Petrographic Study of a Paleozoic, Blue Amphibole- and Ferriphengite-bearing Schist from the Southern Taunus Mountains, Germany
by Christian Röhr, Friedberg-Ockstadt
With 4 figures and 9 tables in the text
(Original paper written 1991, minor updates 1995, published in the Web in 2002)
Abstract: The Southern Taunus is an area that experienced Carboniferous low grade metamorphism under elevated pressures. A sample of greywacke composition, described in detail, consists of feldspar phenocrysts, pseudomorphs after an unknown phase and epidote clasts. The matrix comprises quartz, albite, ferriphengite of variable composition, blue Na-amphibole, stilpnomelane, titanite, chlorite, hematite, magnetite, calcite and apatite. Microprobe analyses of the various minerals are given.
Key words: Taunus, Northern Phyllite Zone, riebeckite, ferriphengite, epidote, stilpnomelane, titanite
1. Introduction
The Southern Taunus Mountains are one of the few places of the Variscan orogen where blue amphibole-bearing schists occur. Blue amphibole in the Taunus was discovered by MILCH (1889), whereas stilpnomelane, another low grade index mineral, was recognized even earlier (KOCH 1880), but the first to use these parageneses to bracket the p-T conditions of metamorphism was MEISL (1970). Later MEISL et al. (1982) analyzed the amphibole and found it to be magnesio-riebeckite. Recently ANDERLE et al. (1990) used this sodic amphibole and thermodynamic modeling with an internally consistent dataset to model a p-T path. The result was peak conditions of 6 kbar and 300°C, followed by isothermal uplift accompanied by increasingly reduced water activity. However, up to now, no description of a single sample complete with microprobe analyses has been presented: a situation rectified by the present work.
2. Geological Setting
The Southern Taunus Mountains are situated at the transition from the Saxothuringian Zone in the SE to the Rhenohercynian Zone in the NW and are taken to represent the site of a former suture between the two Variscan zones (Fig. 1, WEBER & BEHR 1983, FRANKE 1989, ANDERLE et al. 1990). The low grade metamorphic rocks of the Southern Taunus belong to the Northern Phyllite Zone that spans the transition from the Hunsrück in the SW to the Wippra Metamorphic Zone (Harz mountains, SIEDEL & THEYE 1993) in the NE of the Taunus.
Fig. 1: Outcrops of the Variscan belt in Germany and position of the site investigated. The transition from Rhenohercynian to Saxothuringian is indicated by the Northern Phyllite Zone (after FRANKE 1989).
The low grade metamorphics of the Taunus were divided in three units by STENGER (1961):
(1) Metavolcanics of rhyolitic, rhyodacitic, dacitic, trachytic, andesitic and rarely of basaltic composition usually regarded as representing a destructive plate boundary (comprehensive geochemical study by MEISL 1990). U/Pb-studies of zircons from these rocks revealed Ordovician to Silurian ages (445-425 Ma, SOMMERMANN et al. 1992, 1994) interpreted as the time of volcanic activity.
(2) The Eppstein Schist Group comprising homogeneous phyllites and varve-like layered phyllites, composed of mm-thick light layers rich in quartz and dark layers rich in phyllosilicates. These varve-like layered phyllites are characterized by clasts of quartz, albite and muscovite (STENGER 1961). The age of sedimentation is still unknown, but it is noteworthy that the nearby sediments of the 'Bunte Schiefer' (Gedinnian, Lower Devonian) bear the same clasts.
(3) The Lorsbach Schist Group enclosing a more variegated sequence of partly graphitic homogeneous phyllites, quartzites and varve-like layered phyllites (but without clasts, STENGER 1961). REITZ (1989) found spores of Lower Devonian (Emsian) age in this unit.
For a review of the stratigraphy and a map of the area refer to ANDERLE (1987).
MEISL et al. (1982) regard the Eppstein and Lorsbach Schist Group to be dominantly mudstones and siltstones and to a lesser extent sandstones and arkoses. The schists with clasts were regarded as badly sorted greywackes. KLÜGEL et al. (1994) analyzed the detrital components and obtained an age of 580 Ma (Cambrian) for the detrital white micas of an Eppstein schist whereas the Lorsbach schists yielded 415 and 440 Ma (Ordovician/Silurian, K/Ar).
The metamorphic minerals derived from the fine-grained matrix for the various protoliths (Table 1) were given by MEISL et al. (1982).
