Late Ordovician Tectono-Sedimentary Evolution of the Changma Area, Forearc Basin in the Western North Qilian Orogen
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摘要:
北祁连早古生代多岛洋盆的古地理格局和构造演化一直存在争议。位于北祁连南、北蛇绿岩之间的昌马地区广泛出露了一套早古生代深水火山-碎屑建造。早期填图工作普遍认为其形成于寒武纪至早奥陶世。本次研究对昌马西部的鹰嘴山和车路沟山南侧剖面开展系统的野外地质调查。并对采集到的浊积岩和火山岩样品开展了锆石LA-ICP-MS U-Pb定年工作。研究结果表明，原填为寒武系a岩组的鹰嘴山剖面所采集的砂岩样品2307NQL-13最年轻的锆石给出了（456±4） Ma的加权平均年龄；车路沟山南侧，原划归为下奥陶统阴沟群的安山岩样品2307NQL-06给出了（450±4） Ma的加权平均年龄，证明研究区大面积存在晚奥陶世沉积地层，现有年代地层方案需要重新审视。砂岩样品中的碎屑锆石以寒武纪—奥陶纪年龄为主，主要源于岩浆弧的剥蚀；32颗新元古代和古元古代碎屑锆石揭示古老基底物质的大量加入，表明晚奥陶世昌马地区沉积了大量来自中祁连的碎屑物质。来自中祁连基底的碎屑物质暗示北祁连洋的南分支已经闭合；碎屑物质形成于俯冲相关构造环境暗示北祁连洋的北分支俯冲仍在继续。本次研究给出的新数据将为昌马地区进一步的地层和沉积演化提供可靠的同位素年代约束，也为探讨北祁连洋早古生代复杂的俯冲、闭合过程提供重要依据。
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关键词:
- LA-ICP-MS U-Pb年代学 /
- 晚奥陶世 /
- 弧前盆地 /
- 北祁连洋 /
- 昌马地区
Abstract:The paleogeography and tectonic evolution history of the Early Paleozoic archipelago-type North Qilian Ocean remain controversial. A set of Early Paleozoic deep-water clastic-volcanic succussion are well exposed in the Changma Area between the north and south ophiolitic belt of the North Qilian Orogen. Previous geologic mappings generally assigned a Cambrian to Early Ordovician stratigraphic age for these strata. In this study, we conducted systemic field-based investigations on outcrops in the Yingzuishan and Chelugou Section in the western Changma, and conducted zircon LA-ICP-MS U-Pb datings on collected turbidites and volcanics. Geochronologic results indicate that the sandstone sample 2307NQL-13 from the previously termed Cambrian Number a unit of the Yingzuishan Section yielded a well-defined weighted mean age of (456±4) Ma for the youngest detrital zircons. An andesite sample 2307NQL-06 from the previously termed Early Ordovician Yingou Formation yielded a well-defined weighted mean age of (450±4) Ma. These new data solidly demonstrated the existence of Late Ordovician sedimentary strata in the Changma Area, requiring further consideration of previous mapping proposals. Detrital zircons from sandstones are mostly of Cambrian-Ordovician ages, indicating major sedimentary derivations form newly-formed magmatic arcs. Appearance of 32 Neo- to Paleoproterozoic aged zircons imply additional supply form metamorphic basement complexes, revealing depositor of detritus from the Central Qilian. This clue hints at that the southern branch of the North Qilian Ocean should have been closed in Late Ordovician to allow sedimentary materials from the Central Qilian to be transported to and deposited in the Changma Area. The subduction-related depositional setting indicates that ongoing subduction of the oceanic crust of the northern branch should sustain until then. These clues provide new geochronologic data and related observations on stratigraphy and sedimentary evolution history of the Changma Area, and yield further constraints on the composite Early Paleozoic subduction-closure history of the North Qilian Ocean.
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Key words:
- LA−ICP−MS U−Pb dating /
- late Ordovician /
- forearc basin /
- north Qilian Ocean /
- Changma area
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图 7 锆石微量元素Th/U-Nb/Hf判别图解（a） 、锆石微量元素Th/ Nb-Hf/Th判别图解（b）（据Yang et al., 2012） 、研究区地壳厚度随锆石年龄的变化图（c）
Figure 7.
图 8 早古生代北祁连洋构造演化模式图（据 Dong et al., 2021修）
Figure 8.
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