Basic magmatism of early Devonian rifting 85 Tuvinian Trough

Data on content of petrogenic, rare, and rare-earth elements, as well as results of isotopic studies (Rb-Sr, Sm-Nd) of basic part in Devonian magmatic associations of the Tuvinian trough have been presented in the paper. The formation of Tuvinian trough was associated with rifting in rear of Devonian continental margin of Middle Paleozoic North-Asian continent. Two groups of spatially separated basic rocks have been distinguished according to their petro geochemical features. The first group - West Tannu-Ola - is close to OID by composition and is enriched in incompatible elements. The second group -Ulug-Khem - is closer to MORB basalts. According to mantle source types, the basic rocks are compositionally close to primitive mantle and subcontinental litho-sphere. Intensity of magmatic activity, enrichment in incompatible elements, and variety of vulcanite compositions let us suggest that a plume component took active part in formation of Tuvinian rifting trough. The two groups distinguished suppose the presence of a zonation in participation of mantle sources and in a degree of mantle material melting. The vulcanites are also divided into two groups by isotopic characteristics. This was mainly caused by variations in Rb-Sr ratios (eNd from -12.5 to +18) and in a less degree by a range of Sm-Nd ratios (eNd from +6.3 to +8.6). Such variations indicate isotopic heterogeneity of magma sources and interaction of subduction and plume components. The basic rocks of Tuvinian trough generally have low alkalinity, low degree of REE fractionating, deep Ta-Nb minimum, and zonation of melt sources. According to these characteristics, they are closer to Lower Devonian volcanic zones of Northwestern Mongolia than to Lower Devonian volcanites of Minusa troughs and Vilyui rift.

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