THE ROLE OF A PYROXENITE SOURCE IN THE GENESIS OF LATE CENOZOIC BASALTS IN THE IYA-UDA VOLCANIC FIELD ( Biryusa Block, Siberian Craton )

Analysis of the hypsometric positions of lava flows of different ages (~8 and ~4 Ma) in the Iya-Uda volcanic field indicates that the main phase of intense relief dissection and activation of block movements along the Main Sayan Fault within the Biryusa Block occurred from the Late Miocene to the Pliocene. The geochemical characteristics of the lavas, including elevated FCKANTMS values (0.46–0.77) [Yang et al., 2019] and the positions of data points on CaO–MgO and TiO2/Al2O3–SiO2 diagrams, point to melting of a garnet-pyroxenite mantle source rather than a typical peridotite mantle. The trace-element composition of the rocks is consistent with OIB-type intraplate basalts. Variations in Th/Nb and TiO2/Yb ratios suggest that the role of garnet in the melting zone increases for the 4 Ma lavas, whereas the contribution of the lithospheric mantle is more significant for the 8 Ma magmas. The origin of the enriched pyroxenite component in the lithosphere of the Biryusa Block is most likely linked to Cenozoic tectonic convergence and rifting. These processes led to foundering dipping of lower crustal material into the mantle of the Siberian Craton. Volcanism in the Iya-Uda interfluve is thus the result of melting of a heterogeneous and enriched lithospheric mantle.