The role of earth mantle in the development of early Paleozoic oceanic islands volcanism (for the geochemical data of OIB from SE Altai Mountains)
In southeastern part of Gorny Altai (Irbistu River), high-Ti K-subalkaline basalts (TiO<sub>2</sub> 3-4; Na<sub>2</sub>O + K<sub>2</sub>O 3,4-7,7 wt. %; K<sub>2</sub>O/Na<sub>2</sub>O up to 1,8-2,2) were found among Early Paleozoic metaflysch. Volcanics have similar level of enrichment in LILE and HFSE (Rb 3040, Ba 240-377, Zr 350-485, Nb 55-100, REE 190-300 ppm), fractioning of lanthanides (La/Yb = 12-14), and number of indicator ratios (Nb/U 45-56; Th/La 0,11-0,13; Th/Nb 0,07-0,08; Rb/Nb 0,31-0,54; Ba/Nb 3-4,7; La/Nb 0,57-0,64) to derivatives of OIB mag-matism with intraplate plume nature. Wide ranges of isotopic composition of Nd and Sr (е<sub>ш</sub>Т 1,7-5,0; <sup>87</sup>Sr/<sup>86</sup>Sr<sub>T</sub> 0,7026-0,7039, s<sub>Sr</sub>T from -18 to +0,1) suggest heterogeneity of sources of mantle basalt melt and absence of its interaction with supracrustal material. Primary ratios of Pb isotopes (<sup>206</sup>Pb/<sup>204</sup>Pb 17,75-17,85, <sup>207</sup>Pb/<sup>204</sup>Pb 15,48-15,49, <sup>208</sup>Pb/<sup>204</sup>Pb 37,35-37,62) also indicate the above scenario. We are considering possibility of magma generation under conditions of PREMA mixing with EM 1, but without participation of HIMU component, which often initiates OIB volcanism. Distribution of LREE and HREE determined in the rocks could be caused by 7-8% equilibrium melting of standard garnet-bearing mantle lherzolite, which lead to generating of primary magma similar to middle OIB. Presence of insignificant amount of spinel in the source or mixing of peridotite with different composition is also possible.