Accessory Cr-spinel from peridotite massifs of the South Urals: morphology, composition and origin

Abstract

The features of morphology and composition of accessory Cr-spinels from four ophiolitic peridotite massifs of the Southern Urals are considered. Massifs are localized in the Main Uralian Fault zone (Nurali, Mindyak), at its junction with the Sakmar zone (Kempirsai) and in the northern part of the Zilair zone (Kraka). The Kraka, Nurali and Mindyak massifs are composed mainly of lherzolites with subordinate harzburgites and dunites, while harzburgites predominate in the Kempirsai massif and dunites with large chromitite deposits are significantly developed in its southeastern part. The PT–fO₂ formation conditions of lherzolites correspond to the upper mantle below a rift structure: temperature of 700–1000 °C, pressure of 5–12 kbar, and oxygen fugacity varying from –2 to + 0.5 ΔFMQ. The compositional variations of Cr-spinels from primitive lherzolite (Cr# 0.15–0.30, Mg#0.6–0.8) to harzburgite (Cr# 0.3–0.6, Mg#0.5–0.7) and dunite (Cr# 0.6–0.8, Mg#0.4–0.7) and the increase in Mg# value of olivine are a result of synchronous processes of partial melting and plastic flow of the material in the upper mantle. Four main morphological Cr-spinel types are distinguished in lherzolites: (1) fine rods and lamellae within silicate grains and along their boundaries, (2) anhedral and holly-leaf grains closely intergrown with restitic olivine and enstatite, (3) anhedral and subhedral grains in an assemblage with plagioclase and diopside and (4) euhedral grains in dunites. The formation of type 1 grains is interpreted as a result of deformation-induced segregation of trace elements on structural defects of silicates with consequent crystallization of newly formed minerals. The advanced stages of solid-phase transformation produce the larger anhedral and holly-leaf grains in peridotites and euhedral grains in dunites (types 2 and 4 grains). The decompression replacement of a precursor high-pressure mineral (garnet?) is suggested for the formation of the Cr-spinel–plagioclase aggregates. The subhedral and euhedral grains in the assemblage with plagioclase and clinopyroxene could have formed as a result of crystallization from percolating melts or their reaction with restite.