Representative analyses of titanite in titanite-spots granodiorites and tonalites in Mühlviertel area, Austria. Concentration of OH was calculated based on OH´ = (Al + Fe3+)−F, according to [19].
Abstract
Titanite-rich granodiorites occurring in the Austrian Mühlviertel are intimately associated with the I/S-granites of the Mauthausen/Freistadt granite suite. These rocks form small irregular bodies in granites of this granitic suite of the Moldanubian batholith that are represented by usually fine-grained and dark granodiorites, which contain a large amount of titanite hell “spots” formed by aggregates of plagioclases and quartz. Titanite as a relative plentiful accessory mineral exists in the center of these “spots” as idiomorphic and sphenoidal grains. The composition of titanite ranges from 89 to 92 mol.% titanite end-member. According to its Al concentration, the analyzed titanites could be considered as low-Al titanites (Al = 0.05–0.08 atoms per formula unit). Titanite contains low concentrations of both (Al + Fe3+)–OH (2–9 mol.%) and (Al + Fe3+)–F (0–8 mol.%). Titanite together with Na-enriched plagioclase and quartz is originated during late-magmatic evolution of titanite-spots granodiorites.
Keywords
- titanite
- petrology
- geochemistry
- mineralogy
- Bohemian Massif
- Moldanubian zone
1. Introduction
In the SW part of the Moldanubian batholith, titanite-rich granitic rocks occur, which were described in the past as titanite-spots granites and diorites. These rocks were described in detail at the Austrian Mühlviertel, SSW of Aigen [1], and at the southern part of the Fürstenstein pluton [2, 3]. For titanite-spots granitic rocks at the Fürstenstein pluton, a local name “engelburgite” was used [2, 4]. In these usually fine-grained and dark granitic rocks, a significant large amount of titanite clots mantled by hell aggregates of feldspars and quartz are found. Although many petrographic studies on these titanite-spots granitic rocks have been performed several decades ago, the origin of these clots remains uncertain [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The main aim of the present paper is the detailed significance of the petrology and mineralogy of these rocks, in addition to the chemical composition of titanite enclosed in hell “spots”.
2. Geological setting
The titanite-spots granodiorites at the northern Mühlviertel (Austria) occur as small irregular intrusive bodies, concentrated in a local SSW-NNE zone, W of Schlägl. According the most recent geological map 1:200,000 [11], a biggest body that was sampled in the Schlägl quarry forms N-S intrusion, W of Schlägl. These magmatic bodies are enclosed in bigger intrusions of the fine- to medium-grained Mauthausen granites (Figure 1). The Mauthausen granites are part of the youngest Freistadt/Mauthausen I/S-type suite (320–310 Ma) of the Moldanubian batholith [12, 13, 14, 15, 16]. The titanite-spots granodiorites form small irregular bodies enclosed in these granites.
The titanite-spots granodiorites and tonalites from the Fürstenstein pluton occur in the SSW part of the Bavarian Forest (Germany) forming smaller bodies and blocks enclosed in the medium-grained, porphyritic Saldenburg biotite granites, which form the main part of the Fürstenstein pluton [3, 10, 17]. This pluton represents the largest intrusive body in the Bavarian Forest (Figure 2). These granites are equivalent to the Weinsberg medium- to coarse-grained, porphyritic biotite granites at the southern part of the Moldanubian batholith [3, 11]. The titanite-spots granodiorites and tonalites occur predominantly in contact zones between the Saldenburg granites and the medium- to coarse-grained Tittling biotite granites and granodiorites. Their age as determined by Chen and Siebel [10] is 321 ± 4 Ma, by the U/Pb dating method in titanite.
