Geometry And Emplacement Mechanisms Of The Permian Mafic Dyke Swarms In The Eastern And Western Tianshan

Dykes are sheetlike intrusions of magma that cut across crustal fractures in the country rock, and primarily form in the extensional setting. Mafic dyke swarms contain abundant information of deep source and diagenetic process, and are closely related to magmation and magmatic events. A large number of mafic dykes were found in the eastern and western Tianshan, NW China, and many scholars have made a prospective study on geochemistry and isotope chronology of dyke swarms. However, there were few research on geometry and emplacement mechanisms of dykes in these areas. It is difficult to obtain comprehensive data by conventional geological survey because of the remote location and the bad weather of the eastern and western Tianshan, which belongs to the military restricted zones. Thus, this paper mainly discussed the geometry and emplacement mechanisms of dyke swarms in Beishan area and Keping area based on remote sensing images. The main content and conclusions are following:(1)Based on remote sensing images, we extracted 1171 and 117 dykes respectively in Beisahn and Keping area, which both show the dominated NNW-NNE strike and the subparallel arrangement. The average thickness of Beishan dykes is about 7.5m, and the length ranges from 0.3km to 21km with the mean value 3.3km. The average thickness of Keping dykes is about 3.8m, and the length ranges from 0.1km to 17.1km with the mean value 1.8km. The length and thickness distributions of the two areas both have good fit to negative exponent.(2)The research on the shape of dykes in the two areas showed that, the shape of dykes in Beishan area is more complicated and diverse than that in Keping area, indicating that the former dykes were likely formed in the shear-tensional extensional setting, while the latter in the tensional tectonic setting.(3)The estimation of crustal extension of the study area showed, dykes in Beishan area mainly intruded intrusive rocks with the average crustal extension 7.2%, and the mean value of all rocks is 1.9%. Dykes in Keping area mainly intruded sedimentary rocks with the mean value 0.25%.(4)The estimation of the depth of magma source indicated that the majority dykes have a depth of 0.6-40km, and the minority have a depth of 70-100km in Beishan area. The estimation results also indicated that the majority dykes have a depth of 1-30km, and the minority can reach 80-100km in Keping area. We supposed that a magma chamber may be formed at the depth of 80-100km firstly. When magma started to flow, the initial magma chamber may be splitted to form any other chambers by magma migration. The exposed dykes in the study area appeared to be products of emplacement of dykes from multiple magma chambers, and to be principal conduits of magma chamber, sill and basalt.(5) It can be inferred that vertical and lateral emplacement were both recognized in Beishan and Keping area, by comparing the estimation of the depth of magma source and the general characteristics of dyke emplacement.