Gabbro of the mantle part of the ophiolite section in the Alapaevsky dunite-harzburgite-gabbro massif (Eastern zone of the Middle Urals)

Research subject. An intrusive body of gabbroids breaking through mantle peridotites (dunites and harzburgites) of the Alapaevsky ophiolite massif (Eastern zone of the Middle Urals). Methods. The contents of petrogenic elements were determined by the X-ray fluorescence method; the contents of rare and scattered elements were studied by the ICP-MS method. The age of gabbroids was determined by 147Sm-143Nd ID-TIMS by isotope dating. Results. The gabbro and their containing ultramafic rocks were established to be of almost the same age of about 580 Ма, which indicates their belonging to a single ophiolite association of the Vendian age. At the same time, the gabbroids of the Alapaevsky massif differ sharply from both isotropic and stratified gabbro of the crustal part of the ophiolite section, fragments of which are observed within the Eastern zone of the Middle Urals, in terms of a significantly reduced content of light REE, rare alkalis, barium, uranium and thorium, as well as the absence of Pb maxima on spider diagrams. Conclusions. The established features in the composition and intrusive occurrence of the gabbro of the Alapaevsky massif among mantle ultramafic rocks indicate that the studied rocks cannot be identified with gabbroids of the crustal part of the ophiolite association. This suggests that this rather large intrusion of gabbro may be analogous to small veins and dikes of gabbroids observed among mantle peridotites in a number of ophiolite massifs, such as Voikar-Synyinsky massif in the Polar Urals and the Samail ophiolite in Oman. The distribution specifics of rare elements on the spider diagrams of the gabbroids of the Alapaevsky massif, i.e., the presence of Sr and Ba maxima and Nd and Th minima, as well as the Harzburgite composition of the mantle restite containing the gabbro body under consideration, indicate that the Vendian ophiolite association of the Eastern zone of the Middle Urals formed in a suprasubduction (pre-arc) environment. A model for the formation of large masses of gabbro in the mantle part of the ophiolite section is proposed.