关键词: 枕状玄武岩, 淬碎角砾岩, 陆相, 近火口相, 察右后旗

Abstract:

Marine pillow basalts are widely developed, while large-scale continental pillow basalts are especially rare in China. The continental pillow basalt newly discovered in Chahar Right Back Banner, Inner Mongolia, is mainly tholeiite and a part of the Hannuoba basalt in Pliocene. In the accumulation sequence, from the bottom to the top, there are grayish-white calcareous mudstone of deep lacustrine, pillow basalt, stomatal basalt, and massive basalt. The pillow basalt has the thickness of about 10~12m and is mainly composed of black pillow body and yellow quenched clastics. The pillow bodies are preserved well and rare in China with complete structure. In detail, most of them are cylindrical, long ellipsoidal, of different sizes, about 0.8~1.5m long, and the largest pillow is about 2m long. Most of the cross sections are nearly circular, with a diameter of about 0.6m, up to 1m. The pillow bodies have obvious concentric layered structure, which can be divided into crust, middle layer and core. The degree of crystallization gradually becomes better from outside to inside. The crust is glassy, the middle layer is mesocrypt structure, and the core has relatively good crystallization, which is of intergranular-intersertal structure. Radial and discontinuous concentric ring fractures often occur in the pillow bodies, of which radial fractures are the most developed. The number of fractures varies from 10 to 20, with a width of 3~5mm. Most of them are filled with calcium and silica. In terms of composition, the pillow body is mainly olivine tholeiite, with porphyritic texture, stomatal-almond and massive structures. Phenocrysts are mainly plagioclase, clinopyroxene and olivine. Plagioclase is in the shape of self-shaped and plated strip, with the size of 1~3mm, the length-width ratio of 3︰1 to 5︰1, and the content of polysynthetic twin is 10%~15%; Clinopyroxene is short columnar, with a size of 0.6~1mm and a content of 5%~8%; Olivine is granular, with the size of 1~2mm and the content of 3%~5%. The matrix is composed of glass-based interlaced structure and intergranular-intersertal structure. It is mainly composed of microcrystalline plagioclase, pyroxene and glassy, accounting for 70%~85%. The basalt has SiO₂ of 52.84% and(Na₂O+K₂O)of 5.46%, belonging to calc alkaline rock(Rittman index σ=3.0<3.3), with obvious fractionation of light and heavy rare earth elements(LREE/HREE=17.52, La_N/Yb_N=24)and weakly Eu negative anomaly(δEu=0.89), enriching large ion lithophile elements(Rb, Sr, Ba, etc.). The pillow bodies are mainly filled with calcareous cemented basaltic quenched clastics, including agglomerate, breccia, and tuff grades, mainly orange basaltic glassy quenched breccia. The determination of tuffaceous quenched clastics enriches the genetic types of volcanic ash. The cements are mainly calcareous and siliceous precipitated by hydrochemistry, a small amount of clay minerals and gypsum can be seen locally. The quenched breccia also contains some calcareous mudstone fragments. It shows that both marine and continental facies can form pillow basalt, and water is a necessary condition, but its formation is not related to water depth, and it is mainly controlled by the temperature and velocity of lava. When the temperature of underwater basaltic magma is between 1150℃ and 1000℃, pillow structure is easy to form, but it is difficult to form pillow lava below 1000℃, and relatively slow velocity is conducive to the formation of pillow body. Continental pillow basalts are usually distributed around craters, which belong to near-crater deposits. They are of definite significance of facies, which is of great practical significances for remodeling the morphology of continental volcanic edifice and studying the volcanic eruption process.

Key words: pillow basalt, quenched breccia, continental facies, near-crater facies, Chahar Right Back Banner