Reactions behaviour during kelyphite and symplectite formation: a case study of mafic granulites and eclogites from the Bohemian Massif
Messiga, Bruno; Bettini, Emanuela
European Journal of Mineralogy Volume 2 Number 1 (1990), p. 125 - 144
published: Mar 8, 1990
manuscript accepted: Oct 13, 1989
manuscript received: Jul 10, 1989
ArtNo. ESP147050201007, Price: 29.00 €
Abstract The retrogression of high-grade metamorphic assemblages (mainly garnet + clinopyroxene ± quartz) in mafic rocks (granulites and eclogites) takes place by the development of: 1) coronite rims between garnets and pyroxenes ; 2) symplectite (plagioclase + clinopyroxene) replacements after sodic pyroxene ; 3) kelyphite overgrowths on garnet. Kelyphitic textures consists of fibrous intergrowths of both anhydrous (orthopyroxene + plagioclase + spinel) and hydrous (amphibole + plagioclase ± Fe-spinel) mineral assemblages. The symplectites are composed of vermicular intergrowths of plagioclase and clinopyroxene. An important microtextural aspect of such rocks is the fact that garnet show an outer coronitic reaction boundary and an inner kelyphitic rim. Coronitic reactions can produce plagioclase + orthopyroxene (by reaction between clinopyroxene and garnet in granulites), plagioclase + orthopyroxene + clinopyroxene (by reaction between garnet and quartz in eclogites) or plagioclase + clinopyroxene + spinel (by reaction between garnet and sodic clinopyroxene in eclogites). The coronitic plagioclase occurs as internal rims close to the garnet. According to mass balance calculations, coronitic textures result from short range diffusion controlled reactions. On the contrary, kelyphitic textures may be regarded as isochemical break-down products from garnet. Therefore, the kelyphite blastesis seems to be a consequence of lower diffusion rate of chemical elements through the corona. The symplectites are derived from an exsolution reaction involving sodic clinopyroxene which yields plagioclase + clinopyroxene. The P-T path of these rocks is consistent with a retrograde metamorphism in which the decrease in pressure is accompanied by variable rates of undercooling. The increased undercooling completely suppresses the transformation processes, thus preserving mineral relics. Therefore, the rocks experienced a metamorphic history characterized by a re-equilibration rate lower than the rate of change of state variables. The influx of water during metamorphic re-equilibration enhances the degree of chemical communication and increases the reaction rate giving an enlargement of the reaction domains.