Chrome–spinels in gabbro–wehrlite intrusions of the Pechenga area, Kola Peninsula, Russia: emphasis on alteration features

Abstract

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 Fe 3+. 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.