Abstract
Research subject. The research covered the geological position, material composition and rock age of a comparatively small (about 32 km2) Bystrinskii gabbro massif. This massif underlies a tectonic plate package consisting of serpentinized dunits and harzburgites of the ophiolite association in the Eastern boundary of the Middle Urals. Methods. The content of petrogenic elements was measured by the Х-ray fluorescence method using an Х-ray multichannel spectrometer CPM-35. The analysis of rare-earth element contents was conducted using a NexION 300S mass-spectrometer. The composition of rock-forming minerals was studied using an Х-ray microanalyzer CamecaSX100. The age of the massif was determined by the 147Sm-143Nd method of isotope dating. Amphibole geobaometers were used to determine the depth of the rock formation. Results. According to the petro- and geochemical features and composition of the rock-forming minerals, the 147Sm-143Nd isotope age of the rocks was found to be 587 Ma. It was shown that gabbroids in the massif are represented by two petrographic varieties. The predominant type of the rocks is gabbrodolerites, which are similar to the isotropic gabbros of undisturbed ophiolite sections in terms of mineral composition, structure, geochemical features and the depth of formation (not more than 2–3 km). The mapping results showed the massif under study to be the largest among those described thus far. The medium grained gabbroids, which are present in lesser quantities, differ sharply from gabbrodolerites in terms of lower contents of Fe, Ti, both rockforming (K, Na) and rare (Li, Rb, Cs) alkalis, Ba, V, Y, Nb, Zr, Hf and elements of rare-earth group, as well as by significantly higher quantities of Ca, Mg and Cr. The depth of their formation is 10–12 km, which corresponds to the upper mantle. Conclusions. The obtained information demonstrates that fragments of two levels of the ophiolite section are tectonically aligned in the Bystrinskii massif: relatively shallow isotropic gabbros of the upper part of the ophiolite section and deep gabbros of the mantle part of the ophiolite section.
Highlights
The research covered the geological position, material composition and rock age of a comparatively small Bystrinskii gabbro massif. This massif underlies a tectonic plate package consisting of serpentinized dunits and harzburgites of the ophiolite association in the Eastern boundary of the Middle Urals
It was shown that gabbroids in the massif are represented by two petrographic varieties
The predominant type of the rocks is gabbrodolerites, which are similar to the isotropic gabbros of undisturbed ophiolite sections in terms of mineral composition, structure, geochemical features and the depth of formation
Summary
Статья подготовлена в процессе проведения работ по госбюджетной теме ИГГ УрО РАН No АААА-А18118052590032-6 “Палеогеодинамика и эволюция структурно-вещественных комплексов при формировании земной коры континентального типа (на примере Урало-Монгольского складчатого пояса и Западно-Сибирской платформы)”. For citation: Smirnov V.N., Kazakov I.I., Ponomarev V.S., Ronkin Yu.L., Storozgenko E.V. The research covered the geological position, material composition and rock age of a comparatively small (about 32 km2) Bystrinskii gabbro massif. Пространственная совмещенность рассматриваемых габброидов с типичными для офиолитовых комплексов Урала петрографическими разновидностями: серпентинизированными дунитами и гарцбургитами – позволяет уверенно отождествлять эти образования с ассоциациями офиолитового типа (Казаков и др., 2017; Петров, 2019). Однако первые же результаты изучения состава габброидов Быстринского массива показали, что они существенно отличаются от габбро, типичных для офиолитовых массивов как этого региона, так и других частей Урала, что побудило авторов провести более детальное изучение пород массива в целях объяснения наблюдаемых особенностей состава этих пород, результаты которого изложены в настоящей публикации
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