MODELING OF THE EQUILIBRIUM CRYSTALLIZATION OF ULTRAMAFIC ROCKS WITH APPLICATION TO THE PROBLEMS OF FORMATION OF PHASE LAYERING IN THE DOVYREN PLUTON, NORTHERN BAIKAL REGION, RUSSIA

Abstract

Phase equilibria and chemical characteristics of the initial magma of the Dovyren layered dunitetroctolite-gabbro pluton were modeled using the COMAGMAT-3.52 (2000) computer program. The chemical characteristics of marginal rocks and plagioperidotites of the basal zone and the weighted average composition of the intrusion were used for the calculations. Modeling was performed under thermodynamic parameters approaching natural solidification conditions (P = 0.5 kbar and WM buffer) with a crystallization increment of 1 mol %. The results of these calculations suggest an anchieutectic nature (Ol + Pl ± Cpx ± Opx) of the Dovyren magma. At the moment of emplacement, the magma had a temperature of 1180-1190°C and contained at least 40% intratelluric crystals. The liquid portion of this mixture was silica saturated (54-55 wt % SiO2) and relatively high in MgO (∼ 8 wt %). Owing to its high viscosity (∼104-105 P) and the high degree of crystallization of the system, the melt could not fractionate via the convective cumulative mechanism. This inference is supported by minor variations in the compositions of rock-forming minerals in the section of the layered series and the occurrence of undisturbed xenoliths of magnesian skarn after carbonates. It is suggested that the main mechanism of formation of the layering included compaction of a primary crystalline precipitate, adcumulus growth, and residual melt infiltration toward the roof with its partial burial near the bottom of the magma reservoir.