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Combining detrital zircon shape and U–Pb–Hf isotope analyses for provenance studies – An example from the Aquiri region, Amazon Craton, Brazil
Precambrian Research ( IF 3.8 ) Pub Date : 2021-08-22 , DOI: 10.1016/j.precamres.2021.106343
Armin Zeh 1 , Alexandre Raphael Cabral 2, 3
Affiliation  

Provenance studies are commonly based on in situ U–Pb dating and Hf isotope analyses of detrital zircon grains, while leaving shape parameters unconsidered, even though these can provide valuable information about the hinterland and detritus transport. This is demonstrated herein by presenting a comprehensive data set derived from approximately 950 detrital zircon grains from three quartzite samples of the Cuxiú Formation of the Aquiri region, which is part of the eastern realm of the Amazon Craton. The data set combines results of zircon U–Th–Pb–Hf isotope analyses with zircon shape, and some physical parameters, including length, width, aspect ratio, roundness, typology, alpha-decay dose and density. Zircon U–Pb ages and field relationships indicate that pebbly sandstones of the Cuxiú Formation were deposited during the Neoarchaean between 2705 and 2665 Ma, likely in a fluvial or near-shoreline environment of a rift basin, with zircon grains supplied from igneous rocks of the surrounding Amazon Craton. The detrital zircon grains record crystallisation ages of 3100–2800 Ma (74%), and subordinately of 2800–2700 Ma (13%) and 3600–3100 Ma (3%). Hafnium isotope analyses further suggest crust formation in the hinterland at 4.01–3.65 Ga (14%), 3.60–3.10 Ga (85%) and 2.95 Ga (1%). Well preserved euhedral shapes of more than 50% of the detrital zircon grains, independent of U–Pb ages, suggest fast, direct supply from heterogeneous Archaean basement sources to the sedimentary basin, with a short-distance transport, plausibly in a fluvial setting. Correlation of zircon roundness with width in the size range from 50 to 150 µm indicates more intense abrasion of larger zircon grains compared to smaller ones, reflecting difference in transport mechanisms – i.e., abrasive-rolling transport vs. non-abrasive suspension transport. Similar average values for length (~120 µm), width (~66 µm), aspect ratio (1.9) and roundness (~2.5), obtained from zircon grains of very different ages (3600–2700 Ma), as well as log-normal distribution of the width parameter, suggest thorough hydrodynamic sorting during transport. Zircon typology analysis reveals a wide range of combinations of {1 0 0}:{1 1 0} prism faces with{1 0 1}:{3 0 1}:{2 1 1} pyramids, pointing to zircon crystallisation from 900 to 600 °C (average 755 °C). Uranium–Th contents indicate low α-decay doses of ≪1 × 10−15 decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation.



中文翻译:

结合碎屑锆石形状和 U-Pb-Hf 同位素分析进行​​物源研究——以巴西亚马逊克拉通 Aquiri 地区为例

出处研究通常基于原位碎屑锆石颗粒的 U-Pb 测年和 Hf 同位素分析,同时不考虑形状参数,尽管这些可以提供有关内陆和碎屑运输的有价值的信息。本文通过展示来自阿基里地区 Cuxiú 组(亚马逊克拉通东部地区的一部分)的三个石英岩样品的大约 950 个碎屑锆石颗粒的综合数据集来证明这一点。该数据集将锆石 U-Th-Pb-Hf 同位素分析结果与锆石形状以及一些物理参数(包括长度、宽度、纵横比、圆度、类型学、α-衰变剂量和密度)相结合。锆石 U-Pb 年龄和现场关系表明,在 2705 至 2665 Ma 之间的新太古代期间,Cuxiú 组的卵石砂岩沉积,可能在裂谷盆地的河流或近海岸线环境中,锆石颗粒来自亚马逊克拉通周围的火成岩。碎屑锆石颗粒记录的结晶年龄为 3100-2800 Ma(74%),其次为 2800-2700 Ma(13%)和 3600-3100 Ma(3%)。铪同位素分析进一步表明在 4.01-3.65 Ga (14%)、3.60-3.10 Ga (85%) 和 2.95 Ga (1%) 的腹地形成地壳。超过 50% 的碎屑锆石颗粒保存完好的自形形状,与 U-Pb 年龄无关,表明从异质太古代基底源快速、直接供应到沉积盆地,具有短距离输送,似乎是在河流环境中。锆石圆度与宽度在 50 到 150 µm 之间的相关性表明,与较小的锆石颗粒相比,较大的锆石颗粒磨损更剧烈,反映了运输机制的差异——即磨料滚动运输与非磨料悬浮运输。相似的长度 (~120 µm)、宽度 (~66 µm)、纵横比 (1.9) 和圆度 (~2.5) 平均值,从年龄非常不同 (3600–2700 Ma) 的锆石颗粒中获得,以及 log-宽度参数的正态分布,建议在运输过程中进行彻底的水动力分类。锆石类型分析揭示了 {1 建议在运输过程中进行彻底的水动力分类。锆石类型分析揭示了 {1 建议在运输过程中进行彻底的水动力分类。锆石类型分析揭示了 {1 0  0}:{1  1  0} 棱柱面带有{1  0  1}:{3  0  1}:{2  1  1} 棱锥,指向 900 至 600 °C(平均 755 °C)的锆石结晶。Uranium–Th 含量表明 ,到沉积时所有锆石颗粒的α-衰变剂量为 ≪1 × 10 -15衰变事件/mg,为锆石的出色保存提供了额外的解释。

更新日期:2021-08-23
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