西天山伊犁地块早石炭世典型沉积序列及对天山古生代洋陆转换时限的制约

Abstract: Early Carboniferous volcanic-sedimentary rocks are widespread in the Yili block, western Tianshan. They are the most direct carrier in the study of Early Carboniferous palaeogeography of western Tianshan and Paleozoic orogenic evolution of Tianshan. Furthermore, fine study on which is expected to more precisely limits the closure time of the Paleozoic oceanic basin in Tianshan. Preliminary study shows that there are typical non-marine coarse clastic rocks developed in the volcanic-sedimentary sequence, and its sedimentary sequences consist of, from the bottom to the top, conglomerate, coarse sandstone, sandstone, siltstone, mudstones and carbonate rocks, accordingly, the sedimentary environment changes from fan-delta at the base to shallow sea in the upper. These suggest that there may be significant paleogeographic environment change events which extremely confine the time limit of the closing of Tianshan Paleozoic oceanic basin during the Early Carboniferous. This project intends to focus on sedimentary sequence in Yili block. In addition, based on comprehensive study of the sediment composition, sedimentary facies, provenance, paleontology, times, etc., and combined with the research on petrology and geochemistry of the volcanic strata in the sedimentary rocks, which will be helpful to completely reveal the process of the Early Carboniferous sedimentary evolution in the Yili block, we will establish the combination of typical sedimentary sequences and investigate the tectonic setting in the Yili block during the Early Carboniferous. Furthermore, in conjunction with previously data, we will provide basic sedimentary constraints on the Paleozoic ocean-land transition period of Tianshan orogenic belt.

摘要：西天山伊犁地块上广泛出露的一套早石炭世火山-沉积岩系，是研究西天山早石炭世古地理格局和天山古生代造山作用演化过程最为直接的载体；通过对其进一步的精细研究可以较为精确的限定天山古生代洋盆闭合时限。初步研究表明，伊犁地块早石炭世火山-沉积岩系发育十分典型的非海相粗碎屑岩，自下而上依次发育砾岩、粗砂岩、砂岩、粉砂岩、泥岩、碳酸盐岩，沉积环境经历了由下部扇三角洲环境到上部浅海环境的转变。这些暗示着伊犁地块在早石炭世时期可能存在重大的古地理格局变迁事件，该事件极有可能是天山古生代洋盆闭合的时限。本项目拟以伊犁地块早石炭世沉积序列为重点，通过对沉积相、沉积物组成、物源、古生物、形成时代等方面的综合研究，并结合与沉积岩共生火山岩岩石学和地球化学研究，完整地揭示伊犁地块早石炭世沉积演化过程，建立典型沉积组合序列，探讨其形成构造环境，为天山造山带古生代洋陆转换时限提供基本沉积地质约束。

西天山伊犁地块早石炭世火山—沉积岩系，是研究西天山早石炭世盆地性质和天山古生代造山作用演化过程最为直接的载体。本项目通过大量剖面测制，建立了伊犁地区早石炭世区域地层格架；沉积学和层序序列分析表明，早石炭世典型沉积组合序列为：自下而上依次发育砾岩、粗砂岩、砂岩、粉砂岩、泥岩、碳酸盐岩，沉积物由粗到细、水体由浅到深，沉积环境由陆相变为海相；盆地充填序列及演化旋回阶段性表现为：大哈拉军山组底部扇三角洲相、向上出现2次爆发相—喷溢相的火山喷发旋回，阿克沙克组底部扇三角洲相，向上过渡为滨海相—开阔台地相—滨浅海相，这反映了一种伸展—裂陷的动力学背景。结合早石炭世底部区域性角度不整合特征，本项目认为天山古生代洋陆转换时限在晚泥盆—早石炭世之交，早石炭世开始，天山造山带进入后碰撞裂谷演化阶段。以伊犁地块早石炭世沉积序列为重点，通过对沉积相、沉积物组成、物源、古生物、形成时代等方面的综合研究，并结合与沉积岩共生火山岩岩石学和地球化学研究，结果表明：西天山伊犁地区早石炭世火山—沉积建造形成于后碰撞裂谷构造环境。本项研究对于解决天山乃至整个北疆地区早石炭世构造环境及古生代洋盆闭合时限，确定洋陆转化过程具有重要的科学意义。