Table 1: Metamorphic parageneses (fine grained matrix) for the various protoliths of the Southern Taunus, after MEISL et al. (1982).
meta-rhyolites | meta-andesites | meta-sediments | |
minerals always present | quartz | quartz | quartz |
albite | albite | albite | |
chlorite | chlorite | chlorite | |
stilpnomelane | stilpnomelane | sericite | |
K-feldspar | actinolite | ||
sericite | clinozoisite | ||
epidote | |||
minerals partly present | clinozoisite | K-feldspar | K-feldspar |
titanite | titanite | titanite | |
sericite | epidote | ||
Mg-riebeckite | tourmaline | ||
rutile |
The metamorphism is of Carboniferous age as indicated by K/Ar-dating of white mica (325 - 310 Ma, AHRENDT et al. 1983) and was recognized to be subduction zone-related by MASSONNE & SCHREYER (1983). In a structural study of the Southern Taunus, DOUTSOS & PRÜFERT (1986) mentioned four homoaxially deformations following each other more or less continuously with decreasing intensity. An overview of the structural situation is given by KLÜGEL et al. (1994).
3. Sample location
The sample investigated was collected in February 1986 when a basin for retention of rain water was under construction. The site is situated at the southern slope of the Kocherfels immediately N of the street Kronberg - Falkenstein (sheet 5816 Königstein im Taunus, R 346355 H 556150). The artificial outcrop exposed heterogeneous, vaguely interlayered, very dark, greenish and greyish, very fine-grained, almost massive rocks. Using the local nomenclature they correspond to 'Grünschiefer' and 'Felso-Keratophyr', both forming part of the metavolcanic unit. It is noteworthy, that MEISL & SACHTLEBEN (1992) described unusual porphyroblasts of axinite from the 'Grünschiefer' nearby.
4. Bulk rock analysis
The chemical and modal composition of the sample (Table 2) was analyzed by XRF and XRD using a single powder pellet. The determination of major elements from powder pellets is not of the highest accuracy but thought to be sufficient for the present purpose. The same holds true for quantitative XRD after STROH (1988), which has a lower detection limit of around 3 wt.-%.
Table 2: Bulk rock XRF and XRD analysis of the schist near Falkenstein (analyst: Jörg Hansmann, KTB Feldlabor, Windischeschenbach. Lower detection limit of quantitative XRD-analysis around 3 wt.-%).
SiO2 | 69.9 | wt.-% | Sr | 51 | ppm | plagioclase | 45 | wt.-% |
TiO2 | 0.50 | Rb | 114 | quartz | 33 | |||
Al2O3 | 12.4 | Y | 28 | mica | 13 | |||
Fe2O3 tot. | 5.50 | Zr | 151 | amphibole | 10 | |||
MnO | 0.13 | Nb | 9 | |||||
MgO | 2.19 | Cr | 57 | |||||
CaO | 1.36 | Ni | 67 | |||||
Na2O | 5.01 | Zn | 48 | |||||
K2O | 2.82 | V | 67 | |||||
P2O5 | 0.17 | Cu | 169 | |||||
Total | 99.98 |
In order to find a possible protolith, the major elements were compared with a database of 2150 analyses of igneous, sedimentary and pyroclastic rocks assuming constant composition through metamorphism (ANCOMP). This analysis revealed that greywackes have the most similiar composition when compared to the schist investigated. This is in accordance with the heterogeneous texture and also one would expect to find greywackes in conjunction with the volcanics of a destructive plate margin setting. Alternatively assuming a volcanic origin, the analysis was plotted in the Zr/TiO2 versus Nb/Y diagram of WINCHESTER & FLOYD (1977) which revealed a composition near the andesite/dacite border. But the content of Cr and Ni is rather high for such volcanic rocks when compared with the data in EWART (1979).
The elevated Cr-content of the Taunus schist is expressed by the presence of Cr-bearing magnetite: Cr-bearing titanite and Cr-bearing mica grew at the border of that magnetite. The few feldspar phenocrysts clearly show the volcanic derivation of the rock, but these could be redeposited. In conclusion, no clear decision can be made whether the rock was of volcanic, pyroclastic or of greywacke origin, although HENTSCHEL & MEISL (1966) and MEISL (1970) from textural observation concluded the existence of pyroclastics and ignimbrites from this area.
5. Texture
The heterogeneous appearence of the rock in outcrop is reflected also in its appearence in thin section (Fig. 2) where it can be subdivided in partly folded layers and lenses distinguishable by colour, dominating minerals, grain size and frequency of characteristic pseudomorphs. A few phenocrysts of feldspar (0.1 to 1 mm in size) are scattered in a fine-grained matrix. The rock is cut by the dominating foliation s1 and a second oblique to it (s2).
Fig. 2: Sketch of 20 x 30 mm large thin section of the Taunus schist investigated. Note the heterogeneous appearence by varying amounts of the minerals present.
6. Description and chemical composition of the minerals
The following minerals were identified: albite, K-feldspar, blue amphibole, mica, chlorite, stilpnomelane, epidote, calcite, titanite, hematite, magnetite and apatite. The minerals were analysed with the Cameca SX-50 microprobe of the Bayerisches Geoinstitut, Bayreuth, using standard techniques.
