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Geochemistry and tectonic setting of amphibolites in the Paleoproterozoic Beidahe Group-complex in the western segment of the Qilian Mountains

FU Guo-min1,SU Jian-ping2,HU Neng-gao1,WANG Yun-bin1(1.Chang'an University,Xi'an 710054,Shaanxi,China;2.Institute of Geological Survey,Gansu Province,Lanzhou 730000,Gansu,China)  
The Paleoproterozoic Beidahe Group-complex remains as continental shivers in the interior of the North Qilian orogen.In its early stage of development,there occurred abundant amphibolite.The protolith of the amphibolite is basalts.The Alk-∑FeO-MgO plot,REE features and Rittmann index show that the protolith belongs to the calc-alkaline suite and exceptionally shows the subalkaline character.The rock is characterized by higher Al2O3 and ΣFeO and lower Mg#,indicating that it has a higher degree of compositional evolution and that the formation of volcanic rocks is related to activation of older crust.Highly fractionated REE patterns and amphibolite on the Ti-Zr-Y plot,Ni-FeO/MgO plot and Ta/Yb-Th/Yb plot all indicate the characteristics of volcanic arcs and active continental margins.On the geochemical diagram of basalt,the overall REE features are in the main analogous to those of calc-alkaline island-arc basalt,with Sr87/Sr86=0.71617-0.72033,Nd143/Nd144=0.512414-0.512495 and εNd=-2.79 to-4.37,εSr=245.78-305.3,showing that the magma was derived from the crust or subjected to contamination with the lower crust below the continent.So it may be inferred that the protolith of the amphibolite is the product of different degrees of partial melting of the mantle in a magma arc.This suggests that subduction of a paleo-ocean basin occurred between the Paleoproterozoic North China craton and Qaidam craton.
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