敦煌地区晚震旦世-早寒武世沉积地层的发现及其大地构造意义

Discovery of Late Sinian-Early Cambrian sedimentary strata in Dunhuang area and its tectonic significances

近年来,随着敦煌地区越来越多古生代岩浆作用和变质作用等地质信息相继被揭示,许多学者据此提出了"敦煌造山带"的认识,并受到地质学界的广泛关注。本次工作在敦煌地区东北缘发现了大规模露头连续的沉积地层,岩石组合主要为变砂岩和变泥质砂岩,夹少量变砂质泥岩,偶夹变砂砾岩和变硅质岩。通过U-Pb同位素法对该套沉积岩碎屑锆石和侵入其中的花岗岩进行年龄测定,限定其沉积时代为517.3-574.4Ma,属于晚震旦世-早寒武世,由此证明敦煌地区新元古代-古生代沉积地层的存在。综合岩石组合、重矿物特征、岩石地球化学、碎屑锆石结构及年龄频谱等分析,推测该套沉积岩形成于大陆边缘滨海环境,其物源来自敦煌地块甚至塔里木板块前寒武变质基底,揭示了敦煌地区当时应存在着大规模前寒武基底,同时说明敦煌地块的北部古边界应位于疏勒河断裂附近,为确定敦煌地区构造属性、划分区域构造单元提供了关键信息。

In recent years,due to the disclosure of much information of Paleozoic magmatism and metamorphism in Dunhuang area,some scholars have proposed the"Dunhuang Orogenic Belt".In this work,large-scale sedimentary strata with a continuous sequence were discovered in the northeastern margin of Dunhuang area.Lithologically,these strata consist mainly of metamorphotic coarse-to medium-sandstones and muddy sandstones,interlayered with some metamorphotic sandy mudstones and a little pebbly sandstones and siliceous rocks.According to zircon U-Pb dating on the strata and granite intruded in them,the formation age of the strata is limited to 517.3-574.4Ma,belonging to the Late Sinian-Early Cambrian,which proves the existence of the Neoproterozoic-Paleozoic sedimentary cover.Based on the rock and heavy mineral assemblages and geochemistry characteristics of the strata,combined with regional geological information,it can be inferred that these strata should be formed in a continental margin environment,and their sedimentary sources are derived from the Precambrian metamorphic basements of the Dunhuang Massif,or even the Tarim Block,which proved the existence of large-scale Precambrian basements in Dunhuang area in Late Sinian-Early Cambrian.Moreover,combined with regional geological background analysis,these strata should represent the north boundary of the Dunhuang Massif,which provided key information for the determining of the tectonic attribute of the Dunhuang Massif and its regional tectonic unit division.