Feldspar phenocrysts
The few feldspar relict phenocrysts (or relict sedimentary clasts, 100 - 1000 µm) scattered in the fine-grained matrix are single twinned K-feldspar (Or98) and albite (Ab98). The albites bear characteristic irregular hematite exsolutions and inclusions of blue amphibole needles.
Pseudomorphs of unknown origin
With a size of 100 - 200 µm, lying between the fine-grained matrix and the feldspar phenocrysts, there are characteristic pseudomorphs with isometric to slightly elongated, hypidiomorphic sections (Fig. 3): in some mm-sized lenses they form a major part of the rock. They consist of a nearly opaque rim of titanite and a core of granoblastic quartz and minor K-feldspar as well as some white mica and blue amphibole needles.
Fig. 3: Photomicrograph of a group of pseudomorphs of unknown origin, 100 - 200 µm in size. They consist of a nearly opaque rim of titanite and a translucent core of granoblastic quartz, minor K-feldspar and little mica and blue amphibole. The predominant constituents of the surrounding matrix besides quartz and albite are blue amphibole (almost black in the picture) and pleochroic colourless-green mica (grey in the picture). Taunus schist near Falkenstein, plane polars.
Epidote clasts
The few epidotes of this rock are characterized by their relatively large size of 100 to 200 µm, clearly larger than the surrounding matrix almost free of epidote. The foliation s1 flows around the broken epidote grains, giving them the appearence of clasts. One minute epidote grain of the matrix was found in an area clouded by titanite dust. Using back scattered electron (BSE) images, some epidotes show a core of two different phases, but too small to be analyzed individually. The rims of the clasts are often brighter than the cores (BSE-image). The respective analyses revealed the presence of Mg and a lack of Ca in the bright rim, both indicating some REE content of allanitic Fe-epidotes (Table 3). The pistacite content of 36 to 42 mole-% is at the upper limit of epidotes when compared to Ps39 found by NAKAJIMA et al. (1977). Especially in the allanitic epidotes, some of the iron will be in the divalent state.
Table 3: Chemical composition (wt.-%) of epidote clasts from Taunus schist (REE not determined). The cations were calculated assuming O=12.5.
code of analysis | 8 | 9 | 44 | 5 |
core --- rim | ||||
SiO2 | 36.15 | 33.27 | 36.54 | 33.15 |
TiO2 | 0.09 | 0.06 | 0.09 | 0.12 |
Al2O3 | 19.35 | 15.85 | 19.40 | 16.62 |
Cr2O3 | 0.02 | 0.00 | 0.02 | 0.00 |
Fe2O3 total | 17.07 | 18.32 | 17.69 | 16.67 |
MnO | 0.78 | 0.77 | 0.76 | 0.75 |
MgO | 0.01 | 0.34 | 0.02 | 0.14 |
CaO | 18.94 | 14.35 | 21.32 | 16.55 |
Na2O | 0.01 | 0.00 | 0.01 | 0.01 |
K2O | 0.01 | 0.23 | 0.01 | 0.05 |
Total | 92.43 | 83.19 | 95.86 | 84.06 |
Al | 1.94 | 1.78 | 1.89 | 1.84 |
Ti | 0.01 | 0.00 | 0.01 | 0.01 |
Fe3+ total | 1.09 | 1.31 | 1.10 | 1.18 |
Total | 3.04 | 3.09 | 3.00 | 3.03 |
Mn | 0.06 | 0.06 | 0.05 | 0.06 |
Ca | 1.73 | 1.46 | 1.89 | 1.67 |
Total | 1.78 | 1.53 | 1.95 | 1.73 |
Si | 3.08 | 3.17 | 3.03 | 3.11 |
Fe/(Fe+Al) | 0.36 | 0.42 | 0.37 | 0.39 |
Matrix
The fine-grained, heterogeneous matrix enclosing the phenocrysts of feldspar, pseudomorphs and epidote clasts consists of granoblastic quartz and albite with a grainsize in the range 1 to 50 µm. Small columnar apatites (10 x 15 µm) are dispersed throughout the rock. Further matrix-minerals are described in more detail.
