Online Resource 2: Petrology and mineral chemistry
Janoušek V, Aichler J, Hanžl P, Gerdes A, Erban V, Žáček V, Pecina V, Pudilová M, Hrdličková K, Mixa P,
Žáčková E. Constraining genesis and geotectonic setting of metavolcanic complexes: a multidisciplinary study
of the Devonian Vrbno Group (Hrubý Jeseník Mts., Czech Republic). International Journal of Earth Sciences.
Corresponding author: Vojtěch Janoušek, Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech
Republic; vojtech.janousek@geology.cz
The volcanic rocks studied show throughout the volcanic belts similar and very simple
mineral assemblages, following the unifying metamorphic overprint under the lower
greenschist facies. The following minerals were found based on microscopy and electron
microprobe: quartz, albite, potassium feldspar, actinolite, biotite, chlorite, white mica,
epidote, allanite, titanite, ilmenite, titanohematite, magnetite, calcite, rutile, tourmaline,
zircon, monazite, apatite, tourmaline, pyrite, and, “limonite”.
Western Volcanic Belt
Felsic rocks are composed of quartz + potassium feldspar + albite + white mica; biotite,
chlorite (clinochlore), epidote, titanite, ilmenite, rutile and scarce calcite are minor to
accessory minerals.
The rocks of mafic WVB of andesite to dacite composition are composed of
albite + quartz + biotite + chlorite + epidote, sometimes also with some white mica, with
minor to accessory titanite, calcite, potassium feldspar, ilmenite and REE-enriched epidote.
Mafic WVB rocks of basaltic composition contain albite + chlorite + Fe-rich epidote with
minor to accessory actinolite, titanite, calcite, opaques and tourmaline.
The rock-forming minerals show very uniform compositions throughout wide
spectrum of chemical compositions. Plagioclase in all lithologies is pure albite with less than
1 mol. % of anorthite component. Potassium feldspar contains up to 0.8 wt. % Na2O and up to
0.3 wt. % BaO. Biotite from is Mg-annite (XMg = 0.39), Altot = 3.22–3.29 apfu, with 2.0–2.4
wt. % TiO2, poor in Cr2O3 (up to 0.2 wt. %) and Na2O (up to 0.4 wt. %). White mica is Napoor muscovite – (ferro-) aluminoceladonite solid solution with extremely high
concentrations of FeO (~7 wt. %) and low MgO (~0.5 wt. %) – see EVB for details. Epidote
is Fe-rich (14–16 wt. % Fe2O3) with 0.4–0.8 wt. % MnO. Opaque mineral is represented
mostly Mn–Mg-poor ilmenite (containing up to 4 mol. % of pyrophanite and up to 0.6 mol. %
of geikilite). Titanohematite–rutile pseudomorphs occur in the sample GV52 as the only
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opaque phase. The pseudomorphs are composed of about 80 % of Ti-hematite (5.2–10.2 wt.
% TiO2 with low Cr, Al, Mn and Mg (up to 0.3 wt. %) mixed with 20 % of thin pure rutile
laths. Tourmaline was found solely in the sample GV52 as a single prismatic crystal 50 μm
long. It is Fe-dravite (XMg = 0.52–0.61, Na = 0.8–0.9, Ca = 0.05, Al = 5.8–6.2, all apfu, poor
in Mn, Cr as well as Ti). Towards the rim, XMg and Na + Ca contents decrease, being
compensated by growing Al.
Eastern Volcanic Belt
Characteristic mineral assemblage in the felsic types includes: quartz + albite + potassium
feldspar + muscovite as dominant constituents with minor biotite, chlorite and epidote/REEenriched epidote, along with accessory magnetite, ilmenite, titanite, limonitized pyrite,
monazite and zircon.
Intermediate rocks are composed of albite + quartz + chlorite + epidote ± potassiumfeldspar ± muscovite ± biotite ± titanite ± calcite with apatite, allanite, rutile and ilmenite as
common accessories. The metabasalts (greenschists) consist of actinolite + albite + chlorite +
epidote + titanite.
