On the northeastern slope of the Kuznetsk Alatau, small differentiated alkaline basic intrusive massifs form an isometric area ~100 km across. They are composed of subalkalic and alkali gabbroids, basic and ultrabasic foidolites, nepheline and alkali syenites, and carbonatites. Results of complex (U-Pb, Sm-Nd, and Rb-Sr) isotope dating suggest that alkaline basic magmatism developed at two stages, in the Middle Cambrian-Early Ordovician (~510-480 Ma) and in the Early-Middle Devonian (~410-385 Ma). Finding of accessory zircons (age 1.3-2.0 Ga) in alkaline rocks suggests that the ascent of mantle plume was accompanied by the melting of fragments of Proterozoic mature continental crust composing the basement of the Caledonian orogen of the Kuznetsk Alatau. Probably, parental Cambrian-Ordovician alkaline mafic melts initiated metasomatism and lithosphere erosion. During the next melting of lithosphere substrate in ~100 Myr, this caused the generation of magmas of similar composition with inherited isotope parameters (ε Nd (T) ≈ +4.8 to +5.7, T Nd (DM) ≈ 0.8-0.9 Ga) pointing to the similar nature of their matter sources in the moderately depleted mantle.
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 P2O5 (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 (ɛNd(T) = +6.56 to +6.85). According to isotope data (87Sr/86Sr(T) ∼ 0.7032–0.7039; δ18O ∼ 7.5–14.9‰; δ13C ∼ –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.
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