Mica
The mica of this rock forms 200 x 400 µm sericitic aggregates of subparallel aligned flakes with high birefringence. It is pleochroic from colourless to light green. The microprobe analyses revealed iron-rich compositions (Table 4). The paragenesis with hematite suggests that most Fe is trivalent, so all Fe was calculated as Fe3+. Six of the ten analyses revealed a composition close to ferriphengite K2(Mg,Fe2+)Fe3+Al2(AlSi7O20)(OH)2 as first described by KANEHIRA & BANNO (1960) from a ferriphengite-aegirinejadeite-microcline-albite-quartz schist of the Sanbagawa metamorphic complex. A very similiar composition was obtained by JAKOB (1929) from a light red sericite from the Starlera iron-manganese deposit from Graubünden, Swiss Alps. These analyses are given in Table 4 for comparison. Calculating Fe as Fe2+ increases the octahedral occupancy away from the dioctahedral composition, thus supporting the dominantly trivalent state of Fe. The content of divalent iron was estimated by the equation Fe2+ = (Si-6)-Mg assuming ideal Tschermak substitution. This procedure resulted in Fe3+/(Fe3++Fe2+) values in the range 0.77 to 0.83, comparable to 0.80 of the ferriphengite of KANEHIRA & BANNO. The XII-site deficiency around 0.2 p.f.u. is in the normal range for low grade white micas and increases slightly the Si-content by the substitution KXII + AlIV = []XII (vacancy)+SiIV (WANG & BANNO 1987). Similiar iron-rich white micas are known from several low grade areas such as Western Greece (basic metavolcanics, PE-PIPER 1985), Franciscan Terrane, California (amphibolite, HERMES 1973), Eastern Otago, New Zealand (quartzo-feldspatic schists and greenschists, BROWN 1967) and North Carolina (arkose, FOSTER et al. 1960).
Besides these clearly dioctahedral micas four analyses deviate from this composition in a linear manner. The Si-content decreases from 7 to 6.3 p.f.u. whereas the octahedral occupancy increases away from the dioctahedral compostion (from 4 to 4.6 p.f.u.). This spread can be the result of a variable composition of a single phase or the result of submicroscopic interlayering between two micas in varying portions, but a TEM-study is needed to prove this. Interlayering of phyllosilicates is a common feature on the microscopic and submicroscopic scale in low-grade rocks, e.g. chlorite-muscovite (LEE et al. 1984, FRANCESCHELLI et al. 1986), paragonite-phengite (AHN et al. 1985) or glauconite-celadonite (DUPLAY et al.1986). Because of the normal K-content of the analyzed micas, an interlayering with chlorite is unlikely. The plot of Si versus octahedral occupancy (Fig. 4) is in accordance with the possibility, that the micas deviating from dioctahedral composition, consist of an interlayering of varying proportions of phengite and biotite.
Fig. 4: Plot of Si versus octahedral occupancy of micas from the Taunus schist and similar glauconites from metamorphic limestones of the Swiss helvetic alps (FREY et al. 1973). All Fe was calculated as Fe3+. The plot demonstrates the trend from dioctahedral towards triocahedral compositions. For comparision the compositions of the endmembers muscovite, celadonite, Al-phlogopite and phlogopite are given.
A similiar spread in composition was observed in the analyses of glauconites by FREY et al. (1973). There, glauconitic limestone horizons of the Helvetic Zone of the Glarus Alps bear the very similiar assemblage of stilpnomelane + glauconite + K-feldspar + calcite + quarz +/- chlorite +/- riebeckite (Zone II of that area, ca. 250 °C, 2 kbar). Another similiar series of composition of micas (celadonites) was observed in altered basaltic rocks from Iceland (MEHEGAN et al. 1982).
Two analyses of micas from the immediate neighbourhood of Cr-bearing magnetite revealed the rather high contents of 4 wt.-% Cr2O3. They cannot be distinguished from the Cr-poor mica optically. Nearly half of the analyzed micas bear elevated contents of 0.35 wt.-% MnO, whereas one mica revealed some elevated Na2O and CaO content respectively.
Table 4: Chemical composition (wt.-%) of micas from the Taunus schist. The cations were calculated assuming O=22. For comparison, the original ferriphengite of KANEHIRA & BANNO (1960) from an aegirinejadeite-ferriphengite-microcline-albite-quartz schist of the Sanbagawa metamorphic complex as well as the analysis of a light red sericite from the Stalera iron-manganese deposit of the Swiss Alps (JAKOB 1929) is given.
