Acta Musei Moraviae

Corroded garnets in the association of heavy minerals of Balinka conglomerates: their morphology and chemical compositon (Upper Carboniferous, Boskovice Basin)

Two populations of garnet with oriented corroded surfaces were found in the assemblage of the heavy minerals of the Upper Carboniferous Balinka conglomerates at the base of the sedimentary formation of the Boskovice Basin. The dominant almandine garnets Alm_65–85 Prp_13–18 Sps_1–2 Grs_1–5 can be traced to gneisses and mica-schists of the adjacent geological units. Minor pyrope dominant garnets Prp_68–71 Alm_13–20 Grs_11–12 Uvt_5–9 Sps_1–2 come from serpentinized peridotites. Both types of garnets were affected by intrastratal dissolution and exhibit significant oriented corrosion of their surface. There are known types of micro-textures such as etch pits, mamillae features and parqueting, and imbricate wedge marks (IWMs) with hacksaw terminations. Etching on (110) dominates on crystallographically oriented faceted surfaces. No authigenic overgrowth of garnets were found, which would differ in different morphology and especially distinct core-rim chemical composition. The pores after the decomposition of the garnets were filled by calcite and clay minerals and sole Cr-illite has been identified. Strong corrosion of garnet grains probably took place during the diagenesis of sediment at depths of ~2500 meters. The depth of sediment burial, along with the greater permeability of grain-heterogeneous conglomerates, in contrast to sandstones and aleuropelites, also caused an overall depletion of garnet-containing heavy-mineral assemblages.