EARLY DEVONIAN VOLCANICS OF SOUTHEASTERN GORNY ALTAI: GEOCHEMISTRY, ISOTOPE (Sr, Nd, and O) COMPOSITION, AND PETROGENESIS (<i>Aksai complex</i>)
Geological, geochemical, and isotope (Sr, Nd, and O) parameters of Early Devonian (405 Ma) volcanics of southeastern Gorny Altai (Aksai and Kalguty volcanotectonic structures) are discussed. The studied igneous rock association comprises magnesian andesitoids, Nb-enriched basaltic andesites, and A-type peraluminous silicic rocks (dacites, rhyolites, granites, and leucogranites). Magnesian andesitoids (mg# > 50) are characterized by a predominance of Na among alkalies (K2O/Na2O ≈ 0.1-0.7), medium contents of TiO2 (~0.8-1.3 wt.%) and Al2O3 (~12-15 wt.%), enrichment in Cr (up to 216 ppm), and low Sr/Y ratios (4-15). The Nb-enriched (Nb = 10-17 ppm) basaltic andesites have high contents of TiO2 (1.7-2.7 wt.%) and P2O5 (0.4-1.4 wt.). The A-type granitoids are characterized by high contents of K (K2O/Na2O ≤ 60) and alumina (ASI ≤ 2.9) and depletion in Ba, Sr, P, and Ti. The magnesian andesitoids and Nb-enriched basaltic andesites are products of melts generated in the metasomatized lithospheric mantle; silicic magmas formed through the melting of Cambrian-Ordovician metaturbidites of the Gorny Altai Group and, partly, Early-Middle Cambrian island-arc metabasites. The above rock association might have resulted from a plume impact on the lithospheric substrates of the continental paleo-margin during the evolution of the Altai-Sayan rift system.