Abstract

A study of the composition and the age (U–Th–Pb method for zircon, SIMS) of the Neoarchean volcanic rocks of the Khedozero-Bolshozero greenstone structure of Central Karelia was carried out. The volcanogenic sequence of this structure is composed primarily of dacites and andesite-dacites, and to a lesser extent of andesites and andesite basalts. The ultra-felsic volcanic rocks, previously attributed to the sequence, compose intersecting dikes and are represented by rhyolite porphyry. According to geochemical characteristics, the volcanic rocks of the sequence are similar to the basalt-andesite-dacite-rhyolite rock series of modern island arcs, their chemical composition being comparable with the high-silica and low-silica adakites. The age of andesibasalts and dacites of the Khedozero-Bolshozero structure is Neoarchean: 2705 ± 9 Ma for andesibasalts and from 2712 ± 6 to 2703 ± 7 Ma for dacites. The Nd isotopic composition shows that the sources of melts for volcanic rocks were mixed owing to the mantle–crust interaction. The ancient Sm–Nd model ages and low ε_Nd values (0.4 and 0.3) of dacites indicate a significant part of the crustal source in the initial melt. The younger model age of meta-andesibasalts and the high value of ε_Nd (2.8) indicate the predominantly mantle nature of lavas with a minor crustal contamination. According to the age of crystallization, dikes of rhyolite porphyry (2704 ± 12 Ma) are close to the volcanic rocks of the sequence, but they have geochemical differences and a more ancient crustal protolith (3.17 Ga). As compared to the rocks of the Kontok Group of the Kostomuksha greenstone belt, the Khedozero-Bolshozero volcanic rocks have a different composition, a younger age, and a higher position in the stratigraphic scale. It is possible that alternative geodynamic settings, such as an active continental margin, turning into collision, or the combined action of the mantle plume and plate tectonics, led to the formation of the rocks studied.

中文翻译：

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Khedozero-Bolshozero绿岩结构（卡累利阿中部）的新古宙火山岩：组成，年龄和构造环境

摘要

进行了卡累利阿中部Khedozero-Bolshozero绿岩结构的新archarean火山岩的成分和年龄（锆石的U–Th–Pb方法，SIMS）的研究。该结构的火山成因序列主要由水辉石和安山岩-达克石组成，少量程度由安山岩和安山岩玄武岩组成。早先归因于该序列的超长石火山岩组成相交的堤防，并以流纹岩斑岩为代表。根据地球化学特征，该层序的火山岩与现代岛弧的玄武岩-安山岩-d石-流纹岩岩系相似，其化学组成可与高硅质和低硅质ad石相提并论。Khedozero-Bolshozero结构的安第斯玄武岩和dacits的年龄是新旧世纪的：安第斯山玄武岩为2705±9 Ma，Dacites为2712±6至2703±7 Ma。Nd同位素组成表明，由于地幔与地壳的相互作用，火山岩的熔体来源混合。古老的Sm–Nd模型年代久远且ε低_数据的Nd值（0.4和0.3）表明初始熔体中地壳源的很大一部分。元andesibasalts的年轻模型的年龄和ε值高_的Nd（2.8）表明熔岩的主要地幔性质，地壳污染较小。根据结晶时代，流纹岩斑岩（2704±12 Ma）的堤防与该序列的火山岩接近，但它们具有地球化学差异和更古老的地壳原生岩（3.17 Ga）。与Kostomuksha绿岩带的Kontok组的岩石相比，Khedozero-Bolshozero火山岩具有不同的组成，更年轻的年龄和更高的地层位置。替代的地球动力学设置（例如活跃的大陆边缘，变成碰撞或地幔柱与板块构造的共同作用）可能导致了所研究岩石的形成。