Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Stanislav Vrána

Manganese-rich garnet-quartz rocks and gneisses in the Bohemian part of the Moldanubian Zone: lithostratigraphic markers

Journal of Geosciences, volume 56 (2011), issue 4, 359 - 374

DOI: http://doi.org/10.3190/jgeosci.106

Manganese-rich garnet-quartz rocks and gneisses occur in the Varied Group of the Moldanubian Zone, Bohemian Massif, in close association with amphibolites, marbles and accompanying graphite gneisses. Fine-grained garnets contain (mol. %) 26-37 spessartine, 36.8-45.9 almandine, 11.1-14.3 pyrope, 2.9-21.0 grossular. Minor amphibole present in some samples is ferrimagnesiohornblende with 0.17-0.22 Mn pfu. Accessory ilmenite contains 24-34 mol. % pyrophanite and 1.7-5.8 hematite. Some closely associated impure calcite marbles (or amphibolites) carry Ti-bearing andradite, epidote, diopside-hedenbergite, and accessory magnetite.
Data from the Varied Group indicate that manganese enrichment took place both under oxidizing and reducing conditions, but the Mn-garnet-quartz rocks are oxidic. Normalization of major-element contents in the Mn-rich rocks by average abundances in Varied Group paragneisses shows ten- to hundred-fold enrichment in MnO and a slight to moderate increase in CaO and P₂O₅. Values for Na₂O and K₂O indicate severe depletion in some samples, but contents of other oxides are close to unity. Comparison of chondrite-normalized REE patterns in Mn-rich rocks with data for ordinary paragneisses (Varied Group) also indicates that detrital component in Mn-rich rocks was closely comparable to material supplied for protolith of paragneisses. This permits to ascribe the spike of Mn to likely short-lived exhalative processes.
Manganese-rich garnet-quartz rocks and gneisses remained unnoticed till now mainly owing to a small thickness of their layers, typically less than 1 m. Structural relations in regions carrying Mn-rich rocks indicate their occurrence in relict domains of preserved D₁ structures with NW-SE trending foliation. It is suggested that in regionally prevalent areas dominated by superimposed refoliation (D₂, D₃) the thin layers of Mn-rich rocks were likely reduced to boudins dispersed in paragneiss matrix. Comparison with published data for similar Mn-rich rocks abroad (including the so-called coticules) strongly indicates a lithostratigraphic correlation potential of Mn-rich garnet-quartz rocks.