Nitrogen-bearing fluids, brines and carbonate liquids in Variscan migmatites of the Tatra Mountains, Western Carpathians - heritage of high-pressure metamorphism

Abstract

Gaseous inclusions in migmatites of the Tatra Mountains contain 5–100 mol.% nitrogen, contrasting with essentially the nitrogen-absent, mostly CO 2 -CH 4 composition of fluid inclusions in other migmatites. While CO 2 predominates in the Tatra metapelite migmatites, pure nitrogen inclusions are typical of the associated metabasite migmatites. CaCl 2 -rich brines (>30 wt.% NaCl eq.), halite, graphite and carbonate globules are the additional phases accompanying the nitrogen-rich fluids in the migmatite leucosomes. Monophase, crack-bound carbonate inclusions, consisting of calcite in metabasites and of Mg-bearing siderite in Fe-rich metapelites, are believed to represent a carbonate liquid, coexisting with and exsolving from the CO 2 -N 2 -brine fluid at temperatures between 550–700°C. Compositions and densities of the N 2 -CO 2 fluids and associated brines are similar to those in high-pressure granulites and eclogites. Therefore, these volatiles are interpreted to have been inherited from a high-pressure stage, pre-dating the migmatisation. Together with mineral textures and assemblages, the fluid-inclusion record is indicative of a substantial crustal thickening in the Western Carpathians during the Variscan orogeny. Essentially pure CO 2 -N 2 , graphite-saturated fluids in metapelites have been generated at dry conditions ( X H 2 O = 0.2-0.25), resembling those of typical granulite-facies metamorphism.