NEW DATA ON THE AGE AND MATERIAL COMPOSITION OF THE KARA-OS PERIDOTITE-PYROXENITE-GABBRONORITE MASSIF (TUVA)
Ar-Ar age of the Kara-Os massif is 437.2 ± 4.2 Ma (amphibole of websterite M-47-98). This age is not much younger than the age of the Shui gabbroid massif located 45 km to the Southeast - 449 ± 4.2 Ma. Earlier studies showed that about 450 Ma with the participation of increased heat flow associated with the introduction of mantle melts, there was a large-scale formation of acid melts, sealed in the O3 Kaachem polychronous batholith. At the same time, the dating obtained by us testifies to the activity of mantle magmatism not only in the O3, but also in the S1. However, the absence of large-scale manifestations of S1 magmatism on the territory of Tuva indicates the local nature of magmatism at this time. Probably, the formation of the Kara-Os massif was associated with the tectonic activation of the Kaachem rift, within which the massif is located. Geochemical and isotopic data for the Kara-Os massif indicate the presence in its mantle source of a certain proportion of both enriched and depleted mantle source, while the latter could be modified by suprasubduction fluids. The sample M-47-98 is εNd (T) = + 3.0, which is a reflection of a higher proportion of the radiogenic Nd-enriched mantle, the supplier of which was most likely the mantle plume. However, the reason for the melting of mantle sources was most likely not the plume, since the S1 of Tuva is almost amagmatic, but the decompression caused by the transform-shear deformation of the Kaakhem rift in the Early Silurian.