Mineralogy, petrography and geology of the Nedvědice marbles, Svratka Crystalline Complex; a reviewArticle PDF
Mineralogy, petrography and geology of the Nedvědice marbles, Svratka Crystalline Complex; a review
Nedvědice marbles occur in a narrow belt situated along the eastern and northeastern margin of the Svratka Crystalline Complex (Moldanubian Zone). Elongated bodies of calcite marbles up to 150 m long and 40 m thick, with abundant boudins of various calc-silicate rocks, are enclosed in muscovite-biotite paragneisses. The following types of marbles were distinguished: a) silicates-poor marbles with the assemblage: Cal + Phl + Tr ± Di ± Dol ± Gr; b) silicates-enriched skarnized marbles (sky blue Cal + Ves + Wo ± Grs ± Di ± Czo) and c) rare pale red marbles (Cal + Bt + Ms + Ep ± Di ± Qtz ± Kfs). Calc-silicate rocks include diopside gneiss and diffusive reaction skarns (Di + Qtz + Kfs + Czo + Pl + Ttn ± Amf ± Grs) and a variety of hornfelses, skarnoids and skarns with the mineral assemblage – Di + Grs + Ves + Wo ± Kfs. The mineral assemblages given above are products of complex metamorphic evolution. (I) The early stage – isochemical regional metamorphism of amphibolite facies – is characterized by the relic equilibrium assemblage Dol + Tr. (II) Next stage is related to intrusion of peraluminous granites into host metapelite complex. Associated fluids rich in H2O, F and locally also B, K and Na reacted with marbles and intercalations of diopside gneiss and hornfels (T > 550°C at P = 200 MPa, XCO2 < 0.2–0.1) and produced skarnoids and skarns with the assemblages involving vesuvianite, grossular, diopside and wollastonite, as dominant minerals, and minor to accessory K-feldspar, clinozoisite, fluorite and Fe–Cu–Sn–As–Sb–Bi mineralization. (III) postskarns with ferroaxinite, tourmaline and part of K-feldspar and late vesuvianite, locally filling fissures. (IV) Subsequent metamorphism is related to ductile deformation of marbles at low temperature (T < 450°C for 200 MPa or eventually < 650°C for 500 MPa) along the zone about 50 km long but only several tens m thick. Formation of sky blue calcite and equilibrium assemblage Ves + Wo specific for the Nedvědice marbles at high activity of H2O (XCO2 < 0.05) is related to this process as well. Source of external H2O-rich fluids is not clear but it seems to be related to the Variscan nappe development. (V) This retrograde stage involves namely formation of pectolite on fissures. The Nedvědice marbles differ from other marbles of the Svratka Crystalline Complex in dissimilar geological position in the outermost part of the unit, low MgO and SiO2 contents, and by the assemblage – sky blue Cal + Ves + Wo. The mineral assemblage of Nedvědice marbles with sky blue calcite, abundant vesuvianite and wollastonite, accessory malayaite, zircon and native Bi is unique in the Bohemian Massif as well as regional distribution of the equilibrium assemblage Wo + Ves in ductile domains in marbles located in the narrow belt about 50 km long.
Stanislav Houzar, Hana Doležalová, Vladimír Hrazdil, Anna Pfeiferová, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic, e-mail: firstname.lastname@example.org
Milan Novák, Institute of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, e-mail: email@example.com