The ultrabasites and amphibolites of the Uglovsky-Turukhansk block of the Irtysh shear zone

The oceanic stage of development of the Big Altai structures and, in particular, the Irtysh shear zone is fixed by the presence in it of tectonic wedges of ultrabasites represented by dunites and peridotites known as the Sarbasskiy complex. The dunites and harzburgites of this complex are generated as a part of the ophiolitic associations of an oceanic crust in a mid-oceanic ridge of the Paleoasian ocean. It was exhumed as a result of later subduction and collision processes at the margin of the Siberian continent. Oceanic ophiolitic association of the given stage also included metamorphosed oceanic tholeiitic basalts of N-type at the moment transformed in apobasitic amphibolitic slates of the Irtysh metamorphic complex. These rocks, in the form of isolated blocks, are localized in the Devonian active continental margin's accretion prism, which in the modern geological structure of the Altai is allocated as the Irtysh shear zone. Exploration of the chemical composition of ultrabasites and apomafitic amphibolites allowed making several conclusions about their origin. The ultrabasites have a composition close to the dunite-peridotite formation of the rift zones of mid-oceanic ridges. The amphibolites are similar to the N-type oceanic basalts which were also most probably formed in the mid-oceanic ridge and represent fragments of an oceanic plate subducted under the active continental margin of the Altai. Also, in a small amount, there were observed biotite plagiogneiss and crystalline schists with high silica content. They are probably the result of metamorphism of the siliceous-argillaceous and siliceous rock sediment layer of the oceanic crust or the cumulative complex plagiogranites. Thus, the above-mentioned rock complexes can be considered as part of a unified ophiolite association which fixes the presence of remains of the ancient (Early and Middle Paleozoic) metamorphosed and intensely tectonized oceanic crust. At the moment, there remains a little studied complex of gabbro, also spatially associated with the ultrabasites and amphibolites in the eastern part of the Uglovsko-Turukhansk block. Given the fact that gabbro-amphibolites with gneissic-banded textures compose independent massifs (Gorshinsky massif), it is likely they belonged to the cumulative gabbroic complex of the ophiolite association.

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