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

摘要：

海相枕状玄武岩分布广泛, 而大规模陆相枕状玄武岩在国内罕见。内蒙古察右后旗陆相枕状玄武岩属上新世汉诺坝玄武岩。枕状玄武岩厚10~12m, 堆积序列自下而上为深湖相灰白色钙质泥岩、 枕状玄武岩、 气孔状玄武岩和块状玄武岩。枕状玄武岩由黑色枕状体和黄色淬碎角砾组成, 枕状体结构完整, 国内罕见。橄榄拉斑玄武岩枕状体, 呈斑状结构, 气孔、 杏仁状及块状构造, 斑晶主要为斜长石、 单斜辉石和橄榄石。基质为玻晶交织结构和间隐-间粒结构, 主要由微晶斜长石、 辉石和玻璃质组成, 含量为70%~85%。枕体间的填充物主要为玄武质淬碎碎屑, 包括集块、 角砾和凝灰级碎屑, 其中凝灰级淬碎碎屑的确定丰富了火山灰的成因类型。海相和陆相均可形成枕状玄武岩, 水是必要条件, 但其形成与水深关系不大, 主要受熔岩的温度和流速控制。水下玄武质熔浆在1150~1000℃时易形成枕状构造, 而低于1000℃则很难形成枕状熔岩, 相对缓慢的流速利于枕状体形成。陆相枕状玄武岩常分布于火山口周围, 属近火口相堆积物, 具有明确的指相意义, 这对恢复火山机构面貌及火山喷发过程具有重要的实际意义。

关键词: 枕状玄武岩, 淬碎角砾岩, 陆相, 近火口相, 察右后旗