Gabbroids of the Bazhenov gabbro-hyperbasite massif (Middle Urals): Material composition and age

Abstract

Research subject. Saussurite-amphibole rocks (metagabbroids) of the Bazhenov gabbro-hyperbasite massif (eastern sector of the Middle Urals).Materials and methods. The material composition of metagabbroids and their mineralogy were studied in the analytical laboratories of the Institute of Geological Geology, Ural Branch, Russian Academy of Sciences (Ekaterinburg), Urals State Mining University (Ekaterinburg), Institute of Mineralogy, Ural Branch, Russian Academy of Sciences (Miass), Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences (Moscow), and Moscow State University (Moscow). Isotope analyzes of zircon were performed on a SHRIMP-II ion microanalyzer at the Center for Isotope Research of VSEGEI (St.Petersburg).Results. The metagabbroids under study feature the following mineral composition: clinozoisite (zoisite, epidote), magnesian hornblende (pargasite, tremolite), albite, clinochlore, and dolomite. According to petro-geochemistry data, saussurite-amphibole rocks were formed after gabbronorites and olivine gabbro-norites, belonging to the alpine-type formation of gabbro-hyperbasite complexes. U-Pb dating showed the ancient age of zircons from metagabbroids, most of the analyzes are grouped below the concordia line, forming a discordia line, the upper intersection of which with the concordia corresponds to the age of 1992 Ma. Two points on the diagram located below the concordia line, although not falling on the continuation of the discordia line, apparently reflect the influence of intermediate stages of metamorphism, and two points with concordant ages of 256 and 251 Ma record the time of the last tectono-thermal episode.Conclusions. The formation of saussurite-amphibole rocks took place under the conditions of greenschist facies. In local areas of the massif, metamorphism reached the zeolite facies with signs of hydrothermal reworking. The primary gabbroids of the Bazhenov massif were most likely formed under the conditions of a suprasubduction basin. The age of zircons of about 2 Ga obtained by the U-Pb method indicates that the crystallization of the studied zircons occurred in the mantle rocks. It is not yet possible to unambiguously explain the presence of grains of these zircons in the metagabbroids of the Bazhenov massif; several possible explanations have been proposed. Zircons with the youngest age (256 and 251 Ma) were most likely formed as a result of metamorphic transformations under thermal influence from the nearby Aduy granite-leucogranite complex.