The Riphean Meteshikha island-arc peridotite–gabbro massif (western Transbaikalia)

Abstract

Abstract 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.