Metamorphic history of skarns, origin of their protolith and implications for genetic interpretation; an example from three units of the Bohemian Massif

Abstract

Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutná Hora Complex were studied with respect to their metamorphic evolution, major-and trace-element geochemistry, oxygen isotopic composition and zircon ages. Skarns form competent lenses and layers in metamorphosed siliciclastic rocks and preserve some early deformation structures and several equilibrium assemblages representing the products of successive metamorphic reactions. The main rock-forming minerals, garnet and clinopyroxene, are accompanied by less abundant magnetite, amphibole, plagioclase, epidote ± quartz. In the Svratka Unit the early prograde M 1 , prograde/peak M 2 , and retrograde M 3 metamorphic stages have been distinguished. Metamorphic conditions in skarns of the Moldanubian Zone are limited to a relatively narrow interval of amphibolite facies. The prograde and retrograde events in the Kutná Hora Complex skarns probably took place under amphibolite-facies conditions. The presence of magnetite and the increasing proportion of the andradite component in the garnet indicate locally increased oxygen fugacity. Skarn geochemistry does not show systematic differences in the skarn composition among the three units. The regional variations are exceeded by differences among samples from individual localities. The Al 2 O 3 /TiO 2 , Al 2 O 3 /Zr, TiO 2 /Nb ratios point to the variable proportion of the detrital material, combined in skarn protoliths with CaO and FeO, the major non-detrital components. The skarns exhibit elevated abundances of Cu, Zn, Sn and As. The Eu/Eu* ratio varies in the range of 0.5-8.6, the total REE contents vary from 8 to 345 ppm. The lowest ΣREE values (< 100 ppm) occur in skarns with magnetite mineralization. The wide intervals of ΣREE and Eu/Eu* values are interpreted to indicate variations in the temperature and redox conditions among layers of the same locality and at various localities. The oxygen isotope compositions of garnets, pyroxenes and amphiboles from skarns of the Svratka Unit exhibit a range of δ 18 O = 0.1 to 4.1 ‰. In situ (laser-ablation ICP-MS) U-Pb dating of zircon from one of the Svratka Unit skarn bodies yielded a wide range of ages (0.5-2.6 Ga), supporting the detrital origin of this zircon population. The skarn protoliths were probably rocks of mixed detrital-exhalative origin deposited on the sea floor. The geological position of skarns, with their structural and metamorphic record, probably reflect tectono-metamorphic evolution shared with that of their host rocks. The geochemical characteristics, including oxygen isotopic compositions and the presence of detrital zircons with a wide range of ages exclude metasomatic, and point to a sedimentary-exhalative mode of origin for the studied skarns.

Geological position, mineral assemblages and contamination of granitic pegmatites in the Moldanubian Zone, Czech Republic; examples from the Vlastějovice region

“…Carswell and O'Brien 1991;Janoušek and Holub 2007). It was more or less overprinted during a rapid decompression by (ii) a MP-HT event at T < ~700 °C and P ~0.4-0.6 GPa (e.g., Pertoldová et al 2009). This stage was very likely connected to partial melting of the rocks.…”

“…%), which originated during an early stage of MP-HT regional metamorphism of the host Fe-skarn (Žáček 1997; Žáček et al 2003). Fluorine-rich garnet is known also from other Fe-skarn localities in the Moldanubian Zone (Pertoldová et al 2009;S. Houzar, pers.…”

Comparison of lithology and tectonometamorphic evolution of units at the northern margin of the Moldanubian Zone: implications for geodynamic evolution in the northeastern part of the Bohemian Massif

“…The δ 18 O (SMOW) values for garnet and clinopyroxene from skarn localities in the Moldanubian Zone and the Svratka Unit are low, ranging from -0.1 up to +4.0 (Pertold et al 1997;Pertoldová et al 1998Pertoldová et al , 2009Drahota et al 2005). Such low values do not support the idea of contact metamorphic genesis ).…”

“…* U-Pb age determinations on zircon are interpreted as dating the HP events ** unpublished data R. Nahodilová *** van Breemen et al (1982) attributed this monazite age to low-T shearing and fluid activity indicate that the previously identified HP-HT event in skarns (Buriánek et al 2009b;Pertoldová et al 2009;Verner et al 2009) must have been of pre-Variscan age. On a regional scale, the HP-HT processes including extensive partial melting and emplacement of calc-alkaline granitoids could be related with crustal exhumation in final stages of the Cadomian orogenic processes, or with continental extension during early Cambrian rifting (Linnemann et al 2000).…”

Nový Výskyt Skarnu v Gföhlské Jednotce U Vevčic U Jevišovic: Minerální Asociace Skarnu a Kontaminovaných Amfibolických Pegmatitů

“…Zatímco na převládající složení protolitu těchto hornin a tedy na podíl a rozsah metasomatických procesů při jejich vzniku nepanuje mezi badateli jednotný názor, shoda existuje v názoru na silné polymetamorfní přepracování těchto skarnů (srov. Němec 1991;Pertold et al 2000;Pertoldová et al 2009). …”