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We report data on the geology, mineralogy, petrography, and chemistry of 733 Ma gabbro-peridotite sills from the Late Riphean Dovyren plutonic complex. Thick sills were differentiated into plagiolherzolite to olivine gabbronorite compositions by fractional crystallization of the K-Na series high-Mg low-alkali low-Ti picritic parental magma. The magma already contained up to 5% of intratelluric olivine crystals when entering the reservoir. The sills emplaced before the whole complex, judging by the presence of their fragments as plagiolherzolite xenoliths in the gabbro zone of the Yoko-Dovyren layered pluton. The gabbro-peridotite sills are products of high-temperature within-plate magmatism. High heat flow during the generation of the magma, evident from its high-Mg composition, was likely maintained by the activity of a mantle plume associated with the Neoproterozoic Franklin large igneous province. [ABSTRACT FROM AUTHOR]

New data are presented on the geologic structure, age, petrogeochemical composition, and conditions of formation of the Late Proterozoic Meteshikha ultramafic-mafic pluton of the Ikat complex. Mafic rocks are the main rocks of the massif, whereas ultramafic rocks are secondary; both of them correspond to two intrusive phases. The first phase includes a layered rock series enriched in intercumulus amphibole, which varies in composition from olivine gabbro to leucocratic gabbro-anorthosite; the second is composed of wehrlite, plagiowehrlite, and olivine clinopyroxenite. Mineralogical, petrographic, geochemical, and isotope studies show that the rocks of both phases crystallized from the same mantle melt; note that the PT-conditions of their formation were considerably different. We suppose that they were separated in the intermediate chamber during fractional crystallization and the accumulation of early minerals (olivine and, probably, clinopyroxene) in the lower part of the chamber. Using the COMAGMAT software, we have found the composition of the parental melt for the rocks of the first phase—normal tholeiitic basalt with 0.2-0.5 wt.% water, which might have crystallized at 3.0-3.5 kbar and the oxygen activity controlled by the QFM buffer. The differentiated series is characterized by gradual depletion with Cr and Ni and enrichment with Sr, Ti, Cu, and REE during the evolution of melt. The REE patterns for the massif rocks have a similar low-fractionation trend with domination of light lanthanides over heavy ones and (La/Yb) N = 1.25-2.75. Multielement spectra are characterized by negative anomalies of K, Th, Nb, and Zr and positive anomalies of Ba, U, Sm, and Sr. The geochemical characteristics of the rocks are similar to those of the tholeiitic basalts of present-day island arcs. Studies show that the Meteshikha massif formed in the subduction setting of the active margin of the Siberian continent in the Late Riphean (809 Ma). © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. [ABSTRACT FROM AUTHOR]

Abstract: Based on new data on the age, mineralogy, and geochemistry of ultramafic–mafic complexes in the Precambrian structures of the southern periphery of the Siberian Platform, the East Siberian metallogenic (PGE–Cu–Ni) province is recognized. It includes the Yenisei Ridge, Precambrian Kan uplift, Alkhadyr terrane with the adjacent structures of the Biryusa block, and northern Baikal region (Yoko-Dovyren and other massifs of the Baikal–Patom basin). We have established that the U–Pb and Ar–Ar ages of ore-bearing complexes of dunite–peridotite– pyroxenite–gabbro association correspond to the Late Riphean (728–710 Ma). The mineralogical and geochemical similarity of ore-bearing complexes in different areas testifies to their genetic entity. All parental melts were similar in composition to picrites. The calculation results and the PGE enrichment of rocks and ores show high degrees of melting of the mantle source, which agrees with the plume model of formation of the ore-magmatic system. The recognized province is similar in the type of magmatism and time of its occurrence to the Franklin LIP in northern Canada. It is one of the highly promising ore districts of East Siberia. [Copyright &y& Elsevier]

Abstract: The petrographic, petrochemical, and mineralogical compositions of the Kuvalorog Ni-bearing cortlandite-norite intrusion and endocontact leucodiorites hosting pyrope-almandine garnet are considered. Lamprophyre dikes with scarce pyrope-almandine garnet, first discovered in the massif, and plagioclase-pyroxene symplectites in garnet porphyroblast areas are studied. Judging from the petrography of rocks and the composition of inclusions rich in incompatible elements in the garnet, the mineral was produced by the reaction of orthopyroxene with the anorthite component of plagioclase at the subsolidus stage of formation of endocontact diorites. This reaction was probably favored by the fluid phase abundant in the parental magma of the Kuvalorog intrusion and, especially, in the zones near its contact with relics of terrigenous rocks, where it was produced as a result of the rock dehydration under the thermal effect of the intrusion. [Copyright &y& Elsevier]