Chrome-spinels are common accessory minerals of the Pechenga ultramafic rocks in NW Russia, varying in metamorphic grade from sub-greenschist to amphibolite facies. Magmatic chrome-spinels can be grouped into two main types. Spinel-1, which crystallized first, is represented by Al,Ti-chromite. It occurs mainly as inclusions in olivine and, rather less commonly, enclosed within interstitial minerals. Spinel-2 crystallized after spinel-1 and has composition varying from Ti-rich chromite to Ti-rich chrome-magnetite. Spinel-2 occurs in the interstitial spaces between olivine crystals and enclosed within intercumulus silicates. Zoned crystals, with spinel-1 in the core, overgrown by spinel-2 are also common. Cr-poor titanomagnetite occurring in pyroxenites and gabbro apparently represents further evolution of spinel-2 phases. Metamorphic chrome-spinels are represented by ferritchromit and Cr-magnetite which are the dominant chrome-spinel phases in the amphibolite facies rocks. The continuous trend of the magmatic chrome-spinels composition is indicative of their intensive post-cumulus alteration by reaction with evolving intercumulus liquid. This reaction led to depletion of the earlier crystallized chrome-spinels in Mg, Cr, and Al and their enrichment in Ti and Fe3+. Oxidation-exsolution of ilmenite from Ti-rich chrome-spinels is another common process of post-crystallization alterations. Serpentinization of olivine led to formation of magnetite, which overgrew and cut chrome-spinel grains. Low-temperature hydrothermal processes, in particular talc-carbonate alterations, could cause a local loss of Ti and further redistribution of Mg, Mn and Zn. Prograde metamorphic reactions led to formation of ferritchromit and magnetite replacing primary chrome-spinels. Another common process, affecting Ti-rich chrome-spinels, is a prograde metamorphic recrystallization of exsolved ilmenite, which changes from submicron lamellae through a regular grid of coarse ilmenite crystals and further to their irregular blebs and rims. Metamorphism of epidote-amphibolite and amphibolite facies has led to the Liberation of most of the elements. Metamorphically liberated Cr was captured by antigorite. (C) 1998 Elsevier Science B.V. All rights reserved.
