaly,the?Eu=0.74-1.18,,the above features show that the granite of the South belt with the characteristics of Adakite that produced by partial melting through the subduction of oceanic lithosphere plate,and obvious metasomatism occurred with mantle peridotite after the formation of magma.3.Combining with regional geological research,the west section of paleo ocean in the North Qilian was in the evolutionary phase of double subduction,the subduction to south happened later than to north(about 500Ma).The western Central Qilian is a arc accretionary complex composed of remaining micro-continent in Early Paleozoic.