Nizhneteberdinskiy serpentinite massif: composition and evolution (Greater Caucasus)

Abstract

Relevance. In the tectonic folded-boulder structure of the Greater Caucasus serpentinites are mainly associated with the zone of the Peredovoy Ridge, where they are associated with Hercynian tectonic sheets or large faults limiting blocks of pre-Paleozoic crystalline rocks. Information on the conditions of formation of the Caucasian apohyperbasites protoliths (including their formational affiliation and geodynamic confinement), as well as their metamorphic transformations in crustal conditions remain debatable, which is largely due to the insufficient study of the mineral composition. The aim of the work is to gain data on the mineral composition of serpentinites of the Nizhnetberdinskiy massif, to analyze specific features of the zonal chromespinelides’ composition, to recieve data on formation conditions and transformation of apohyperbasites. Methods. X-ray phase analysis, electron-probe microanalysis and electron microscopy, Mössbauer spectroscopy. Results. The Nizhneberdinskiy massif’ serpentinites have a loop texture and chrysotile-lysardite composition; magnetite, chrome-spinelides, hematite, chlorites, quartz, calcite, magnesite, dolomite, and brucite are associated with serpentines. Chrome-spinelides by structure and composition are subdivided into two groups. One group consists of rounded homogeneous massive grains of alumochromite composition (Cr 0.69-0.73, Mg 0.47-0.51), which are confined to the viscoplastic deformation zones of the rocks. The other is irregularly shaped zoned grains with chromepicotite cores (Cr 0.36-0.43, Mg 0.66-0.72) and ferrichomite – chrome-magnetite rims. The formation of the chrysotile-lysardite association occurred in the low-temperature zone of the greenschist facies in an oxidizing environment under the influence of hydrothermal solutions saturated with trivalent iron, silica, and manganese. Under these conditions, characteristic “atoll” porous chromespinelides enriched with SiO2 and MnO were formed due to reprocessing by hydrothermal solutions of already zoned (affected by metamorphic recrystallization) chromespinelides. The composition of relic chromespinelide cores corresponds to the ophiolitic trend. The compositions of the Nizhneteberdinskiy massif are “shifted” to an area of abyssal peridotites. This trend is consistent with the association of serpentinites and Middle Paleozoic phyllites and graywacke rocks of the Tohan tectonic cover, referred to the complex of the pre-arc basin by some authors.