GEODYNAMIC SETTING, STRUCTURE, AND COMPOSITION OF CONTINUOUS TRACHYBASALT- TRACHYANDEZITE-RHYOLITE SERIES IN THE NORTH OF THE ALTAI-SAYAN AREA: THE ROLE OF CRUST-MANTLE INTERACTION IN CONTINENTAL MAGMA FORMATION
A.A. Vorontsov1,2, O.Yu. Perfilova3, N.N. Kruk4,5
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Irkutsk State University, ul. Karla Marksa 1, Irkutsk, 664003, Russia
3Siberian Federal University, pr. Imeni Gazety Krasnoyarskii Rabochii 95, Krasnoyarsk, 660025, Russia
4V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
5Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Keywords: Трахибазальт, трахиандезит, риолит, ордовик, силур, девон, Минусинский прогиб, редкие литофильные элементы, изотопный состав Sr, Nd, магматические источники, фракционирование, коровая контаминация, Trachybasalt, trachyandesite, rhyolite, Ordovician, Silurian, Devonian, Minusa basin, rare lithophile elements, isotopic compositions of Sr and Nd, magmatic sources, fractionation, crustal contamination
Abstract
The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO2 contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO2 (<1.7 wt.%) and Fe2O3tot and higher contents of Al2O3 in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.
|
