SIMULATION OF THE GEOCHEMICAL STRUCTURE OF THE IOKO-DOVYREN LAYERED INTRUSION, NORTHWESTERN BAIKAL AREA

Abstract

The processes of differentiation in the magmatic chamber of the Ioko-Dovyren layered dunite-troctolite-gabbro-gabbronorite massif were simulated using the COMAGMAT-3.5 software package, which is based on the convection-accumulation model for the crystallization of magmatic intrusions. The initial magma composition was assumed to be equal to the weighted mean composition of the rocks composing the intrusion (wt %: 43.92 SiO2, 9.72 Al2O3, 10.53 FeO, 27.88 MgO, 6.99 CaO, 0.59 Na2O, 0.07 K2O, and 0.11 TiO2). The results obtained by simulating the crystallization of this composition within a pressure range of 0-10 kbar indicate that the crystallization sequence determined for the rocks Ol + Chr → Ol + Pl + Chr → Ol + Pl + CPχ → ± Ol + Pl + CPχ + LowCaPx in an anhydrous system takes place under pressures of 0-2 kbar. A series of simulations for a system closed with respect to oxygen yielded estimates for the phase and chemical composition of the emplaced magma and the parameters of the optimum model, which reproduces accurately enough the geochemical structure of the Ioko-Dovyren intrusion: the naturally occurring distributions of minerals and components in its vertical section. The correlation coefficients between the concentrations of oxides determined in the rocks and calculated within the model are rMgO,Al2O3+,CaO ≥0.9 and rFeO,SiO2+,Na2O ≥0.6. The simulated phase composition of the magma during its emplacement corresponded to melt + olivine (Fo89). The crystallinity of the parental magma was determined to have been equal to approximately 40 vol % at an assumed cumulus density of 90% near the lower contact and 70% near the upper one. The temperature of the magma during its emplacement was close to 1340°C at a pressure of 1 kbar. In the model, plagioclase and clinopyroxene appear on the liquidus at T ≅ 1255°C at T ≅ 1210°C, respectively, and the crystallization sequence of cumulus minerals corresponds to that observed in nature. The liquid phase (melt) of the parental magma during its emplacement had the following composition (wt %): 45.95 SiO2, 15.93 Al2O3, 14.49 MgO, 10.88 FeO, 11.46 CaO, 0.97 Na2O, 0.11 K2O, and 0.18 TiO2. Our results confirm the plausibility of the hypothesis that the inner structure of the Ioko-Dovyren intrusion was formed by the emplacement and differentiation of a single magma portion with no less than 40 vol% crystallinity. © Pleiades Publishing, Ltd. 2007.