3. Analytical methods
Approximately 30 quantitative electron probe microanalyses of titanite were performed in the three representative samples of titanite-spots granodiorites from the western Mühlviertel area (Austria). Minerals were analyzed in polished thin sections, and back-scattered electron images (BSE) were acquired to study the interaction of examined titanite with surrounding minerals and the internal structure of individual mineral grains. Element abundances of Al, Ca, Ce, F, Fe, La, Mg, Na, Nb, Nd, Pr, Si, Ta, Th, Ti, Y, and Zr in selected accessory titanite were determined using a CAMECA SX 100 electron probe microanalyzer (EPMA) operated in wavelength-dispersive mode. The contents of the abovementioned elements were determined using an accelerating voltage and beam current of 15 keV and 20 nA, respectively, with a beam diameter of 2–5 μm. The following standards, X-ray lines, and crystals (in parentheses) were used: AlKα, sanidine (TAP); CaKα, fluorapatite (PET); CeLα, CePO4 (PET); FKα, topaz (PC1); FeKα, almandine (LIF); LaLα, LaPO4 (PET); MgKα, Mg2SiO4 (TAP); NbLα, columbite (PET); NdLβ, NdPO4 (LIF); PrLβ, PrPO4 (LIF); SiKα, sanidine (TAP); TaLα, CrTa2O6 (LIF); TbLα, TbPO4 (LIF); ThMα, CaTh(PO4)2 (PET); TiKα, anatas (PET); YLα, YPO4 (PET); and ZrLα, zircon (TAP). Intra-REE overlaps were partially resolved using Lα and Lβ lines. The raw data were converted into concentrations using appropriate PAP matrix corrections [18]. The detection limits were approximately 400 ppm for Y and 180–1700 ppm for REE.
4. Results
4.1 Petrography
The titanite-spots granodiorites from the northern Mühlviertel are dark, fine-grained rocks with many light spots formed by plagioclase and bigger, usually idiomorphic titanite crystals (Figures 3 and 4).
The titanite-spots granodiorites contain plagioclase (An30–55) (36–46 vol.%), biotite (15–32 vol.%), quartz (21–28 vol.%), K-feldspar (6–16 vol.%), rare amphibole (0–1 vol.%), and pyroxene (0–1 vol.%). Plagioclase grains in light “spots,” which are enriched in albite component (An33–39), contain bigger titanite crystals in their cores (Figures 4 and 5). Biotite is annite with Fe/(Fe + Mg) = 0.51–0.54 and 0.24–0.54 apfu (atoms per formula unit). Amphibole is cummingtonite with Mg/(Mg + Fe) 0.44–0.45 apfu. The accessory minerals are titanite, apatite, zircon, magnetite, ilmenite, allanite, and pyrite.
4.2 Mineralogy and mineral chemistry of titanite
Titanite as a relatively plentiful accessory mineral usually has up to 1–2 mm long. The idiomorphic, sphenoidal crystals are enclosed in light “spots” of fine-grained aggregates of plagioclase and quartz (Figures 3 and 4). In some cases, larger titanite crystals contain irregular inclusions of plagioclase (Figures 4 and 5). Titanite in thin sections is usually dark brown and indistinctly pleochroic. It is compositionally homogeneous, that is, without obvious zoning. The electron microprobe data show that the chemical composition of titanite in these titanite-spots granodiorites and tonalites varies over a wide range. The formula of titanite is calculated on the basis of 1 Si as suggested by Harlov et al. [19] (Table 1).
Sample, wt.% | 1719–66 | 1725–15 | 1725–16 | 1725–21 | 1725–23 | 1725–27 |
---|---|---|---|---|---|---|
SiO2 | 30.43 | 30.55 | 30.68 | 30.18 | 29.99 | 30.44 |
TiO2 | 37.20 | 37.13 | 36.70 | 36.91 | 36.25 | 37.06 |
Nb2O5 | 0.30 | 0.28 | 0.19 | 0.28 | 0.41 | 0.38 |
Al2O3 | 1.61 | 1.30 | 1.57 | 1.37 | 1.55 | 1.39 |
Fe2O3 | 1.08 | 1.33 | 1.52 | 1.37 | 1.57 | 1.43 |
CaO | 28.51 | 27.49 | 28.08 | 27.28 | 26.85 | 27.21 |
Na2O | 0.01 | b.d.l. | 0.01 | 0.02 | 0.02 | 0.01 |
La2O3 | b.d.l. | 0.08 | 0.02 | 0.10 | 0.12 | 0.15 |
