Abstract

Changes in (La/Yb)_N, Eu/Eu*, t_Nd(DM), and ε_Nd(t) values in Upper Precambrian clay rocks on the western slope of the Southern Urals are considered. The (La/Yb)_N, Eu/Eu*, and ε_Nd(t) average values in Riphean and Vendian clay rocks are 5.7–15.1, 0.58–0.74, and –14.6 to –5.1, respectively. This reflects the change in the composition of the source areas and fits well into the general outline of subglobal events established by traditional geological methods in the conjunction area of the eastern regions of the East European Platform and the modern Southern Urals. The Early and Middle Riphean sedimentary sequences (1750–1250 Ma) were formed mainly owing to the erosion products of the mature continental crust of the East European Platform (t_Nd(DM) = 2.8–2.4 Ga). However, the pre-Upper Riphean hiatus led, probably, to a significant change in the composition of source areas about 1 Ga. The increase in magnitude of ε_Nd(t) to –5.9 and decrease in t_Nd(DM) to 2.0 Ga in clay rocks of the Biryan Subformation of the Upper Riphean Zilmerdak Formation compared to underlying deposits suggest the appearance of the juvenile crust in the erosion area. This indicates the accumulation of fine-grained sediments at the beginning of the Late Riphean under the influence of active rifting processes, which is not recorded by traditional geological methods. The significant increase in (La/Yb)_N (to 13.1 on average), decrease in ε_Nd(t) to –14.6, and increase in t_Nd(DM) up to 2.5 Ga in clay rocks of the Vendian Bakeevo Formation compared to the Riphean rocks are interpreted as the result of the accumulation of the Bakeevo deposits due to the supply of the products of glacial exaration of the basement mature rocks during the Marino glacial period. The significant increase in ε_Nd(t) to –6.8 and rejuvenation of t_Nd(DM) to 1.8 Ga in Vendian mudstones of the Basa and Zigan formations compared to the rocks of the lower part of the Asha Group reflect the appearance of a new mantle or volcanogenic material in the catchment areas during the Middle Vendian.

中文翻译：

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南乌拉尔上前寒武统粘土岩的物源：地球化学和 Sm-Nd 同位素地球化学调查的结果

摘要

考虑了南乌拉尔西坡上前寒武统粘土岩中 (La/Yb) _N、Eu/Eu*、t _Nd (DM) 和 ε _{Nd (t) 值的变化。}(La/Yb) _N、Eu/Eu* 和 ε _Nd(t) Riphean 和 Vendian 粘土岩的平均值分别为 5.7–15.1、0.58–0.74 和 –14.6 至 –5.1。这反映了震源区组成的变化，与传统地质方法建立的东欧地台东部与现代南乌拉尔结合区次全球事件的总体轮廓吻合。早、中里弗世沉积层序（1750～1250 Ma）的形成主要是东欧地台成熟陆壳（t _Nd（DM）=2.8～2.4 Ga）的侵蚀产物。然而，上河岸前的中断很可能导致源区组成在 1 Ga 左右发生了显着变化。ε _{Nd (t) 的量级增加到 –5.9 并且 t Nd}减小与下伏沉积物相比，上 Riphean Zilmerdak 组的 Biryan 亚组粘土岩中的 (DM) 至 2.0 Ga 表明在侵蚀区出现了幼壳。这表明晚里弗期开始时细粒沉积物在活动裂谷作用的影响下堆积，这是传统地质方法没有记录的。(La/Yb) _N显着增加（平均为 13.1），ε _Nd (t) 减少至 –14.6，t _Nd增加(DM) 与 Riphean 岩石相比，Vendian Bakeevo 组的粘土岩石中高达 2.5 Ga 被解释为由于在 Marino 期间基底成熟岩石的冰川分解产物供应而导致 Bakeevo 沉积物积累的结果冰期。Basa 组和 Zigan 组 Vendian 泥岩与 Asha 组下部岩石相比，ε _Nd (t) 显着增加至–6.8，t _Nd (DM) 恢复至 1.8 Ga，反映了一个新的岩石的出现。中文迪安时期集水区的地幔或火山物质。