Olivine-Chromspinel Oxythermobarometry of Ultramafic Rocks of the Ioko-Dovyren Layered Massif

摘要

Recent years have been characterized by the accumulation of a large body of information pertaining to redox conditions of ultramafic rocks, including peridot-ites from mid-oceanic ridges and island arcs, orogenic lherzolites, mantle nodules in alkaline basalts and kim-berlites, ultramafic rocks and chromitites in ophiolite complexes, dunites and associated Cr-Pt ores in concentric-zonal dunite-clinopyroxenite-gabbro complexes of the Ural-Alaska type, and different types of basalt [1,3, 8-12, 15, and others]. However, the redox state of layered ultramafic-mafic massifs has not been estimated by modern methods of Oxythermobarometry. The present communication is devoted to solution of the following interrelated tasks: (ⅰ) elucidation of oliv-ine-chromspinel equilibrium in ultramafic rocks of the Ioko-Dovyren layered massif (northern Baikal region); (ⅱ) estimation of temperature and oxygen fugacity at the final stage of subsolidus exchange reactions; and (ⅲ) assessment of changes in the fluid regime of rocks during their interaction with metasedimentary carbonate rocks (a well-known and comprehensively investigated phenomenon in the loko-Dovyren Massif). The Ioko-Dovyren dunite-troctolite-gabbro massif, a large sill-shaped body located 70 km north and northeast of Nizhneangarsk, is conformable with Pale-oproterozoic carbonate-terrigenous rocks developed in the axial part of the Synnyr riftogenic structure. The massif includes plagioperidotite, dunite, troctolite, and gabbronorite units (Fig. 1). The upper section of the dunite unit contains abundant xenoliths of magnesian skarns related to the interaction of the picritic basalt melt with dolomite fragments entrapped in the course of pluton intrusion. Assimilation of carbonate material by ultramafic rocks produced a contaminated rock zone near the skarns. The geological setting, internal structure, rock composition, mineralization mode, and genetic model of the loko-Dovyren Massif are scrutinized in[4].