塔里木盆地中央隆起带:新元古代造山型基底拼合带——来自深钻孔的碎屑记录证据

Central uplift of Tarim Basin being the Neoproterozoic basement structure zone of orogeny origin: Evidences from clastic samples of deep holes

在塔里木盆地巴楚隆起,新近有同1井、夏河1井和巴探5井在寒武系碳酸盐岩之下钻遇碎屑沉积岩。在对碎屑岩进行详细岩相学观测、碎屑组分统计和地球化学分析的基础上,深入探讨了碎屑岩源区及其形成的构造环境。砾岩和砂岩碎屑组成与泥岩地球化学特征共同表明,这3口井寒武系碳酸盐岩之下的碎屑岩物源区包括沉积岩、火成岩和变质岩,以中酸性火山岩和侵入岩为主。砾石及砂岩中碎屑颗粒棱角状-次圆状的外貌、砾石中易磨蚀风化组分(碳酸盐岩和片岩)的存在以及泥岩较低的Th/U比值都反映这些碎屑岩为成熟度较低的近源堆积物。通过碎屑成分、地球化学特征和碎屑锆石年龄结构对比,确定这些碎屑岩的碎屑组分主要源自其下伏基底。砾岩和砂岩碎屑模式、泥岩地球化学特征及碎屑锆石原位微量元素分析结果表明,碎屑源区(即下伏基底)在新元古代(峰期约790～730 Ma)处于大陆边缘弧构造环境。本文新获取的碎屑记录证据与前人对同1井底安山岩、英安岩及卡塔克隆起上塔参1井底闪长岩、花岗闪长岩形成时代及构造环境的研究结论完全一致,表明横亘塔里木盆地中部的中央隆起带是一条新元古代大陆边缘弧,塔里木盆地的基底是由新元古代造山作用形成的拼合基底。

Three deep holes(Well TX1,XH1 and BT5)encountered clastic sedimentary rocks underlying the Cambrian carbonates in the Bachu uplift of the Tarim Basin.After detailed study of these clastic rocks through petrographic observations,clastic composition statistics and geo-chemical analyses,their provenances and the tectonic setting for their formation were discussed in depth.The clastic compositions of conglomerates and sandstones together with the geochemical characteristics of mudstones indicate that the provenances of the clastic sedimentary rocks underlying the Cambrian carbonates in the three wells include sedimentary rocks,igneous rocks,metamorphic rocks,among which the intermediate-acidic volcanic rocks and intrusive rocks are predominant.The angular-to-sub-rounded appearance of the clastic particles in the conglomerates and sandstones,the presence of erodible compositions in gravels(such as carbonates,schists)and the lower Th/U ratio of mudstones collectively show that these clastic rocks are near-source deposits with low maturity.By comparing the clastic composition,geochemical characteristics,and age structure of detrital zircons,we could determine that the detrital components of these clastic rocks are mainly derived from the underlying basement.The detrital models of conglomerates and sandstones,geochemical characteristics of mudstones and in-situ trace element characteristics of detrital zircons suggest that the source area,that is the underlying basement,was located in an arc-related tectonic setting on the active continental margin in the Neoproterozoic(peak:790-730 Ma).The newly obtained evidences of clastic rocks are well consistent with the previous studies on the age and tectonic setting of the andesites and dacites at the bottom of Well Tong-1 and the granodiorites and diorites at the bottom of Well Tashen-1 in the Katake uplift,demonstrating that the central uplift zone(Bachu uplift-Katake uplift-Guchengxu uplift)across the central Tarim Basin is a Neoproterozoic continental marginal arc,while the basement of the Tarim Basin is the result of amalgamation caused by Neoproterozoic orogeny.