type | dioctahedral ferriphengite | array towards trioctahedral | Kanehira & Banno (1960) | Jakob (1929) | |||||||
code of analysis | 9 | 8 | 14b | 29 | 43 | 28 | 42 | 10 | 4 | ||
SiO2 | 50.96 | 52.46 | 51.18 | 51.22 | 50.59 | 48.70 | 46.55 | 44.85 | 44.45 | 48.70 | 50.20 |
TiO2 | 0.15 | 0.18 | 0.18 | 0.13 | 0.21 | 0.19 | 0.31 | 0.14 | 0.04 | 0.58 | 0.81 |
Al2O3 | 18.34 | 19.55 | 19.29 | 16.28 | 19.03 | 15.67 | 18.20 | 17.62 | 15.13 | 21.08 | 19.69 |
Cr2O3 | 0.00 | 0.10 | 0.04 | 4.38 | 0.01 | 3.94 | 0.04 | 0.00 | 0.00 | - | - |
Fe2O3tot. | 10.77 | 10.32 | 11.06 | 9.44 | 11.11 | 11.02 | 13.26 | 14.22 | 18.23 | 11.28 | 9.62 |
MnO | 0.13 | 0.11 | 0.36 | 0.00 | 0.10 | 0.04 | 0.18 | 0.31 | 0.36 | 0.33 | 0.60 |
MgO | 4.02 | 4.09 | 3.52 | 4.27 | 3.48 | 5.60 | 5.71 | 7.58 | 8.38 | 2.87 | 3.82 |
CaO | 0.03 | 0.03 | 0.01 | 0.04 | 0.06 | 0.03 | 0.04 | 0.04 | 0.22 | - | 0.00 |
Na2O | 1.04 | 0.05 | 0.03 | 0.03 | 0.02 | 0.03 | 0.03 | 0.04 | 0.13 | 0.37 | 1.50 |
K2O | 8.78 | 10.04 | 10.26 | 9.60 | 10.28 | 9.72 | 9.71 | 9.18 | 9.04 | 9.85 | 9.76 |
Total | 94.22 | 96.93 | 95.93 | 95.38 | 94.89 | 94.94 | 94.03 | 93.98 | 95.98 | 95.06 | 96.00 |
K | 1.55 | 1.72 | 1.78 | 1.69 | 1.81 | 1.74 | 1.75 | 1.67 | 1.64 | 1.73 | 1.70 |
Na | 0.28 | 0.01 | 0.01 | 0.01 | 0.00 | 0.01 | 0.01 | 0.01 | 0.04 | 0.10 | 0.40 |
Total | 1.82 | 1.73 | 1.79 | 1.69 | 1.81 | 1.75 | 1.76 | 1.68 | 1.67 | 1.83 | 2.10 |
Fe3+total | 1.12 | 1.04 | 1.13 | 0.98 | 1.15 | 1.16 | 1.41 | 1.52 | 1.95 | 1.17 | 0.99 |
Mg | 0.83 | 0.82 | 0.72 | 0.88 | 0.72 | 1.17 | 1.20 | 1.61 | 1.77 | 0.59 | 0.78 |
Mn | 0.02 | 0.01 | 0.04 | 0.00 | 0.01 | 0.01 | 0.02 | 0.04 | 0.04 | 0.04 | 0.07 |
Ti | 0.02 | 0.02 | 0.02 | 0.01 | 0.02 | 0.02 | 0.03 | 0.02 | 0.00 | 0.06 | 0.08 |
Cr | 0.00 | 0.01 | 0.00 | 0.48 | 0.00 | 0.44 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
AlVI | 2.02 | 2.12 | 2.08 | 1.69 | 2.07 | 1.40 | 1.61 | 1.34 | 0.84 | 2.15 | 2.02 |
Total | 3.99 | 4.02 | 3.99 | 4.04 | 3.97 | 4.20 | 4.28 | 4.52 | 4.61 | 4.01 | 3.94 |
Si | 7.03 | 7.03 | 6.97 | 7.05 | 6.98 | 6.82 | 6.58 | 6.38 | 6.31 | 6.72 | 6.85 |
AlIV | 0.97 | 0.97 | 1.02 | 0.95 | 1.02 | 1.18 | 1.42 | 1.62 | 1.69 | 1.28 | 1.15 |
Blue Na-amphibole
The needle-shaped, 10 to 40 µm thick blue amphiboles appear throughout the whole rock but are enriched in some lenses. In the few larger grains some albite inclusions were found. The amphiboles are virtually unzoned. The composition of the analyzed amphiboles is uniform (Table 5) but plots across the riebeckite/magnesio-riebeckite border of the classification diagram after LEAKE (1978).
Table 5: Chemical composition (wt.-%) of blue amphiboles (magnesio-riebeckite to riebeckite) from Taunus schist. The cations were normalized to 15 excluding K.
