The alkaline and carbonatitic rocks of Gorny Altai (Edel'veis complex) as indicators of Early Paleozoic plume magmatism in the Central Asian Fold Belt

摘要

The minor intrusions of the Edel'veis alkaline-carbonatite complex are bounded by the spurs of the North Chuya Ridge in southeastern Gorny Altai. According to Ar-Ar isotope data, the complex formed in the Middle Cambrian (~507 Ma). All of its components (alkali clinopyroxenite-melanogabbro-alkali syenite + Ca-carbonatite) occur in only one pluton. Silicate igneous rocks are equivalent in silica content and alkalinity to potassic alkaline and subalkalic mafic rocks. Apatite-phlogopitic Ca-carbonatites are enriched in P _2O _5 (up to 3.6 wt.%), Sr (~2500-5500 ppm), and REE (up to ~2000 ppm) and are, presumably, of liquation genesis. A PREMA-type plume component was a predominant magma source for the complex ({parallelogram open} _(Nd)(T) = +6.56 to +6.85). According to isotope data (~(87)Sr/ ~(86)Sr(T) ~ 0.7032-0.7039; δ ~(18)O ~ 7.5-14.9- δ ~(13)C ~ -2.7 to -8.4-), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the rocks suggests that the complex developed on a continental margin and its development was accompanied by late-collisional rifting and the mixing of moderately depleted (PREMA) and enriched suprasubductional lithospheric mantle (EM I or EM II) with continental crust. It is presumed that the alkaline and carbonatite complexes in the western Central Asian Fold Belt are of primary plume origin and form a LIP within this belt together with other associations produced by Early Paleozoic (510-470 Ma) magmatism.