The rock-forming minerals, in the same way like in WVB, show very uniform
compositions throughout wide spectrum of chemical compositions. Albite and potassium
feldspar occur as major components of a fine-crystalline matrix but sometimes relics of
original phenocrysts also do occur. The concentration of anorthite component in albite reaches
1.5 mol. %, BaO is low (<0.2 wt. %); notable are increased concentrations of MgO and FeOtot
(both up to 0.9 wt. %). Potassium feldspar is also poor in BaO (up to 0.3 wt. %) and it
contains significant Na2O (up to 0.55 wt. %) with MgO and MnO (both up to 0.24 wt. %).
Chlorite is Fe-clinochlore, rarely Mg-chamosite (XMg = 0.41–0.55, Si = 2.76–2.99 apfu), with
concentrations of MnO ranging between 0.25 and 1.0 wt. %; Cr and Ti are low, mostly below
0.1 wt. %. The biotite forms brown, mostly tiny flakes. It is Fe-rich, with a strong prevalence
of annite, KFe32+AlSi3O10(OH)2 component (XMg = 0.12–0.20, Altot = 1.31–1.58 apfu;
following classification of Rieder et al. (1998). It contains detectable TiO2 (2.0–3.3 wt. %),
Na2O (up to 0.8 wt. %), MnO (up to 0.34 wt. %) and Cr2O3 (up to 0.2 wt. %). Presence of
tetra-ferri-annite KFe32+Fe3+Si3O10(OH)2 component is highly probable due to the high
oxygen activity during metamorphism (presence of magnetite). White mica contains
extremely high concentrations of FeOtot (5.0–8.5 wt. %) along with some MgO (1.2–3.1 wt.
%; XMg = 0.20–0.50) and it has therefore high (ferro-) aluminoceladonite, KAl (Fe2+, Mg)
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Si4O10(OH)2 content. The VIR2+/(VIR2++VIAl) ratio ranges between 0.23 and 0.31. Majority of
the samples with this ratio exceeding 0.25 in fact corresponds, following classification of
Rieder et al. (1998), to (ferro-) aluminoceladonite. Atomic abundances (calculated on the
basis of 10 oxygen and 2 OH atoms) are as follows: Si = 3.20–3.34, Altot = 2.02–2.41, AlIV =
0.65–0.81, AlVI = 1.43–1.60, all apfu. The remaining elements occur in low concentrations:
TiO2 (0.3–0.8 wt. %), Cr2O3 with MnO (up to 0.25 wt. %) and Na2O (up to 0.44 wt. %).
Epidote is Fe-rich (14.5–16.5 wt. % Fe2O3), poor in MnO (up to 0.6 wt. %); Mg, Cr, Ti do not
exceed 0.4 wt. %, but mostly are below the respective detection limits. Allanite with REEenriched epidote of Ca-La-Nd type (sum of REE oxide contents ranging between 8 and 20 wt.
%) occur as thin skeletal rims of normal epidote in rocks of the felsic suite. Accessory titanite
from felsic rock types is characterized by high concentrations of FeOtot (4.3–5.6 wt. %) along
with low Al2O3 (1.1–2.0 wt. %). Titanite from mafic rocks on the other hand has lower FeOtot
(1.0–3.0 wt. %) and higher Al2O3 (mostly 2.6–4.6 wt. %). Ilmenite occurs as relics replaced
by titanite in mafic rocks; in felsic types it forms small, newly formed grains. Ilmenite
contains 7–18 mol. % of pyrophanite; concentrations of Cr, Mg and Al oxides are low, < 0.24
wt. %.
Felsic dykes cutting the Cadomian basement
These rocks carry also secondary metamorphic assemblage of quartz + albite + potassium
feldspar ± white mica ± biotite. Minor and accessory minerals are represented by zircon,
magnetite and allanite.
Albite is mostly nearly Ca-free; K-feldspar contains up to 0.6 wt. % Na2O. White mica
is poor in both Na and Ti as well as rich in Fe (FeOtot = 6.6–8.1 wt. %) along with some MgO
(1.0–1.5 wt. %). It corresponds to ferroaluminoceladonite (for details see the EVB section
above). Biotite is Fe-rich, close to the end-member annite (XMg = 0.12). It contains c. 2.5 wt.
% TiO2 and low Na2O (mostly below detection limit). Zircon crystals are tiny, skeletal.