code of analysis | 2 | 3 | 6 | 36 | 37 | 39 | 40 |
core --- rim | |||||||
SiO2 | 53.68 | 53.12 | 53.95 | 54.13 | 53.53 | 53.29 | 52.96 |
TiO2 | 0.08 | 0.13 | 0.10 | 0.07 | 0.18 | 0.15 | 0.06 |
Al2O3 | 1.24 | 1.75 | 1.43 | 2.07 | 1.72 | 1.41 | 1.58 |
Cr2O3 | 0.01 | 0.01 | 0.01 | 0.03 | 0.00 | 0.12 | 0.00 |
FeOtotal | 25.53 | 24.79 | 25.19 | 23.55 | 23.64 | 25.72 | 27.66 |
MnO | 0.57 | 0.66 | 0.56 | 0.49 | 0.63 | 0.49 | 0.52 |
MgO | 6.84 | 7.43 | 7.08 | 7.63 | 7.63 | 7.11 | 5.74 |
CaO | 0.89 | 1.44 | 1.26 | 0.79 | 1.12 | 0.96 | 0.71 |
Na2O | 6.11 | 6.12 | 6.53 | 6.70 | 6.49 | 6.57 | 6.66 |
K2O | 0.10 | 0.19 | 0.17 | 0.09 | 0.12 | 0.13 | 0.09 |
Total | 95.06 | 95.64 | 96.29 | 95.55 | 95.05 | 95.95 | 96.00 |
Si | 8.05 | 7.88 | 7.95 | 7.96 | 7.94 | 7.88 | 7.89 |
AlIV | 0.00 | 0.12 | 0.05 | 0.04 | 0.06 | 0.12 | 0.11 |
AlVI | 0.22 | 0.19 | 0.20 | 0.32 | 0.24 | 0.13 | 0.17 |
Ti | 0.01 | 0.02 | 0.01 | 0.01 | 0.02 | 0.02 | 0.01 |
Fe3+ | 1.42 | 1.62 | 1.67 | 1.59 | 1.62 | 1.81 | 1.83 |
Cr | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.01 | 0.00 |
Mg | 1.53 | 1.64 | 1.55 | 1.67 | 1.69 | 1.57 | 1.28 |
Fe2+ | 1.78 | 1.46 | 1.44 | 1.31 | 1.31 | 1.37 | 1.61 |
Mn | 0.04 | 0.07 | 0.07 | 0.06 | 0.08 | 0.06 | 0.07 |
Ca | 0.00 | 0.00 | 0.06 | 0.04 | 0.04 | 0.04 | 0.04 |
Total | 5.00 | 5.00 | 5.00 | 5.00 | 5.00 | 5.00 | 5.00 |
Mn | 0.03 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
Ca | 0.14 | 0.23 | 0.14 | 0.09 | 0.13 | 0.12 | 0.08 |
NaB | 1.77 | 1.76 | 1.87 | 1.91 | 1.87 | 1.88 | 1.92 |
Total | 1.95 | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 |
NaA | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
K | 0.02 | 0.04 | 0.03 | 0.02 | 0.02 | 0.02 | 0.02 |
Stilpnomelane
The brown stilpnomelanes (20 x 200 up to 100 x 1000 µm) appear in three textural sites: (1) as part of small s1-parallel dykes together with quartz and calcite, (2) in areas of the matrix especially rich in blue amphibole together with mica and chlorite. In contrast to the parallel orientated amphibole stilpnomelane grew without preferred orientation. (3) In areas especially rich in mica, stilpnomelane is concentrated at s2-planes. The chemical analysis revealed low K and an elevated content in Mn (Table 6).
Table 6: Chemical composition (wt.-%) of stilpnomelanes from the Taunus schist.The cations were calculated assuming Si=8.
code of analysis | 7 | 16 | 45 |
SiO2 | 47.64 | 48.48 | 49.15 |
TiO2 | 0.02 | 0.00 | 0.00 |
Al2O3 | 5.90 | 6.01 | 5.76 |
Cr2O3 | 0.03 | 0.00 | 0.01 |
Fe2O3 total | 26.00 | 26.74 | 26.53 |
MnO | 3.53 | 3.13 | 3.35 |
MgO | 8.84 | 9.38 | 8.63 |
CaO | 0.25 | 0.23 | 0.04 |
Na2O | 0.02 | 0.01 | 0.02 |
K2O | 0.42 | 0.09 | 0.13 |
Total | 92.65 | 94.07 | 93.62 |
K | 0.09 | 0.02 | 0.03 |
Na | 0.01 | 0.00 | 0.01 |
Ca | 0.05 | 0.04 | 0.01 |
Total | 0.14 | 0.06 | 0.04 |
Fe3+ total | 3.29 | 3.32 | 3.25 |
Mg | 2.21 | 2.31 | 2.09 |
Mn | 0.50 | 0.44 | 0.46 |
Al | 1.17 | 1.17 | 1.10 |
Total | 7.17 | 7.23 | 6.91 |
Si | 8.00 | 8.00 | 8.00 |
Titanite
The titanite forms very small crystals, so that in transmitted light it appears as hardly identifiable nearly opaque dust, whereas it can clearly be observed in reflected light. The small crystals are agglomerated around larger matrix grains and especially at the rim of the pseudomorphs mentioned above. A few grains were large enough to be analyzed by microprobe (Table 7). The analyses show a bimodal Al-content of 0.05 and 0.21 p.f.u. respectively. A similiar bimodal titanite compositional range was observed in amphibolites from the Southern Hunsrück Mountains by MEISL (1986). Such elevated Al-contents are also known from altered volcaniclastic rocks from Iceland (VIERECK et al. 1982) and from eclogites (SMITH 1988). The K2O-content of the titanites from the Taunus schist is unusual, a titanite grain bordering magnetite is rich in Cr.
