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四合屯 大康堡地区所出露的义县组火山 沉积岩可分为上、中、下 3个层位。下部层位是以玄武质、玄武安山岩质以及安山质火山岩类为主的基性、中基性火山 沉积岩 ;中部是以流纹质及英安质火山岩类为主的中酸性火山 沉积岩 ;而义县组的上部层位是以玄武岩、玄武玢岩为代表的基性火山岩。薄层状的沉凝灰岩、凝灰质粉砂岩及钙质页岩等作为火山活动间歇期的产物 ,常不同程度地发育于义县组火山熔岩或集块岩的底部。野外工作发现 :在义县组下部层位的沉凝灰岩层中发育有以鹦鹉嘴龙为主的古脊椎动物类化石 ;四合屯 大康堡一带富含有热河生物群化石的沉积层属义县组火山 沉积岩系的中部层位。稀土元素分析表明 ,在相同层位的火山 沉积岩内 ,沉凝灰岩、凝灰质粉砂岩和钙质页岩及其与火山熔岩存在密切的相关性 ;而不同层位间则存在一定的差异性。稀土元素配分特征还反映出 ,不同的义县组火山 沉积岩层位之间具有相似的物源关系。
The volcanic sedimentary rocks of Yixian Formation exposed in the DaKangBao area of Sihetun can be divided into upper, middle and lower strata. The lower horizon is basaltic, intermediate-basic volcanic sedimentary rocks mainly basalt, basaltic andesite and andesitic volcanic rocks. The middle part is moderately acidic volcanic sedimentary rocks mainly composed of rhyolite and amphibolite. The upper strata of Yixian Formation are basic volcanic rocks represented by basalt and basaltic rock. Thin tuffaceous tuffaceous tuffaceous siliceous rocks and calcareous shales, as the product of the intermittent volcanic activity, often develop to the bottom of the volcanic lavas or agglomerates of the Yixian Formation to varying extents. Field work found that in the lower layers of the Yixian Formation, tuffaceous fossils, mainly composed of psittacotes, are developed in the sedimentary tuff stratum of the Lower Member of the Yixian Formation. Sediments of the Fankangpu area, Yixian Formation volcanic sedimentary rock series of the middle layer. REE analysis shows that tuff, tuffaceous siltstone and calcareous shale are closely related to volcanic lava in volcanic sedimentary rocks of the same horizon; however, there are some differences between different layers. The distribution characteristics of REEs also reflect similar source-related relationships between different volcanic sedimentary layers in the Yixian Formation.