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://dx.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.

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ISSN: 1802-6222

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