The Geological Features Of Granitic Rocks And Tectonic Significance In Wujishan Of Baiyin, Gansu Province

Qilianshan is not only the favorable position to study the major Paleozoic collisional orogenic belt in Gansu Province, but also one of the most complex geological structure in China. The article, based on plate tectonic theory and tectonic-magmatism, primarily provides some important scientific accordings as follow:(1) Plagiogranites has typical O-type adakite characteristics, which, with the same characteristics as xiwan’s adakite’s, is rich in Na and depleted in K, high Sr and low Y, rich in LREE and relatively depleted in HREE, with no Eu anomaly, high oxygen fugacity, etc. The melt, with with the initial composition of adakite, is formed by partial melting under high pressure by the subducted slab in the amphibolite facies.(2) The diorite has characteristics of Guangxidiorite:high-P, Ca, Al; high Sr and high Yb, A/NKC<1.1, which are typical I-type granite characteristics and also very similar with ATG granite. Quartz diorite and granodiorite have characteristics of Himalayan granitoid:low Sr and Yb; rich of Si, Al, Na and depleted of K; rich in LREE yet a weak negative Eu anomaly and flat REE ratios; HREE loss, all of which are similar to Jamalca island arc granites. All above imply that residual phase is mainly granulite (garnet and plagioclase is the major constituent minerals) and source rocks may be formed by partial melting of island arc volcanic rocks or island arc volcanic magmatic evolution from homology.(3)In the later of Early Paleozoic normal island arc magmatic rocks are located in the south of the Wuji Mountain, suggesting that the trench located in north of Wujishan, adakite during that time. Because of the northward subduction of Central Qilian during Early Silurian, a comparatively complete trench-arc-basin system was formed in the north of the subduction belt. As the ocean basin had been shrinked very small, there was still oceanic crust northward subduction, suggesting that in the later of the oceanic crust, typical adakites, which represents the closed time limitation of ocean crust, can also be formed of. During Late Silurian-Early Devonian, with the progression of north vs. south compression the residual basin closed, the rigid continental collision happened, and collisional orogenic belt was formed of. Large-scale of north-south thrusting orogeny made the Middle Qilian continental mass above the North Qilian orogenic belt. From south to North, imbricate thrust nappe constitutes the existing fracture tectonic combination melange belt in the collision zone.