Table 7: Chemical composition (wt.-%) of titanites from Taunus schist. Cr-bearing titanite 26 grows at rim of magnetite. Note the different content of Al. The cations were calculated assuming O=5.
code of analysis | 26 | 30 | 34 | 38 |
SiO2 | 31.57 | 30.58 | 29.89 | 30.51 |
TiO2 | 35.90 | 30.03 | 37.52 | 30.25 |
Al2O3 | 1.38 | 5.20 | 0.98 | 5.30 |
Cr2O3 | 0.49 | 0.01 | 0.02 | 0.07 |
FeO total | 2.48 | 1.41 | 1.50 | 2.25 |
MnO | 0.00 | 0.04 | 0.05 | 0.02 |
MgO | 0.08 | 0.21 | 0.23 | 0.29 |
CaO | 27.00 | 27.64 | 27.69 | 27.60 |
Na2O | 0.03 | 0.03 | 0.02 | 0.01 |
K2O | 0.28 | 0.23 | 0.22 | 0.33 |
Total | 99.20 | 95.38 | 98.11 | 96.61 |
Ca | 0.95 | 1.01 | 0.99 | 0.99 |
Ti | 0.89 | 0.77 | 0.94 | 0.76 |
Al | 0.05 | 0.21 | 0.04 | 0.21 |
Cr | 0.01 | 0.00 | 0.00 | 0.00 |
Fe3+ total | 0.07 | 0.04 | 0.04 | 0.06 |
Mg | 0.00 | 0.01 | 0.01 | 0.02 |
Total | 1.02 | 1.03 | 1.03 | 1.05 |
Si | 1.04 | 1.04 | 1.00 | 1.02 |
Chlorite
The modal content of chlorite is rather low. It is present as patches probably at the sites of former mafic minerals, and is an early phase oriented only parallel to s1 where it seems to have been replaced by mica. Chlorite also appears in the pressure shadows of the few epidote clasts. Areas rich in chlorite are poor in blue amphibole. The paragenesis with hematite and other Fe3+-rich silicates suggests that chlorite also contains a considerable amount of Fe3+ (Table 8), probably more than 4 wt.-% Fe2O3, which would qualify them to be delessite of the leptochlorite group of TRÖGER & TROCHIM (in TRÖGER 1969).
Table 8: Chemical composition (wt.-%) of chlorites from the Taunus schist. The cations were calculated assuming O=28.
code of analysis | 11 | 13a | 10 | 11 | 13b |
SiO2 | 31.18 | 28.54 | 27.88 | 28.13 | 28.14 |
TiO2 | 0.04 | 0.02 | 0.00 | 0.03 | 0.01 |
Al2O3 | 17.02 | 16.91 | 16.14 | 17.02 | 17.63 |
Cr2O3 | 0.01 | 0.02 | 0.00 | 0.02 | 0.00 |
FeO total | 21.88 | 21.96 | 20.78 | 22.37 | 23.28 |
MnO | 0.80 | 0.80 | 0.82 | 0.93 | 1.20 |
NiO | 0.13 | 0.05 | - | - | - |
ZnO | 0.18 | 0.16 | - | - | - |
MgO | 15.86 | 17.77 | 17.93 | 17.42 | 16.28 |
CaO | 0.04 | 0.03 | 0.04 | 0.03 | 0.05 |
Na2O | 0.02 | 0.01 | 0.01 | 0.00 | 0.01 |
K2O | 1.51 | 0.28 | 0.03 | 0.15 | 0.49 |
Total | 88.66 | 86.56 | 83.63 | 86.09 | 87.08 |
Fe2+total | 3.79 | 3.88 | 3.77 | 3.97 | 4.12 |
Mg | 4.90 | 5.60 | 5.80 | 5.52 | 5.14 |
Mn | 0.14 | 0.14 | 0.15 | 0.17 | 0.22 |
AlVI | 2.62 | 2.24 | 2.17 | 2.23 | 2.36 |
Total | 11.46 | 11.86 | 11.89 | 11.89 | 11.84 |
Si | 6.46 | 6.03 | 6.05 | 5.97 | 5.96 |
AlIV | 1.54 | 1.97 | 1.95 | 2.03 | 2.04 |
Iron oxides
Hematite occurs as embayed hypidiomorphic laths with unidentified inclusions and is dispersed throughout the rock. The TiO2-content is around 1 wt.-%. A hematite inclusion in magnetite bears 0.26 wt.-% Cr2O3 (Table 9). The modal amount of magnetite is only about 10 % of that of hematite. Magnetite is always idiomorphic, ca. 50 µm in size, almost free of inclusions and with very low TiO2-contents, but remarkably high Cr2O3 up to 1.5 wt.-%. In places, hematite can be found at the rim or even included in magnetite. Some magnetites are cut by minute meandering veins filled with a high reflective, greyish ore mineral, too small to be determined. The presense of both hematite and magnetite buffers the oxygen fugacity an decreases the degrees of freedom by one. Hematite, titanite and mica from the neigbourhood of magnetite are enriched in Cr.
