Field Relations and Petrology

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;

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 Na-poor 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 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/REE-enriched epidote, along with accessory magnetite, ilmenite, titanite, limonitized pyrite, monazite and zircon.

Intermediate rocks are composed of albite + quartz + chlorite + epidote ± potassium-feldspar ± 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) 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 REE-enriched 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.

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. %). Plagioclaseof dolerites corresponds to albite with the anorthite component varying in the range of 0.5–3.6 mol. %. Titaniteis 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. %.

References

Holland TJB, Blundy JD (1994) Non-ideal interactions in calcicamphiboles and their bearing on amphibole–plagioclase thermometry.Contrib Mineral Petrol 116:433–447

Leake BE, Wooley AR, Arps CES, Birch WD, Gilbert MC, GriceJD, Hawthorne FC, Kato A, Kisch HJ, Krivovichev VG, LinthoutK, Laird J, Mandarino J, Maresch WV, Nickel EH, RockNMS, Schumacher JC, Smith JC, Stephenson NCN, WhittakerEJW, Youzhi G (1997) Nomenclature of amphiboles: report ofthe Subcommittee on Amphiboles of the International MineralogicalAssociation Commission on New Minerals and MineralNames. 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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