Geochemical Characteristics of Neoproterozoic Gabbro from Cuolonggou Area in North Qilian

The basic intrusive rocks represent the extensional and rifting event of continental crust, which can provide the time constraints for the geodynamics study of continental reengineering and orogenic processes.By using the LA-ICP-MS dating method, the obtained U-Pb age of gabbro from Cuolonggou area is (675±31)Ma, suggest that these gabbro was formed in Early Sinian Epoch.Main elements features of these gabbro rocks reflect poor obvious alkali rich magnesium.These rocks have low total rare earth, most of these samples' ∑ REE ranges from 10.19×10^-6 to 21.71×10^-6, a small part of rocks reflect weak light rare earth concentration, but most of them reflect the obvious fractionation of light and heavy rare earth.The δEu values vary from 1.39 to 2.46, all of them are larger than 1, showing the feature of abnormal europium.The Sm/Nd ratios are 0.26~0.41, suggesting that the magma composition of gabbro is consistent with the ones of mantle source rock.Combined with the tectonic environment discrimination diagrams, the gabbro in this area has been classified as the product of mainland or marginal initial rift environment.