RECRYSTALLIZATION OF THE LARBA GRANULITE

Abstract

For zircons of metapelitic gneiss from the Zverev group in the Upper Larba River basin2, uranium-lead dating gave an age of 2.6 b.y. This important estimate is significant because the zircon sample was made up mainly of grains trapped within garnet crystals, so its age, given the absence of any visible retrograde minerals, suggests the age of the principal stage of metamorphism. The authors consider the problem of interpreting this age estimate from petrologic data. The presence of several garnet generations that seem to have been formed at different times, and the pronounced, extensive change in composition of large garnet crystals suggest a prolonged recrystallization of the rock minerals. Among biotite crystals, though, it is virtually impossible to identify 'relic' and 'newly formed' compositions. Such variations in chemical properties of biotite can easily be explained by local changes in the chemical composition of microscale rock patches. The strong recrystallization of our Larba gneiss specimen is indicated, too, by geothermobarometric data.