Paleozoic Alkaline-Mafic Intrusions of the Kuznetsk Alatau, Their Sources and Conditions for Magma Generation

Abstract

Cambrian, Devonian and Permian differentiated intrusions made up of subalkaline and alkaline gabbro, foidolite, nepheline and alkaline syenites, and rare carbonatites compose an area in the northern part of the Kuznetsk Alatau alkaline igneous province. Primary melts of the intrusions were moderately fractionated ((La/Yb)N up to 7–12) and could be derived by 1–10% partial melting of mantle spinel lherzolite. The LILE and HFSE distribution in the mafic rocks (Rb 6–58, Ba 111–2499, Sr 175–1555, REE 28–208, Zr 40–315, Nb 1.5–52, Th 0.3–12, U 0.2–7.4, ppm) indicates the involvement of OIB- and IAB-type oceanic and marginal-continental lithosphere materials in magma generation. A source of the parental melt was heterogeneous and consisted of a mixture of depleted (PREMA and E-MORB) and enriched (EM-type) mantle magmas with an intermediate isotopic ratio eNd(Т) ~ 3–9. Apparent heterogeneity of the source is reflected in the initial Pb isotopic composition of rocks and minerals: 208Pb/204Pb = 37.49–38.12, 207Pb/204Pb = 15.53–15.71, 206Pb/204Pb = 17.92–20.65. The subsequent upper crustal contamination of the melts led to a simultaneous increase in the isotopic ratios (87Sr/86Sr)Т ~ 0.7042–0.7074; δ18ОV-SMOW ~ 6.3–15.5‰, as well as to the enrichment of rocks in 207Pb. Geochemical data suggest that primary melts were formed by impact of a mantle plume (OIB-type hot spot) on accretion-collisional complexes of the former active continental margin. Such geodynamic setting provides the generation of heterogeneous magma sources even within a small igneous province.