Abstract

Gaseous inclusions in migmatites of the Tatra Mountains contain 5–100 mol.% nitrogen, contrasting with essentially the nitrogen-absent, mostly CO2-CH4 composition of fluid inclusions in other migmatites. While CO2 predominates in the Tatra metapelite migmatites, pure nitrogen inclusions are typical of the associated metabasite migmatites.

CaCl2-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 CO2-N2-brine fluid at temperatures between 550–700°C.

Compositions and densities of the N2-CO2 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 CO2-N2, graphite-saturated fluids in metapelites have been generated at dry conditions (XH2O = 0.2-0.25), resembling those of typical granulite-facies metamorphism.

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