Tectonic Evolution Of Alxa Block From The Proterozoic To Early Paleozoic

The geologic structure is complicated about Alxa.Alxa plot is located in the north China plate,the central Asian orogenic belt,Qilian orogenic belt,and the joint of Tarim plate and the north China plate.Although this area's metallogenic condition is superior and mineral resources are rich,which are base on tectonic thermal event development with magmatism,Alxa is mostly covered by desert,the structure is complicated,and the degree of research is low,especially in Precambrian crustal evolution and tectonic attribute is controversial.Based on field geological survey and predecessors' research results,this paper adopts structural geology,zircon U-Pb chronology,geochemistry,petrology,and some comprehensive research methods research and explores magmatic activity and crustal evolution process from Proterozoic eon-Early Paleozoic.The main understanding following:1,This paper obtains and understands the gneissic granite LA-ICP-MS,zircon U-Pb aged between 1180±16Ma and 1396±18Ma the weighted average is 1269±90Ma in aoquan Gneissic Granite in Alxa Mesoproterozoic.This explains that the gneissose granite forming age is in Proterozoic in the middle-late stage.Rock contains high SiO2(74.66%-76.42%),relatively low Al2O3(12.61%-13.06%),low Ca(CaO,about1.04%),low MgO(0.13-0.27%),high Na20+K20(7.77%-8.42%),high K20(4.53%),and low Na(K2O/Na2O=1.08-1.88),with metaluminous(A/NKC=0.84-0.98).Primitive mantle standardization spider diagram microelement shows that rock contains Rb?Th?U?K,but obviously lack Ba?Nb?Ta?Sr?P?Ti,adversely,involves high 10000Ga/Al(2.39-2.71);at the same time,Ree chondrite standardized figure revels that rock contains a handful of LREE,(La/Yb)N=2.17-6.34,strong negative of Eu anomaly(8Eu=0.34-0.44),Zr=118-166(<250×10-6),and Zr+Nb+Ce+Y=199-282(<350×10-6).Thus,the study indicates that rock in Alxa belongs to the A2 type granite in the middle Proterozoic due to inferring rock is formed in orogenic environment,which is result from partial melting of lower crust materials.This area is from the compressive orogenic to stretch the structure.2,Augen gneiss and banded gneiss are distributed on the west of Alxa,such as Su Shan,Habuqigai,Keketuole.The result of ages about four samples have got,for example 913±7Ma?921±7Ma?926±15Ma and 904±7Ma though rock LA-ICP-MS zircon U-Pb analysis.Rock has high SiO2(70.0%and 70.0%),high alkali(Na2O+K20:6.5%-8.5%),low MgO(0.05%-0.05%),T FeO(1.2%-4.2%),CaO(0.50%-1.50%),and A/CNK value between 1-1.2.This belongs to aluminum granite.Furthermore,this area indicates that it contains low Fe elements,lower Ba compared with Rb and Th,high field strength elements like Nb,P,Zr,Ti relative loss;the fractionation of ree is apparent;(La/Yb)N value is greater than 10;and strong negative Eu anomaly(delta Eu=0.19-0.79).Clearly,the study revels that the neoproterozoic magmatic event has characteristics of crustal remelting,and displays the characteristics of tectonic environment or collision of magmatic event.The neoproterozoic magmatic event of Alxa is the result of Rodinia supercontinent's collage and aggregation.3,This study has obtain the early Paleozoic granite 44 samples in Alxa.However,except for the T1-1 and T1-2 two samples,the rest of rock contains abundant SiO2(the average is 72.02%);abundant Na2O(average 3.57%);K2O(average 4.21%);Al2O3(average 14.08%);low CaO(average 1.82%);MgO(average 0.59%);the high value of(Na2O+K2O)/Al2O3 and TFe2O3/MgO;ree is Right deviation;obvious negative Eu anomaly;the strong loss of Ba,U,Nb,Ta,Sr,P and Ti;as well as contains the abundant K.Moreover,the north of Taolai two groups of plagiogranite have low SiO2(58.31%-67.09%),high Al(Al2O3=16.96%-19.23%),the content of Na2O is 3.88%-4.22%,low K(K2O=1.27%-1.52%).Apart from these,they also contains K2O/Na2O=0.32-0.36,relative high CaO(4.03%-6.06%),the content of MgO is 1.19%to 2.77%and the TiO2 content is 0.48%to 0.79%.Simultaneously,research shows light rare earth on plagiogranite;the Eu is weak anomalies;High Sr low Y;Sr/Y ratio is 216-233;Ta,Nb,Zr,Y,Ti is strong loss;and Sr,Ba,U are relative abundant.Base on above mentioned,it could be seen that it is a typical characteristic of Adakite.Combined with the research and regional structural background,it can be speculated that the Adakite is likely formed by the crust molten mass under basaltic erosion and partial melting under the situation of unsaturated water and thickened crust(thickness>at least 40 km).Therefore,the study revels that early Paleozoic granite is A1 type granite and its formation is mainly related to plate with magma activities or rifting.Alxa has experienced crustal thickening-thinning process in neoproterozoic-early Paleozoic era.Basaltic erosion and high Sr low Y type of magma is the main forms of cracking of Rodinia supercontinent.4,To sum up,the paper has made an assumption that Alxa is a very complex geological history landmass by comparing with the structure,stratum,magma,metamorphism and some important geological events in Alxa plot and the north of China-craton at the same period.This bases on the research about basement,which has deduced that Alxa plot could belong to Tarim plate and its formation can be traced back to the early or late archeozoic era,this is with dunhuang plot together has formed rigid block in early Precambrian,then it has been cut by sinistral translation of Altyn Tagh fault.Finally,the block has experienced the three tectonic thermal rock-magma events,such as 2.2?2.4Ga?1.8?2.0Ga and 0.8?1Ga;or piece together and collage together with the north China plate,thus forming the regional pattern of today after middle Ordovician.