赞比西深海扇:富含长石的被动边缘浊积岩的矿物学、REE、Zr/Hf、Nd同位素和锆石年龄变化

IF 2 4区 地球科学 Q1 GEOLOGY
E. Garzanti, G. Bayon, P. Vermeesch, M. Barbarano, G. Pastore, A. Resentini, B. Dennielou, G. Jouet
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引用次数: 1

摘要

基于中更新世至全新世浊积岩的岩相、重矿物、元素地球化学、同位素地球化学和碎屑锆石地质年代分析,我们首次对赞比西深海扇进行了全面的物源研究。赞比西河谷和扇代表了一个约5000公里长的沉积物路由系统的海底部分,从南非高原的中心延伸到印度洋的深海。沉积物不仅来自非洲一侧,而且主要通过Tsiribihina山谷来自马达加斯加岛。莫桑比克海峡两岸提供的碎屑被两个切开的裂谷边缘剥离,具有相似的富含长石的长石-石英组成,尽管在重矿物和地球化学特征方面存在显著差异。马达加斯加砂的εNd值明显更负,TNd模型年龄明显更老。锆石颗粒在非洲衍生砂中产生的U-Pb年龄主要为Irumide(晚斯坦阶),在马达加斯加衍生砂中形成的U-Pb时代主要为泛非(埃迪卡拉纪-成冰纪),也产生了一些与古太古代一样古老的颗粒和许多不和谐的年龄,反映了泛非对太古代克拉通岩石的改造。Lower Valley和Lower Fan矿床具有中等指纹,这表明马达加斯加的沉积物供应量并不比非洲少多少,尽管集水区要小得多,这可以解释为非洲衍生沉积物的显著部分沉积在赞比西大陆架、贝拉高地和艾斯岛之间的中间盆地。通过假设第四纪海底沉积物和现代赞比西河沉积物之间的成分差异主要是由大坝蓄水造成的,我们可以评估人为因素对自然沉积物通量的影响。第四纪浊积岩的石英含量略高,重矿物含量较低,具有较高的耐用ZTR物种相对量,并且产生比现代河砂更多的埃迪卡拉纪、新太古代和石炭纪碎屑锆石年龄。具有中间盆地样本特征的奥罗西亚峰表明,在中更新世,赞比西流域的中部和上部有显著的供应。粗略计算表明,赞比西大坝前沉积物在上游流域产生≤10%,在中间流域产生约60%,在下游流域仅产生≥30%,这提供了今天到达印度洋的全部沉积物。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
The Zambezi deep-sea fan: mineralogical, REE, Zr/Hf, Nd-isotope, and zircon-age variability in feldspar-rich passive-margin turbidites
We here present the first comprehensive provenance study of the Zambezi deep-sea fan, based on integrated petrographic, heavy-mineral, elemental-geochemistry, isotope-geochemistry, and detrital-zircon-geochronology analyses of middle Pleistocene to Holocene turbidites. The Zambezi Valley and Fan represent the submarine part of an ∼ 5000-km-long sediment-routing system, extending from the heart of the South African Plateau to the abyssal depths of the Indian Ocean. Sediment is derived not only from the African side, but also from Madagascar Island mostly via the Tsiribihina Valley. Being shed by two dissected rifted margins, detritus supplied from opposite sides of the Mozambique Channel shares similar feldspar-rich feldspatho-quartzose composition, although with significant differences in heavy-mineral and geochemical signatures. The εNd values of Madagascar sand are markedly more negative and TNd model ages notably older. Zircon grains yield mostly Irumide (late Stenian) U-Pb ages in African-derived sand and mostly Pan-African (Ediacaran–Cryogenian) U-Pb ages in Madagascar-derived sand, which also yields a few grains as old as Paleoarchean and many discordant ages reflecting Pan-African reworking of Archean cratonic rocks. Lower Valley and Lower Fan deposits have intermediate fingerprints, indicating that sediment supply from Madagascar is not much less than from Africa despite a much smaller catchment area, which can be explained by deposition of a conspicuous part of Africa-derived sediment in the Intermediate Basin confined between the Zambezi Shelf, the Beira High, and the Îles Éparses. By assuming that compositional differences between Quaternary submarine deposits and modern Zambezi River sands primarily resulted from sediment impoundment by large dams, we could evaluate the anthropogenic impact on natural sediment fluxes. Quaternary turbidites are somewhat higher in quartz and poorer in heavy minerals with higher relative amounts of durable ZTR species, and yield more Ediacaran, Neoarchean, and Carboniferous detrital-zircon ages than modern river sands. The Orosirian peak characterizing the Intermediate Basin sample points to prominent supply from the middle and upper parts of the Zambezi catchment in the middle Pleistocene. Rough calculations suggest that pre-dam Zambezi sediments were generated ≤ 10% in the upper catchment, ∼ 60% in the middle catchment, and only ≥ 30% in the lower catchment that provides the totality of sediment reaching the Indian Ocean today.
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来源期刊
CiteScore
3.80
自引率
5.00%
发文量
50
审稿时长
3 months
期刊介绍: The journal is broad and international in scope and welcomes contributions that further the fundamental understanding of sedimentary processes, the origin of sedimentary deposits, the workings of sedimentary systems, and the records of earth history contained within sedimentary rocks.
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