Acta Musei Moraviae

Regional typization of the iron skarns of the Bohemian-Moravian Heights (Českomoravská vrchovina)

The skarns of the Bohemian-Moravian Heights can be classified as iron skarns. They are pre-Variscan in age and have been regionally metamorphosed in a migmatization zone under abyssal conditions. Apart from some input of granitic metatect from enclosing gneisses, metamorphism took place isochemically. Due to metamorphism the skarns acquired the petrographical characteristics of metamorphic rocks. During regional metamorphism, almandine, grossularite-almandine and hornblende originated. Diopside-hedenbergite, grossularite-andradite, epidote and magnetite are relict minerals. Minerals of the skarns are characterized on the basis of their chemical composition. Chemical equilibria of some assemblages are discussed by use of partition coefficients and the phase rule. To discuss the premetamorphic genesis, the following features of the skarns are important: Striking geological and partly petrographical similarities to bodies of marbles of the region; spatial relations to massifs and large complexes of orthogneisses (in some of them, presence of relict greisens was proved); evidence of relict metasomatic zoning and stage development, which is especially obvious in the superposition of magnetite mineralization on older silicate assemblages; presence of minerals (or pseudomorphs after them) which can originate only under conditions of contact metamorphism; presence of geochemical features enabling a distinction between skarns-after-carbonates and skarns-after-noncarbonates. All these features suggest that, before regional metamorphism, the skarns were typical primary skarns developed through a high-temperature metasomatism, mostly of carbonate rocks, which was caused by magmatogeneous ore fluids. The hypothesis that they have been originally submarine-exhalative deposits is not sufficiently supported.