松辽盆地东南隆起区营城组一段熔浆胶结复成分砾岩特征及其成因分析

Characteristics and Genetical Study of the Magma-cemented Polymictic Conglomerate in the Cretaceous Yingcheng Formation,Southeastern Uplift Area of the Songliao Basin

对野外剖面实际测量及室内薄片鉴定等资料研究表明,松辽盆地营城组一段复成分砾岩共有4种胶结成岩类型:熔浆胶结、灰泥球胶结、凝灰质胶结和砂泥质胶结。其中以熔浆胶结发育最为广泛。砾岩中砾石成分复杂,以基底花岗岩及二叠系变质岩为主,原地火山岩次之。砾石整体分选性差,磨圆度中等—好,以次圆状—次棱角状最为发育。自基底向火山活动中心,砾岩中外来砾石逐渐减少,原地火山岩砾石逐渐增多。基质成分由底部的砂泥质向上过渡为凝灰质并最终完全过渡到熔浆质胶结。砾岩是冲积扇或辫状河等沉积环境下已经堆积但未固结或半固结的砂砾质沉积物,在火山活动期被火山碎屑流或熔浆流再搬运,这种碎屑流、熔浆流和水介质构成的多相混合物通过岩浆冷凝和压实复合成岩作用而形成的岩石,实质是爆发相热碎屑流亚相的另一种表现形式。由于该类砾岩主要见于断陷盆地边缘火山岩序列下部靠近基底一侧,说明其形成与构造-岩浆活动密切相关,是火山旋回早期所特有的产物。

According to field geological section survey and in door thin-section identification, polymictic conglomerate of the 1st member of the Yingcheng Formation contain 4 types of cementation： magma, micrite, tuffaceous and sand and politic. Magma cementation is a main cementation type in the Songliao Basin. The gravels in the polymitictic conglomerate are quite complex. Most of gravels are granite derived from the base of the Songliao Basin and from Permian metamorphic rocks. In-situ volcanic rocks are secondary in abundance. The gravels are poorly sorted. Their psephicity is from moderate to fine, with sub-round and sub-angular shapes being mostly common. From base to active volcanic center, the amount of the mexotic gravels of polymictic conglomerate is decreasing, and the in-situ volcanic gravels are increasing gradually. Matrix composition is also mulriple. At bottom it is argillo arenaceous, changed into trffaceous upwords and became magmatic at the top. Unconsolidated or semi-consolidated sand-gravel deposits accumulated in some sediment environments such as alluvial fan, braided river and so on, were carried by pyroclastic flow or lava flow during volcanic eruption periods to form mixtures with pyroclastic flow, lava flow and waters. After cooling and compaction, the polymitic conglomerates were formed as a result. Since the conglomerates were often found near the bottom of the volcanic sequence at the fault basin brink, it reveals that the origin of conglomerate is relative both to tectonic activity and volcanic activity, and they are special products at the early stage of the volcanic cycle.