Characteristics Of Basite At Jiling Area In Longshoushan And Its Geological Significance

Mafic dikes are the magmatic intrusions that mainly come from asthenosphere or mantle lithosphere on the background of continental stretch, and as an important symbol of the lithosphere（or crust） stretch, mafic dikes have important significance on deep source mineralization. Jiling area is located in central Gansu province, which is the important part of Longshoushan-Qilianshan uranium metallogenic belt. Geological research in Jiling area began in the 1950 s, though the basite outcropping were found very early in previous geological work, which focused on uranium geology prospecting, the study of basite at Jiling area is rare. Therefore, this article selected basite of Jiling area as the study object, to discuss the petrogenesis, source characteristics, tectonic environment and the possible relationship between basite and uranium mineralization on the basis of the study on petrology and geochemistry characteristics of basite. The main conclusions obtained are as follows:（1）By the research on mineralogy and petrology of basite, make sure the type of rocks is diabase.（2）SiO₂ content of basite is 47.09%⁵1.13%, Al2O3=14.56%¹5.97%,Na2O=3.35%³.69%,K2O=0.347%¹.23%,high Al,content of Na is rich and content of K is low; all the samples fall within the range of sub-alkaline basalt in the TAS and Zr/ TiO2-Nb/Y discrimination diagram. Magnesium index（49.52⁵5.51） and consolidation index（25.93³0.39） are smaller than the levels of original basaltic magma, indicating basite in Jiling area have evolutionary basalt’s characteristics. And the fractional crystallization diagram of monoclinic pyroxene shows that mafic magma experienced fractional crystallization, especially the Clinopyroxene.（3）The total amount of REE（ΣREE） is low, with the right type of LREE distribution patterns. The average（La/Sm） N is 2.20; mean（Gd/Lu） N is 2.30. Both LREE and HREE have obvious fractionation effect and the degrees of fractionation are same. The δEu value is 0.97¹.08, and δCe value is 0.97¹.01. There is no Eu and Ce anomaly. Basite’s loss elements are REE, Rb, Th and the enriched elements are Ba, P, and Ti. REE patterns and trace element spider diagram have a uniform evolution trend and pattern, indicating they are the products of comagmatic evolution.（4） The Sr, Nd, Pb isotopes and trace elements discrimination diagram shows that the basite was formed in extensional environment of intraplate belt, and the magma source area of basite should be mix EMI and EMII enriched mantle, it has the crustal materials mixed in magma source area.（5）This paper shows that basite and uranium ore bodies have a close space-time connection. Some ore-forming fluid has the same source with the mafic magma. The basite provides part of space for the uranium precipitation and enrichment.