Mineralogical features of gabbroic rocks from Mindyak mafic-ultramafic massif

Abstract

The mineralogical features of several samples of gabbroids from the eastern part of the Mindyak mafic-ultramafic massif have been studied. The compositions of rock-forming and accessory minerals are given. The rocks underwent autometamorphic transformations with the replacement of the primary pyroxene-plagioclase association to amphibole and essentially sodium feldspar secondary assemblage. Typical accessory minerals are titanite, ilmenite, titanomagnetite, and apatite. The gabbroids of the massif cover a wide range of compositions from melanogabbro to diorites, also demonstrating significant variations in the contents of petrogenic oxides and trace elements. In terms of REE distribution, most of the studied samples of gabbroids are comparable to basalts of mid-ocean ridges, but often differ from them in high concentrations of strontium and some other rare elements. The gabbroids of the eastern part of the massif have U-Th mineralization in association with aggregates of xenomorphic zircon grains. The origin of these finds has not been established in the present study. Estimates of the PT conditions by the amphibole barometer show the presence of two maxima of values – 2.8–5.1 kbar and less than 2 kbar, which most likely fix two different stages in the formation of rocks – autometamorphism in the magma chamber during cooling and interaction with magmatic fluids (magnesian hornblende), and low-temperature (greenstone) transformations (actinolite).