Ce2O3 | 0.02 | 0.54 | 0.36 | 0.81 | 0.84 | 0.86 |
Pr2O3 | 0.14 | 0.02 | 0.16 | 0.27 | 0.34 | 0.11 |
Nd2O3 | b.d.l. | 0.50 | 0.28 | 0.78 | 0.99 | 0.90 |
Y2O3 | b.d.l. | 0.32 | 0.19 | 0.37 | 0.64 | 0.48 |
F | 0.61 | 0.33 | 0.44 | 0.28 | 0.25 | 0.23 |
O〓F | 0.26 | 0.14 | 0.19 | 0.12 | 0.11 | 0.10 |
Total | 99.65 | 99.70 | 100.02 | 99.90 | 99.71 | 100.55 |
Si | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
Ti | 0.92 | 0.91 | 0.90 | 0.92 | 0.91 | 0.92 |
Nb | 0.00 | 0.00 | 0.00 | 0.00 | 0.06 | 0.01 |
Al | 0.06 | 0.05 | 0.06 | 0.05 | 0.06 | 0.05 |
Fe3+ | 0.03 | 0.03 | 0.04 | 0.03 | 0.04 | 0.04 |
Ca | 1.04 | 0.96 | 0.98 | 0.97 | 0.96 | 0.96 |
Na | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
La | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
Ce | 0.00 | 0.01 | 0.00 | 0.01 | 0.01 | 0.01 |
Pr | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
Nd | 0.00 | 0.01 | 0.00 | 0.01 | 0.01 | 0.01 |
Y | 0.00 | 0.01 | 0.00 | 0.01 | 0.01 | 0.01 |
F | 0.06 | 0.03 | 0.05 | 0.03 | 0.03 | 0.02 |
OH | 0.03 | 0.05 | 0.05 | 0.06 | 0.07 | 0.07 |
X(Ttn) | 0.91 | 0.92 | 0.91 | 0.91 | 0.90 | 0.91 |
X(Al,Fe3+−F) | 0.06 | 0.03 | 0.05 | 0.03 | 0.03 | 0.02 |
X(Al,Fe3+−OH) | 0.03 | 0.05 | 0.05 | 0.06 | 0.07 | 0.06 |
The composition of titanite ranges from 89 to 92 mol.% titanite end-member. The Al and Fe3+ contents range from 0.05 to 0.08 atoms per formula unit (apfu) and 0.02–0.05 apfu, respectively. Titanite shows some Al + Fe3+ excess over F, which indicates the coupled substitution of (Al + Fe3+) below the line (Al + Fe3+) + F− = Ti4+ + O2− (Figure 6). The presence of this substitution in analyzed titanites is shown also in the plot of Ti vs. (Al + Fe3+) (Figure 7). Calculation of OH content allows the estimation of (Al + Fe3+)−OH component from 2 to 9 mol.%. The content of (Al + Fe3+)−F component is lower than the amount of (Al + Fe3+)−OH component and ranges from 0 to 8 mol.%. The content of REE in analyzed titanite is usually below the microprobe detection limits. However, the contents of La, Ce, Pr, and Nd in some cases could be up to 0.99 wt.% (La2O3 up to 0.15 wt.%, Ce2O3 up to 0.86 wt.%, Pr2O3 up to 0.34 wt.%, Nd2O3 up to 0.99 wt.%).
5. Discussion
Titanite, with its considerable variation on chemical composition, is a highly sensitive indicator of oxygen fugacity (
The origin of titanite that is usually concentrated in light spots is thought to be formed with albite-enriched component plagioclases (oligoclase) and quartz on the expense of andesine and Ti-annite as expressed by the following reaction:
Similar late-magmatic reaction is supposed for titanite origin from I-type granites occurred in the Třibeč Mountains (Western Carpathians, Slovakia) [28]. This suggestion for the origin of titanite-spots granodiorites in the Austrian Mühlviertel by late-magmatic crystallization was presented in the past by Rupp et al. [11]. Late-magmatic origin of titanite in titanite-spots granodiorites and tonalites from the Fürstenstein pluton is supposed by Chen and Siebel [10]. In the past, the origin of these titanite-spots granitic rocks in the Fürstenstein pluton was given in detailed discussion of older papers [6, 7, 8] and also supposed by Troll [3]. Especially according to old detailed studies of titanite-spots granodiorites and tonalites from the Fürstenstein pluton, Drescher [6], Fischer [7], and Troll [3] supposed that titanite in the light spots originated by late-magmatic and/or younger hydrothermal crystallization. In all these cases, biotite served as a source of Ti for titanite, and titanite is most likely originated from a reaction involving biotite and probably anorthite component in plagioclase, during its deanorthization.