Dolerites
Five samples studied by electron microprobe show simple and uniform mineral composition:
calcic amphibole + albite + chlorite + epidote + titanite. Ilmenite, quartz, rutile, apatite and
limonitized pyrite are accessoric, some samples in addition contain variable amounts of
calcite.
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Amphibole corresponds to actinolite, exceptionally to magnesiohornblende,
Mg/(Mg+Fe2+) = 0.55–0.76, in the classification scheme of Leake et al. (1997). It is poor in
TiO2 (mostly below 0.20 wt. % but rarely reaching 0.75–0.93 wt. %), MnO (0.10-0.55 wt. %),
and Cr2O3 (mostly below the detection limit, exceptionally up to 0.28 wt. %). Atomic
abundances (calculated on the basis of 23 oxygen atoms following Holland and Blundy
(1994) are as follows: Si = 7.37–7.94 apfu, AlIV = 0.06–0.63 apfu, AlVI = 0.07–0.44 apfu,
(Na+K)A = 0.00–0.16 apfu. The rocks contain exceptionally relics of igneous amphibole,
sometimes with rarely preserved tiny diopside cores. Chlorite corresponds to Fe-rich
clinochlore, rarely to the compositions transitional between clinochlore and chamosite,
Mg/(Mg + Fetot) = 0.50–0.64. Chlorite is poor in MnO (0.00–0.41 wt. %) and Cr2O3 (mostly
below the detection limit of the electron microprobe). Atomic abundances (calculated on the
basis of 10 oxygen atoms and 8 OH) are: Si = 2.76–2.89 and Altot = 2.42–2.58 apfu. Epidote
usually contains 5.9–7.1 wt. % but exceptionally up to 13.0 wt. % Fe2O3. Atomic abundances
(calculated on the basis of 12 O and 1 OH) are nearly stoichiometric with Si = 2.99–3.01, Altot
= 2.26–2.65, Fe3+ = 0.35–0.78 and Ca = 1.99–1.02 apfu. The MnO contents are low (0.00–
0.20 wt. %), as are the concentrations of TiO2, Cr2O3, and MgO (mostly less than 0.2 wt. %).
Plagioclase of dolerites corresponds to albite with the anorthite component varying in the
range of 0.5–3.6 mol. %. Titanite is ubiquitous accessory component of dolerites occurring in
a two morphological types. The most abundant generation represents large irregular
aggregates of tiny “leucoxene” up to 1 (2) mm long. Textural criteria and the phase relations
indicate that this titanite replaced primary ilmenite, preserved as relics only. The second type,
represented by smaller euhedral to subhedral titanite grains, is less frequent. There is no
significant compositional difference between both morphological types. Titanite shows nearly
stoichiometric empirical formulae and rather low Al2O3 (0.63–2.61 wt. %), FeO (0.14–0.88
wt. %), and MgO (0.00–0.30 wt. %) contents. Ilmenite is mostly the only opaque phase (along
with rare limonitized pyrite) and occurs as strongly corroded relics replaced by titanite. It
represents most probably a primary component in the studied dolerites. Ilmenite contains 2.0–
5.7 wt. % MnO, which corresponds to 4–13 mol. % of the pyrophanite component; MgO is
low but detectable in the range of 0.08–0.24 wt. %.
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References
Holland TJB, Blundy JD (1994) Non-ideal interactions in calcic amphiboles and their bearing on amphibole–
plagioclase thermometry. Contrib Mineral Petrol 116:433–447
Leake BE, Wooley AR, Arps CES, Birch WD, Gilbert MC, Grice JD, Hawthorne FC, Kato A, Kisch HJ,
Krivovichev VG, Linthout K, Laird J, Mandarino J, Maresch WV, Nickel EH, Rock NMS, Schumacher JC,
Smith JC, Stephenson NCN, Whittaker EJW, Youzhi G (1997) Nomenclature of amphiboles: report of the
Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals
and Mineral Names. Mineral Mag 61:295–321
Rieder M, Cavazzini G, D'yakonov Y S, Frank-Kamenetskii VA, Gottardi G, Guggenheim S, Koval' PV, Müller
G, Neiva AMR, Radoslovich EW, Robert JL, Sassi FP, Takeda H, Weiss Z, Wones DR (1998) Nomenclature
of the micas. Canad Min 36:905-912
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