Neoproterozoic Metabasalts of the Tyya Complex of the Olokit Rift Trough (Baikal–Muya Belt): Composition, U–Pb Age, Isotope-Geochemical Characteristics, and Geodynamic Effects

Abstract

Abstract —We present results of new mineralogical, geochemical, geochronological, and isotope (Sm–Nd, Rb–Sr, and O) studies of the volcanic rocks of the Tyya complex in the Olokit trough. These are normal tholeiitic basalts and scarcer basaltic andesites forming a fractionated series with mg# = 45–65, medium TiO2 contents (0.73–1.62 wt.%), low P2O5 contents (0.04–0.25 wt.%), and a significant domination of Na over K (Na2O/K2O = 2.1–50.0). The rocks are metamorphosed to greenschists, which are composed of chlorite, actinolite, epidote, and albite with quartz, titanite, ilmenite, and magnetite impurity. The metabasalts have is an age of 915 ± 5 Ma (zircon U–Pb dating) and are characterized by wide variations in εNd(T) (–3.5 to –11.9) and 87Sr/86Sr (0.70602–0.70732) and high δ18O values (9.0–15.2‰) as compared to the mantle ones. According to the isotope-geochemical characteristics, the studied metabasalts have features of both IAB and E-MORB. The Tyya metabasalts might have resulted from the melting of the lithospheric mantle with a subductional component. Comparison of the studied rocks with volcanic rocks of recent geodynamic settings shows their similarity to basalts of back-arc basins. The Tyya metabasalts might belong to a back-arc basin of the late Mesoproterozoic Nyurundukan island arc system.