Abstract

The study of the volcano–sedimentary sequence in the lower part of the Zaonega formation in the Paleoproterozoic Onega structure (Karelian craton, Fennoscandian Shield) has shown that tuffaceous and high-silica rocks predominate in its composition. High-silica rocks (SiO₂ up to 94 wt %) are depleted in all elements and probably represent chemogenic siliceous silts. Tuffaceous rocks vary broadly in composition and were formed during mixing of tuffogenous basalt material and high-silica chemogenic sediments. In terms of levels of contents and the distribution character of trace elements, tuffogenous rocks are close to N-MORB volcanic rocks. This rock association is typical of the early stages of continental rifting in the Phanerozoic and may indicate the formation of volcano–sedimentary complexes of the Zaonega formation in the environment of continental rifting. The tuffaceous rocks in the lower part of the Zaonega formation are geochemically identical to dolerite dikes and MORB-type volcanic rocks 2.10–2.14 Ga in age. Their formation was probably related to this episode of large-scale extension and thinning of the continental lithosphere of the Karelian craton in the Middle Paleoproterozoic. In this case, the age limit of the Zaonega and underlying Tulomozero formations should be somewhat older than the 2.06–2.10 Ga interval accepted in the modern regional stratigraphic schemes of the Paleoproterozoic Fennoscandian shield.

中文翻译：

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古元古代奥涅加盆地的裂谷：扎奥内加组火山-沉积岩的地球化学

摘要

对古元古代奥涅加构造（卡累利阿克拉通、芬诺斯堪地盾）扎奥内加组下部火山-沉积层序的研究表明，其成分以凝灰质和高硅质岩石为主。高硅岩石（SiO ₂高达 94 wt%）的所有元素均已耗尽，可能代表化学成因硅质粉砂。凝灰质岩石的成分差异很大，是在凝灰质玄武岩材料和高硅化学沉积物混合过程中形成的。凝灰岩的含量水平和微量元素分布特征与N-MORB火山岩接近。这种岩石组合是显生宙大陆裂谷早期的典型特征，可能表明在大陆裂谷环境中形成了 Zaonega 组火山-沉积杂岩。Zaonega 组下部的凝灰岩在地球化学上与辉绿岩脉和年龄为 2.10-2.14 Ga 的 MORB 型火山岩相同。它们的形成可能与中古元古代卡累利克拉通大陆岩石圈的大规模扩张和减薄事件有关。在这种情况下，Zaonega 和下面的 Tulomozero 地层的年龄限制应该比古元古代芬诺斯坎迪亚地盾的现代区域地层方案中接受的 2.06-2.10 Ga 区间更老。