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

Abstract

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 and 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 s light to moderate increase in CaO and P 2 O 5 . Values for Na 2 O and K 2 O 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 shortlived 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 D 1 structures with NW–SE trending foliation. It is suggested that in regionally prevalent areas dominated by superimposed refoliation (D 2 , D 3 ) 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.