Origin of the Dunite Block of the Monchegorsk Layered Mafic-Ultramafic Complex, Baltic Shield: Evidence from Mineralogical and Geochemical Data

Abstract

The paper reports the results of detailed mineralogical and geochemical study of the Dunite block in the Paleoproterozoic Monchegorsk complex, Baltic Shield. This block forms an autonomous body at the intersection of the sublatitudinal and sublongitudinal branches of the Moncha pluton. It is composed mainly of adcumulus dunite grading into poikilitic peridotite in the upper part of the section. The central part of the block is occupied by the Sopcheozero chromite deposit. It is shown that the Dunite block is not related to the layered series of the Moncha Pluton by fractionation of a common magma, but was formed through an independent influx of a more primitive high-Mg, Cr-rich (komatiite) magma, which is confirmed by the presence of the highest Mg olivine (Fo₉₃) and higher Cr and Mg spinels compared to the dunites from the ultramafic part of the Layered Series of the Moncha Pluton. These differences are emphasized by the variations of Ni/Cu and Pd/Ir ratios in the bulk rock compositions, as well as by estimated characteristics of the primary melts: 8.44 wt % FeO, 10.26 wt % MgO for the rocks of the Layered Series and 22–25 wt % MgO, 10.5–11.5 wt % FeO for the rocks of the dunite block. The geochemistry and mineralogy of the rocks of the Dunite block reflect fractionation of the primary melt expressed in the upward accumulation of large-ion lithophile elements (LILE) as well as the formation of the olivine–chromite cumulates with the high contents of intercumulus minerals (pyroxenes, more rarely, plagioclase) and an increase of TiO₂ content in spinel in the upper portions of the section. It is suggested that the high-Cr magma emplaced along a channel in the tectonized zone and precipitated chromite with increasing Cr₂O₃ and MgO in the central part of the section. The chemical composition of ore chromite from the Sopcheozero deposit resembles those of the Archean and Paleoproterozoic chromitites, which formed in a magma conduit system and represent a component of ultramafic horizons of the layered complexes (Kemi, Baltic Shield; Uitkomst, South Africa; Sukinda, Indian shield). The similarity of major minerals (olivine and chromite) from dunites to those of peridotites from ophiolite complexes in the diagram (Arai, 1994) is likely explained by the formation of some grains at mantle depths near the crust–mantle boundary, with their subsequent entrapment by forming high-Mg melt, which was parental for the Dunite block. The formation of revealed extremely high-Mg compositions of olivine up to Fo₉₈ with an inverse zoning within chromitites at the absence of typical fractionation trends was related to the subsolidus re-equilibration of olivine with chromite rather than to the crystallization from a melt.