Petrologic Characteristics of Cenozoic Alkaline Basalts from the Azas Plateau, Northeast Tuva (Russia)

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The Azas Plateau is a part of the East-Tuva Highland (West Sayan ridge, the south of Krasnoyarsk Region, Siberia). It is the westernmost lava field of the Baikal Rift System, covering about 2000km^2. A volume of volcanics is estimated as high as 600km^3. The volcanics are represented by massive lava flows and thick hyaloclastite units. Their composition corresponds to alkaline basalts and basanites. The most intensive volcanic activity started about 2 Ma from basaltic fissure eruptions producing voluminous lava flows. The second stage voluminous eruptions formed the upper lava unit on the surface of moderately eroded lower lava flows. During the next stages, the eruptions became centralized so that several volcanic centers were formed along certain lineaments marking deep-seated fissures. The peculiarity of the volcanic activity is that at least 7 volcanoes were formed during extensive glaciation covering the entire area of plateau. Moreover, the subglacial eruptions occurred about 0.7 and 0.1 Ma ago, whereas there was no sign of glaciation about 0.29 Ma ago, when Yurdawa shield volcano erupted. The subglacial eruptions formed table mountains (tuyas) with resembling structure of thick lower hyaloclastite units and armoring upper lava flows. The composition of lavas is rather uniform with slight variations in acidity and alkalinity of volcanics during eruptions. The most voluminous eruptions produced trachybasalts, smaller volcanoes are composed of tephrites/basanites. The basalts have typical for silica undersaturated rocks normalized REE patterns with regular enrichment in LREE and depletion in HREE. REE patterns, in accordance with eruption volumes, show lower degrees of partial melting for tephritic volcanoes (La/Yb=12-17) and higher degrees for trachybasalts (La/Yb=5-9). Correlation between La/Yb and Mg# indicates the formation of basalts by chemically identical source with one definite exception (Yurdawa volcano). The trace element compositions plotted on chondrite-normalized spidergram have OIB-like patterns.Article

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