Geochemistry and petrology of high-pressure kyanite-garnet-albite-K-feldspar felsic gneisses and granulites from the Kutná Hora Complex, Bohemian Massif

Abstract

*Corresponding author The Kutna Hora Complex (KHC), bordering the Moldanubian Zone at the north, represents a crustal stack of several allochthonous units. This study presents new interpretation based on detailed whole-rock geochemical analyses, petrology, mineral analyses and geothermobarometric calculations. Samples were selected from the Malin and the Běstvina units, with preference for kyanite-bearing albitic felsic gneisses with high-pressure metamorphic record. Three types of kyanite-garnet-albite-K-feldspar rocks studied include: (1) an early Variscan felsic granulite from the Běstvina Unit equilibrated under eclogite-facies conditions (P = 1.8–2.1 GPa, metamorphic event M 1 ), (2) a garnetkyanite leucosome from a polymetamorphic migmatite (P ~ 1.6 GPa, metamorphic event M 2 ), and (3) a felsic kyanite–garnet–muscovite gneiss derived from garnetiferous leucogranite, equilibrated under amphibolite-facies conditions (P = 0.9–1.1 GPa, metamorphic event M 3 ). All the three rock types exhibit geochemical characteristics of highly evolved leucogranitic rocks with low Zr/Hf ratios (14 to 19), relatively low FeOt/MnO (10 to 33), and contain < 30 ppm Zr, < 15 ppm Y, and low ΣREE (< 28 ppm) abundances. The kyanite–garnet–muscovite felsic gneiss contains locally garnet porphyroclasts up to 3 cm in diameter, with up to 28 mol. % spessartine and 0.55 wt. % P 2 O 5 , which represent relics from the magmatic stage of the garnetiferous leucogranite precursor. Accessory tourmaline (dravite–schorl) and dumortierite occur in migmatitic gneisses of the Malin Unit, while dumortierite is common in the felsic gneiss (M 3 ). Textural relationships indicate a late, post-kinematic growth of tourmaline and dumortierite in relation to D 3 /M 3 . The repetitive high-pressure record manifested during three metamorphic events in the Kutna Hora Complex represents a unique example in the Bohemian Massif. These relations indicate a separate evolution of the Kutna Hora Complex and the Moldanubian Zone during much of their Variscan tectonometamorphic history. Several aspects of possible correlation among the Kutna Hora Complex (plus the Svratka Unit) and the Orlice–Sněžnik Unit at the Czech–Polish border, which all have a record of Palaeo-Variscan (U)HP evolution, are discussed.