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Union of Soviet Socialist Republics
Devonian of Altai
In the Territory of Altai, accumulation of Devonian deposits took place under a variety of structural conditions. In Devonian time, the central and eastern parts of Altai (Mountain Altai), belonging to the Caledonides of the Altai-Sayan folded region, experienced late stages of geosynclinal development, whereas the southwestern Altai, being a part of the Hercynian Zaysan geosyncline, was in an early stage of development. As a consequence of this, in southwest Altai marine sediments are developed while in Mountain Altai continental sediments are widespread along with marine ones.
The Lower Division of the Altai sequence is represented mainly by sedimentary rocks with characteristic palaeontological suites, including limestones, aleurolites, sandstones and conglomerates; the division attains a thickness of from 500 to 1500 metres.
The Middle Division (5000 to 8000 metres) is characterized by the extensive development of volcanogenic formations, which in the southwestern Altai are mainly acid and, in the Mountain Altai, acid, intermediate, and basic. Interlayers of limestone are rich in fauna, while the terrigenous rocks contain imprints of plants. The strata of Late Eifelian and Late Givetian age are especially fossiliferous.
The Upper Division in Altai (1500-3000 metres) is represented by both sedimentary and volcanic formations, but sedimentary rocks predominate. Those of Early Frasnian age are notably fossiliferous.
The faunal assemblages permit the discrimination of stages, the establishment of a zonal subdivision, and wider correlation with the standard Devonian succession, despite the prevalence of endemic forms.
Effusive activity was accompanied by injection of numerous subvolcanic and hypabyssal intrusions comagmatic with the volcanic formations. Intrusions of granitoids are most prominent in the uplifts and in the marginal parts of the Devonian sags.
Toward the close of the Devonian, instrusive diabase and gabbro-diabase were injected in the most mobile structures. In Mountain Altai the magmatism was accompanied by formation of iron ore, gold-polymetallics, and rare-metallic ores. In southwestern Altai, the Devonian deposits contain veins of polymetallic ores.
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