Vývoj metamorfních fluid pozdního stádia regionální metamorfózy v keprnické klenbě silezika

Abstract

Neo-Proterozoic rocks (Brunovistulian Unit) are dominant in the studied area of Silesicum, which have complex nappe, thick-skinned and thrust sheet geological structures. The Keprník Dome (Keprník nappe) represents the easternmost part of crystalline complexes of the Silesian domain and is dominated by various types of orthogneiss, gneiss, mica schist and phyllite, with a small occurrence of calc-silicate rocks. These rocks are affected by pre-Variscan and Variscan metamorphism, dominantly staurolite-sillimanite and garnet zone and late Variscan to Cenozoic brittle deformation. Hydrothermal Variscan quartz veins and surrounding rocks (biotite-bearing paragneiss and orthogneiss) were studied in the Keprník dome geological unit to confront the type of fluids enclosed in fluid inclusions and the intensity of metamorphism that affected the host rocks. The study of fluid inclusions in quartz veins was supplemented by Raman spectrometry and microscopic study of rocks, EMPA and mineral thermometry. Temperatures 616 to 643 °C obtained by the Ti-in-biotite method for biotite paragneisses are compatible with the peak metamorphic conditions estimated by previous authors. Thermometer for orthogneiss provided temperatures between 688 to 699 °C using the Ti-in-biotite thermometer. The lower temperatures (483 to 529 °C) obtained using magnetite-ilmenite thermometer document later retrogression. Three types of fluids were found in the quartz of the Variscan hydrothermal veins based on the phases present, respectively components: I. L+V, H2O–NaCl ± K+ ± Mg2+ ± Ca2+, II. L+V+S±S2, H2O–NaCl–CaCl2 ± K+ ± Mg2+, III. L1+L2+V, H2O–NaCl–CO2 ± CH4 ± K+ ± Mg2+ ± Ca2+ (L – liquid, V – vapour, S – solid). Type II inclusions contain aqueous fluids with high salinities, which would correspond to the post-Variscan systems found (Slobodník et al. 2010a, 2020) in the Silesicum area. On the other hand, they have high Th temperatures, reaching values of over 300 °C. Only results from type I could be used to calculate isochores and derive P-T conditions for the formation of Variscan veins, because in type III often occur fluid inclusion decrepitation and the impossibility of measuring total homogenization. Types I and III according to the structural position of fluid inclusion (uncertain or primary and pseudosecondary inclusions), microthermometric data and composition, they are considered to be metamorphic types of fluids generated by Variscan metamorphism. The evaluation of isochores of the first type of fluids using a lithostatic thermobaric gradient (50 °C/270 bar/km) indicates the entrapment of fluids, or formation of inclusions under conditions of 190–300 °C and 90–160 MPa. This type of fluid was very likely captured at a later stage of metamorphism, after the crystallization of magnetite and ilmenite, and represents a retrograde stage of metamorphism.