Geochronology and petrology of pyroxene-garnet skarns (eastern Bohemian Massif): implications for the source and evolution of the Variscan continental crust

Abstract

Zircon ages (U-Pb, LA-ICP-MS) obtained from skarns in the Kutná Hora and Svratka units and in the Moldanubian Zone (Bohemian Massif) show significant variations between the lithotectonic units as well as among samples from individual localities. The ages fall between 310 Ma and 3.1 Ga. We suggest that such large spread is a result of both different zircon clastic grain sources and subsequent metamorphic histories. The oldest Neoarchaean to Neoproterozoic ages are interpreted as detrital zircon grains from pre-Cadomian basement. Pronounced age maxima between 600 and 520 Ma (late Neoproterozoic to early Cambrian) were found in skarn samples from the Moldanubian Zone, Svratka and Kutná Hora units. We interpret these as maximum ages of skarn precursor deposition. Apparent Cambrian-OrdovicianSilurian to early Devonian zircon ages were obtained from non-mineralized samples of the Moldanubian Zone and also in the Kutná Hora Unit. This wide temporal interval could reflect maximum ages of skarn precursors related to disintegration of abundant early Palaeozoic magmatic rocks, provided the zircons were not modified by later metamorphism or fluid circulation. The early Carboniferous ages are interpreted to result from strong Variscan HT metamorphism and subsequent circulation of post-metamorphic fluids. Zircons of this age are dominant in skarns with massive magnetite mineralization, while the non-mineralized samples preserve the older ages.

Pre-collisional crustal evolution of the European Variscan periphery: Constraints from detrital zircon U–Pb ages and Hf isotopic record in the Precambrian metasedimentary basement of the Brunovistulian Domain

“…In the Bohemian Massif, detrital zircon age populations have been recently studied in the Saxothuringian Zone (Collett et al, 2020;Linnemann et al, 2014;Tabaud et al, 2021;Žáčková et al, 2012), Teplá-Barrandian Unit (Drost et al, 2011;Hajná et al, 2018) and Moldanubian Zone (Košler et al, 2014;Pertoldová et al, 2014;Soejono et al, 2020;Žák and Sláma, 2018). Detrital zircon ages from the Brunovistulian Domain have so far only been investigated in the metasedimentary nappes of the Moravo-Silesian Zone (Jastrzębski et al, 2021;Košler et al, 2014) and lower Cambrian sandstones of the Upper Silesia Block in the northern Slavkov Terrane (Habryn et al, 2020;Źelażniewicz et al, 2020).…”

Origin of the pegmatite veins within the skarn body at Vevčice near Znojmo (Gföhl Unit, Moldanubian Zone)

“…Amphibole-plagioclase pegmatite veins were found at several other localities in the Moldanubian Zone (including Kutná Hora and Svratka units at its N and NE boundaries;Němec 1991;Pertoldová et al 2009). By contrast they are missing in the Saxothuringian Zone (Němec 1963a).…”

“…By contrast they are missing in the Saxothuringian Zone (Němec 1963a). The skarns associated with pegmatites occur at two tectono-stratigraphic levels in the Moldanubian Zone (Pertoldová et al 2009(Pertoldová et al , 2014: along the contact of the Gföhl Unit with the Varied Unit (Vlastějovice, Županovice), and within the Gföhl Unit (Rešice, Slatina, Vevčice and Kordula).…”

“…However, they are only preserved as rare inclusions in garnet and clinopyroxene. Garnet often contains epidote inclusions (Pertoldová et al 2009), whereas amphibole inclusions are rare. Clinopyroxene from the skarns in the Moldanubian Zone sometimes contains inclusions of the hisingerite-neotocite group minerals (Drahota et al 2005).…”

“…Predominant biotite migmatites contain rare intercalations of skarn (e.g., Němec 1963b(e.g., Němec , 1991Pertold et al 1997;Pertoldová et al 2009) and Ca-rich metasediments (Matějovská 1975;Matějovská et al 1987;Houzar et al 2014).…”