敦煌造山带南部红柳峡混杂带基质的沉积学、地球化学和年代学特征及其大地构造意义

敦煌造山带南部红柳峡混杂带基质的研究,为认识敦煌造山带的形成和演化提供了新的依据。本文从沉积学、地球化学和年代学等方面系统讨论了该混杂带基质的特征和形成环境。结果显示,基质的岩石类型主要包括变泥质岩(云母石英片岩)和变质砂岩,普遍发生强烈变形。局部弱变形变质的基质仍保留有原生沉积构造(如T_(ab)、T_(de)、T_(bde)组合的鲍马序列),反映原岩是一套浊积岩复理石。显微岩相学特征显示,基质碎屑组分以长石、石英和岩屑为主,长石和岩屑含量较高,分别为47%和27%,反映大量火成岩物质的加入,且碎屑颗粒的分选性和磨圆度较差,说明搬运距离较近。地球化学方面,低的化学蚀变指数(CIA=49~67),反映复理石基质物源区母岩经历的风化程度较低。高的成分变化指数(ICV0.8)以及Zr/Sc-Th/Sc投图结果显示,沉积物再循环程度低,为近物源区的初次沉积。基质Sc、Cr、Co、Ni含量低,Eu/Eu*、La/Sc、Th/Sc、La/Co、Th/Co和Cr/Th等元素比值类似于来自长英质源区的沉积物,暗示其物源区母岩以中-酸性岩石为主。La/Sc-Ti/Zr和Th-ScZr/10投图结果显示,复理石基质形成于陆缘弧或活动大陆边缘构造背景。弱变形浅变质砂岩的碎屑模式表明,基质的物源来自"切割型弧-过渡型弧"源区。综上,红柳峡混杂带基质在碎屑组成方面,以再循环程度低、近物源堆积的"切割型弧-过渡型弧"源区长英质碎屑组分为主,在沉积构造方面,发育鲍马序列和深水块体搬运沉积(MTD)构造,表明基质形成于陆缘弧或活动大陆边缘的俯冲带海沟环境。碎屑锆石年代学显示三组年龄:2300Ma、1850Ma和423Ma,结合区域地质背景分析,初步认为物源碎屑可能来自混杂带北侧的三危山弧和东巴兔-蘑菇台弧的古生代花岗岩类以及俯冲折返的变质基性岩岩块。复理石基质的变质砂岩中获得的最年轻的岩浆碎屑锆石年龄为389Ma,说明该砂岩形成于中泥盆世之后,暗示敦煌造山带南部红柳峡地区洋盆尚未俯冲完毕,碰撞作用尚未开始。

Sedimentology, geochemistry, geochronology, and tectonic implication of the matrix in Hongliuxia mélange, southern Dunhuang Orogenic Belt

The integrated investigation of the matrix in Hongliuxia mélange, southern Dunhuang Orogenic Belt (DOB), provides new evidences for understanding the formation and evolution of the DOB. In this contribution, sedimentological, geochemical, and geochronological studies were undertaken on the matrix for constraining its tectonic setting formed. The matrix in Hongliuxia mélange dominantly consists of sheared metapelite (mica-quartz schists) and meta-sandstones. Locally, some of the matrix preserves the original sedimentary structures (such as T_ab, T_de, T_bde divisions of Bouma sequence). This indicates the protolith of the matrix is turbidite flysch. In detrital composition, the matrix are composed of predominate feldspar, quartz, and lithic fragment. High proportion of feldspar (47%) and lithic fragment (27%) indicate abundant magmatic-derived clasts were involved. The clasts are poorly sorted and rounded, suggesting a short distance before deposited. Geochemically, Low CIA values (49~67) of the matrix indicate low-degree of weathering in the provenance. High ICV values (mostly>0.8) and the Zr/Sc-Th/Sc diagram reflect the sediments were not significantly recycled before deposited. The matrix show low Sc, Cr, Co, Ni contents, and similar Eu/Eu^*, La/Sc, Th/Sc, La/Co, Th/Co, and Cr/Th ratios with the clasts shed from the felsic rocks provenance. This indicates the source rocks comprise dominant intermediate-acidic rocks, though few mafic rock clasts involved. The La/Sc-Ti/Zr and Th-Sc-Zr/10 plotting of the matrix suggest they form in the continental arc or active continental margin tectonic setting. Framework modes of slightly deformed and low-grade metamorphosed sandstones indicate the clasts of matrix predominantly derive from the "dissected-transitional arc" provenance. To some up, the composition of low recycled, near sourced "dissected-transitional arc" derived clasts, accompanied by the sedimentary structure of mass-transport deposit (MTD), manifest the matrix deposit in the trench associated with the subduction zone along continental arc or active continental margin. The detrital zircons from the matrix show three age groups:2300Ma, 1850Ma, and 423Ma. Combined with the regional geological background, we preliminary speculate the clasts probably shed from the Paleozoic granitoid and exhumed metamafic rocks in the Sanweishan arc and Dongbatu-Mogutai arc, north to the Hongliuxia mélange. The meta-sandstones of the flysch matrix yield the youngest magmatic detrital zircon age of 389Ma, indicating the protolith (sandstone) formed after Middle Devonian. This further indicates the ocean basin of the Hongliuxia area had not been closed at Middle Devonian and the collision hadn''t initiated at that time.