Table 9: Chemical composition (wt.-%) of iron oxides from the Taunus schist
mineral | hematite | magnetite | |||
code of analysis | 4 | 31 | 32 | 25 | 33 |
SiO2 | 1.19 | 0.06 | 0.09 | 0.21 | 0.22 |
TiO2 | 0.88 | 1.37 | 0.96 | 0.08 | 0.01 |
Al2O3 | 0.10 | 0.01 | 0.04 | 0.02 | 0.01 |
Cr2O3 | 0.02 | 0.06 | 0.26 | 1.53 | 0.37 |
FeO total | 86.72 | 87.05 | 87.78 | 90.12 | 92.37 |
MnO | 0.09 | 0.01 | 0.03 | 0.02 | 0.04 |
MgO | 0.06 | 0.00 | 0.00 | 0.01 | 0.00 |
Total | 89.06 | 88.56 | 89.16 | 91.99 | 93.02 |
Calcite
Calcite is predominantly bound to s1-parallel veins where it is intergrown with stilpnomelane and quartz. Some of the calcites are undeformed whereas others are polysynthetically twinned. Some calcite appears to replace K-feldspar as well as albite phenocrysts.
7. Discussion
The petrographic study revealed that in the schist of probable greywacke-origin albite, quartz, ferriphengite, riebeckite/magnesio-riebeckite, stilpnomelane, titanite, hematite and magnetite form the metamorphic paragenesis proper. K-feldspar is present only as part of the pseudomorphs and as a few phenocrysts. Epidote and chlorite are rare and seem to be relictic phases that were mostly consumed by metamorphic reactions. Other low grade phases such as clinozoisite, actinolite, zeolite, pumpellyite or lawsonite are absent. At the time when foliation s1 formed, s1-parallel veins opened and were filled with calcite, quarz and stilpnomelane. It is noteworthy that probable aragonite was described a few kilometers away by RITTER (1884). With the methods used here, the possibility of the presense of aragonite in the sample investigated, cannot be excluded.
Judging from the hypidiomorphic sections and the frequency of occurrence, the pseudomorphs of unknown origin must have replaced a major former constituent of the rock. But from the phases now present (mainly quartz, K-feldspar, titanite) the mineral replaced stays unknown.
The appearence of blue amphibole is not limited to blueschist facies rocks. Crossite-riebeckite and riebeckitic actinolite appear in the pumpellyite-actinolite facies of Shikoku, Japan (NAKAJIMA et al. 1977, MARUYAMA & LIOU 1985). Taking the reaction epidote + magnesio-riebeckite + chlorite + quartz = tremolite + albite + hematite + water the observed paragenesis of the Taunus schist is on the left hand side, which would indicate a pressure above 4 kbar (MARUYAMA et al. 1986).
The latest geothermobarometric data from the Southern Taunus Mountains were given by ANDERLE et al. (1990). From Na-amphibole zonation patterns, the appearence of calcite and the partial decomposition of titanite they derived peak conditions of 6 kbar and 300 °C followed by isothermal uplift accompanied by increasingly reduced water activity. The p-T-peak-conditions of the schist investigated here, are to be expected in the same range.
The aim of the present investigation was a detailed study of all constituents to get a full insight in the metamorphic paragenesis. However, because of the complicated mica TEM, XRD, Fe2+/Fe3+-determination and a good knowledge seems necessary to apply geothermobarometry. Further problems arise from the presense of considerable amounts of MnO in stilpnomelane, chlorite, epidote, blue amphibole and mica and so geothermobarometric evaluation is left for future workers.
Acknowledgements
I thank F. Seifert and D. Krauße, Bayerisches Geoinstitut Bayreuth, for permission and help with the microprobe. P. O'Brien, Bayreuth, supplied a program to calculate cations of amphiboles and greatly improved the english text. J. Hansmann (KTB Feldlabor, Windischeschenbach) performed the the XRF and XRD analyses.
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