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GEOCHEMISTRY CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF THE HAERTEERDELE GRANITE MASS IN MIDDLE DEVONIAN IN BEISHAN,GANSU

JI Bo;YU Ji-yuan;GUO Lin;GUO Lei;BU Tao;Research Center for Orogenic Geology,Xi'an Center of Geological Survey,Geological Survey of China;Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,Ministry of Land and Resources,Xi'an Center of Geological Survey,Geological Survey of China;  
The Haerteerdele granite mass in Niuquanzi shows a high content of Si(72.2%),alkaline(AR=2.9~5.1,K2O=3.86%~5.49%,Na2O=3.04%~3.83%) and FeOT/Mg(3.29~13.47),and a low content of Al_2O_3(11.48%~13.42%),CaO(0.57%~2.27%),Mg O(0.14%~0.78%) and Fe OT(1.33%~2.34%).It is determined that the granites are peraluminous granitoids of the calc-alkaline suite(A/NK=1.43~1.77,A/CNK=1.35~1.4),with the characteristics that an enriched pattern with a high ΣREE content and a slight high LREE content((La/Yb) N=4.9~16),unapparent fractionation of heavy and rare earth elements,obvious Eu negative abnormality(δEu=0.12~0.21),relatively large concentration of large ion lithophile elements such as Rb,K,Pb,strong depletion of the elements of Ba、Sr、P、Ti,Eu,and weak depletion of Ta,Nb and other elements.Also the granites have the similar value in Nb/Ta and Nd/Th with the earthcrust,showing the characteristic of type A(A2) granite.The chronology result of LA-ICP-MS zircon dating shows the age of~(206)Pb/~(238)U of the Haerteerdele granite mass is 371.7 ± 2.9Ma,which represents the formation age of the granite mass and the formation period is the middle Devonian.Taking into consideration the regional tectonic evolution and the comparison with the contemporary type A granite,it shows that the granite mass is the product of postcollision to extension.
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