Geochemical Characteristics Of Acidic Dike Swarm From The Eastern Segment Of The Ertix Tectonic Belt, Altai Orogeny And Its Geological Implications

The Altai orogeny, as one of notable crustal accretion areas, is famous for its abundant mineral deposits. The geodynamics of the orogeny has been well studied on the regional tectonics, deformation features, and magmatic rocks. However, the tectonic evolution history of the orogeny is still controversial for long time. The Ertix belt is regarded as one of important parts of the Altai orogeny, and its tectonic property and deformation characteristics during the later Hercynian are keys to understand the tectonic evolution of the orogeny.Field investigation revealed series of undeformed acidic dike swarm developed between the Ertix active fault and the Fuyun-Xibodu fault in the eastern segment of the Ertix tectonic belt, Altai orogeny, where Late Hercynian granites and the Upper Carboniferous Ertix Formation outcropped. Because the acidic dike intruded in a special geological setting, and was regarded as the sign of the end of deformation in orogensis, much information on the tectonic evolution of the orogeny was preserved. Therefore, this paper mainly presents geochemical analyses results of acidic dikes and the formation age by U-Pb dating method of zircon in order to discuss the property and the tectonic evolution history of the Altai orogeny.Main achievements in this dissertation are listed as following based on the field investigation and geochemistry measurements and synthetical analyses in laboratory during the past three years.1)The content of SiO2 in dikes is 70.9~75.38%, and that of K2O+Na2O is 7.55~8.99%. For most of samples, the content of Na is larger than that of K. The Rittmann index (δ) varies from 1.8 to 2.5. The content of Al2O3 equals to 12.8~14.53%. The value of A/CNK is 0.9~1.1, and that of NK/A ranges from 0.7 to 0.9, suggesting that the dike should be attributed to A-type granitoids and Sub-alkaline rock with rich of Aluminum and lower Magnesium, higher Kalium and Natrium, lower Calcium and Strontium, and higher n(Fe)/n(Mg).2)Analyses of trace and rare earth elements showed that these rocks are rich of light rare earth elements and relatively rich of great ion lithophile elements with obviously middle minus-Eu abnormity, as well as rich of incompatible elements(Rb, Ba), febrific elements(U,Th) and high field intensity elements(Zr,Hf) and lacking in Nb, Sr, Cr, Ni, indicating obviously fountainhead of crust resource.3)The U-Pb dating of zircon by Second Ion Microprobe revealed the formation age of the dike between 277~286Ma.4)Regional extensional surrounding existed during the Early Permian after the end of Hercynian orogenesis in Altay orogeny.5)The acidic dike resulted from the partial melting of crustal material, and represent the partial extensional surrounding after Hercynian Altay orogenesis.