Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
An interesting mafic dyke from the Holedná - Bosonohy mine adit (Brno Massif): A transitional petrographic type between microdiorites and lamprophyres
Abstract
The Cadomian Brno Massif (the largest exposure of Brunovistulicum) is cut by numerous post-collisional mafic calc-alkaline dykes of supposed Variscan age. From the petrographic point of view, microdiorite dykes strongly prevail over the less abundant lamprophyres. In this article, we describe a new finding of a spessartite dyke which represents a transitional petrographic type between the typical microdiorites and amphibole lamprophyres. The dyke is characterised by abundant phenocryst of anhydrous mafic minerals (clinopyroxene, pseudomorphosed olivine). On the other hand, the dyke also contains euhedral to subhedral matrix amphiboles (mainly magnesiohastingsite) poikilitically enclosed by larger feldspar aggregates. The presence of matrix amphiboles suggests an increase in water activity in the parental melt.
