Trends of Incoherent Elements in High-Magnesian Basalts of the Tolbachinsky Dol (Kamchatka) and the Possible Causes of Their Occurrence

Abstract

This paper describes new data relating to the compositions of major and trace elements in carefully sampled high-magnesian basalts from eruptions of monogenic cone 1004, the second cone of the Northern Vent in the Great Tolbachik Fissure Eruption (GTFE), and Alaid Cone in Tolbachinsky Dol. The data on chemical composition showed that high-magnesian products of eruptions with constant content of major oxides show considerable variations in the concentrations of K₂O, P₂O₅, TiO₂, CaO and the entire range of incoherent trace elements (Ba, Rb, Zr, Ce, La, Lu, Y, Yb, Hf, Th and others). The variations in the concentrations of K₂O, P₂O₅ and of incoherent trace elements in the rocks reach a ratio of 2 and demonstrate direct correlation between themselves. The compositional data points in diagrams make continuous trends within the Tolbachinsky Dol high-magnesian basalts. These geochemical variations in high-magnesian basalts were for the first time described by us within the rocks of individual monogenic eruptions, as far as two types of lava in a single sample, which is inconsistent with the previous hypotheses of a deep-seated origin of these variations. We hypothesize a shallow (h = 2–10 km) character of formation for geochemically contrasting high-magnesian-magmas from an originally common high-magnesian magma as it was crystallizing in the source in situ. Such a process can probably have taken part in the formation of medium-K and high-K trends in the entire rock series of the Tolbachik volcanic massif.