北祁连西段肃南地区下志留统肮脏沟组河流相的发现及其大地构造意义

The discovery and the tectonic significance of fluvial facies of the Lower Silurian Angzanggou Formation in Sunan area of the western section of North Qilian Orogen

北祁连地区位于中国中央造山带中段,具有典型造山带的特征。区内志留系肮脏沟组沉积环境存在较大争议,以致对其沉积期古地理格局的认识就存在很大差异。本研究选取志留系肮脏沟组发育齐全的肃南地区老虎沟剖面为研究对象,通过野外剖面实测、室内薄片鉴定和碎屑岩粒度分析等方法对老虎沟剖面肮脏沟组沉积充填序列、沉积构造和沉积相类型进行了细致分析。北祁连肃南地区老虎沟剖面志留系肮脏沟组发育大套的砾岩、砂砾岩和含砾砂岩,沉积构造以大型槽状交错层理、板状交错层理、平行层理和底冲刷构造为主,其中砂岩碎屑的成分和结构成熟度都较低;砂岩段的滚动组分含量较高,纵向剖面上显示为向上远离物源区,粒度变细的演化序列。沉积特征指示北祁连肃南地区老虎沟剖面志留系肮脏沟组的沉积相为辫状河流相,并非前人认为的造山俯冲阶段或者弧陆碰撞阶段的深水海相沉积,而是已经发生陆—陆碰撞形成的陆相沉积。此时北祁连东部地区还未发生陆—陆碰撞,由此可知北祁连在早志留世碰撞为"西早东晚"的不规则点式碰撞。本研究有助于深化对北祁连造山带志留纪的古地理格局和其大地构造背景的认识。

The North Qilian Belt is located in the Center orogenic belt of China,which has the characteristics of typical orogenic belt.The sedimentary environment of the Silurian Angzanggou Formation is controversial,which leads to a great difference in the understanding of paleogeographic pattern in the northern Qilian area.The succession of Silurian Angzanggou Formation from the well-developed Laohugou Profile in Sunan area was measured,and the sedimentary filling sequence,sedimentary structure and the sedimentary facies types of Angzanggou Formation in Laohugou profile were analyzed in detail by outcrop observation,thin section identification and particle size analysis of clastic rocks.It is considered that Silurian Angzanggou Formation in the Laohugou profile of Sunan area,the western North Qilian area develops a large set of conglomerate,glutenite and pegmatite sandstones,the sedimentary structures are dominated by large trough cross-bedding,plate cross-bedding,parallel bedding and scouring structures,and the composition and structural maturity of sandstone fragments are relatively low.The content of rolling components in sandstone member is high.From bottom to top,the particle size becomes finer,indicating the distance from the source area becoming further.In conclusion,the sedimentary facies of Silurian Angzanggou Formation in Laohugou section of North Qilian Sunan area are braided fluvial facies,which indicates that in the early Silurian the mid-Qilian block has already collision with the Alax block.Meanwhile,the eastern part of North Qilian belt has not yet collided.Thus,it can be inferred that the collision of North Qilian in the early Silurian was an irregular point collision,happening in the western part of the orogen earlier than eastern part.This will directly affect the understanding of the Silurian paleogeographic pattern and tectonic setting of the North Qilian Orogenic belt.