Acta Musei Moraviae

Heavy minerals from moldavite-bearing sediments in the vicinity of Vrábče: paleogeographic implications

The area of Southern Bohemia in vicinity of the Budějovice Basin is known for the occurrence of moldavites. This study provides sedimentological and mineralogical descriptions of the Miocene to Pleistocene moldavitebearing sediments in the vicinity of Vrábče (Vrábče and Koroseky beds); heavy minerals from these sediments were investigated in order to reconstruct their source areas. Both sediment types (Vrábče and Koroseky beds) contain variable amounts of kyanite, tourmaline, garnet, staurolite, and rutile in their transparent heavy mineral assemblages. The heavy minerals were derived mainly from migmatites to paragneisses, and their minor part was sourced most likely from weathering of marbles, calc-silicate rocks, orthogneiss, muscovite granites and/or pegmatites. Eclogites, amphibolites, and ultrabasic rocks were identified as significant sources of material for forming the Vrábče and Koroseky beds based on the chemical composition of garnet and spinel. At least a part of the kyanite probably comes from the granulites. The Miocene Vrábče beds represent the product of alluvial/lacustrine-fan sedimentation. Based on their abrasion degree heavy minerals in these sediments were transported on short distances (max. several km), probably from rocks strongly affected by Cretaceous and Paleogene weathering (Prachatice Mountains). The assemblage and composition of heavy minerals from fluvial sediments of the Koroseky beds show excellent correlation with mineralogy of rocks in their close vicinity. Detrital opaque heavy minerals are significant constituents of Koroseky beds. The chemical composition of ilmenite clasts suggests a source in basic rocks (amphibolites, eclogites). The dominance of quartz and/or feldspars in the clastic material, together with the prevalence of zircon and Th-rich monazite in heavy minerals assemblage, indicate that a significant part of the clastic material was derived from the weathering of granitoids (mainly durbachites). The minor part of the heavy mineral assemblages (e.g., staurolite, cassiterite) either comes from more distant parts of the Moldanubian Unit or represents a recycled material from older sedimentary sequences (e.g., Neogene, Cretaceous sediments).