Geological and Isotope-Geochemical Indicators of Early Devonian Plume–Lithosphere Interactions in the Southwestern Framing of the Siberian Craton: Data Synthesis for Magmatic Associations of the Altai–Sayan Rift System

Abstract ––The Early Devonian Altai–Sayan rift system (ASRS) has spread to the structures of East and West Sayan, Kuznetsk Alatau, and Mongolian Altay. Its largest fragments are the Tuva, Delyun–Yustyd, Kan, Agul, and Minusa basins as well as depressions in north-western Mongolia. The paper summarizes the geologic, geochemical, and Sr–Nd isotope characteristics of the ASRS mafic rocks represented by nappes of moderately alkaline and alkali basalts and their subvolcanic and intrusive rock analogues. They are present in all magmatic associations and are divided into low-Ti (TiO2 = 0.2–2.2 wt.%) and high-Ti (TiO2 = 2.2–4.3 wt.%) subgroups. These rocks are characterized by wide variations in Sr isotope characteristics (εSr(T) = –16 to +30). High-Ti mafic rocks are common at the southern segment of the ASRS; they show a weak positive Ta–Nb anomaly (La/Nb = 0.8–1.1) and are relatively enriched in LREE ((La/Yb)N = 6–14) and radiogenic Nd (εNd(T) = 3.8–8.7). Low-Ti varieties are confined to the northwestern segment of the ASRS; they are enriched in Ba but depleted in Th, U, Nb, Ta (La/Nb = 1.2–2.2), Zr, Hf, LREE ((La/Yb)N = 3–7), and radiogenic Nd (εNd(T) = 2.0–6.0). Taking into account the existence of different terranes, which were combined in the structure of the Altai–Sayan folded area during accretion (ca. 500–480 Ma), we propose a model suggesting different environments of magma formation at the southern and northwestern segments of the ASRS and the relationship of magmatism with a mantle plume within the ASRS. In composition the plume corresponds to the sources of high-Ti magmas. The effect of the melted lithospheric mantle of different compositions beneath different groups of terranes led to the observed isotope-geochemical heterogeneity of mafic rocks within the ASRS, in particular, the absence of high-Ti mafic rocks from the Minusa basin.