Incorporation of the REE in titanite is realized by the coupled substitution:
Partial enrichment of REE, especially Ce and La, is significant for magmatic titanite, especially for titanite occurred in alkalic magmatic rocks and lamprophyres [29, 30]. Incorporation of Nb and Ta is realized by the following reaction:
Both the exchange reactions (2 and 3) are significant for primary magmatic titanite [21]. The Nb and Ta enriched titanite was described from some granitic pegmatites [31].
6. Conclusions
Titanite-rich granodiorites occur in the Austrian Mühlviertel as small irregular bodies enclosed in I/S-type granites of the Freistadt/Mauthausen suite, which are usually fine-grained and dark granitic rocks having significant large amount of titanite spots mantled by hell aggregates of plagioclase and quartz.
Titanite as a relative plentiful accessory mineral usually forms idiomorphic, sphenoidal grains enclosed in light “spots” of fine-grained aggregates of plagioclases and quartz. The composition of titanite ranges from 89 to 92 mol.% titanite end-member. According to its Al concentration, the analyzed titanites could be considered as low-Al titanite with an aluminum of 0.05–0.08 apfu. The Fe3+ contents range from 0.02 to 0.05 apfu. Titanite shows some Al + Fe3+ excess over F, which indicates the presence of coupled substitution of (Al + Fe3+) + (F, OH) = Ti4+ + O2−. Calculation of OH content allows the estimation of (Al + Fe3+)−OH component from 2 to 9 mol.%. The content of (Al + Fe3+)−F component is lower than the amount of (Al + Fe3+)−OH component and ranges from 0 to 8 mol.%. Titanite of these titanite-spots granodiorites together with plagioclase and quartz was originated during late-magmatic evolution. The origin of titanite can be described by the following reaction:
Acknowledgments
Thanks are extended to the support of the Long-Term Conceptual Development Research Organization RVO 67985891 and the project of Ministry of Education, Youth and Sports (ME10083). I am also grateful to P. Gadas and R. Škoda from Institute of Masaryk University for their technical assistance by using electron microprobe analyses of selected minerals (titanite, plagioclase, biotite, amphibole). I am also grateful to the academic editor Ali Al-Juboury for the numerous comments and recommendations that helped to improve this paper.
References
- 1.
Fuchs G, Thiele O. Erläuterungen zur Übersichtskarte des Kristallins in westlichen Mühlviertel und im Sauwald, Oberösterreich. Geologische Bundesanstalt: Wien; 1968. p. 96 - 2.
Frenzel A. Das Passauer Granitmassiv. Geognostische Jahreshefte. 1911; 24 :105-192 - 3.
Troll G. Das Intrusivgebiet von Fürstenstein. Geologica Bavarica. 1964; 52 :1-140 - 4.
Graber HV. Die Diorite des Passauer Waldes. Geologische Rundschau. 1933; 24 :15-27 - 5.
Osann A. Über Titanitfleckengranite. Neues Jahrbuch für Mineralogie (Abhandlungen). 1923; 48 :223-239 - 6.
Drescher RK. Zur Tektonik und Petrographie der Diorite von Fürstenstein. Abhandlungen der Hessischen Geologischen Landes-anstalt. 1925; 8 :1-49 - 7.
Fischer G. Über Verbreitung und Entstehung der Titanitfleckengesteine im Bayerischen Wald. Zentrbl. f. Min. 1926; 25 :155-168 - 8.
Scholtz H. Das Saldenburger Granitmassiv und seine Umgebung. Geological Survey (Palaeontology Monographs). 1927; 2 :1-157 - 9.
Köhler A. Zur Entstehung der Granite der südböhmischen Masse. Mineralogy and Petrology. 1948; 1 :175-184 - 10.
Chen F, Siebel W. Zircon an titanite geochronology of the Fürstenstein granite massif, Bavarian Forest, NW Bohemian Massif: Pulses of the late Variscan magmatic activity. European Journal of Mineralogy. 2004; 16 :777-788. DOI: 10.1127/0935-1221/2004/0016-0777 - 11.
Rupp C, Linner M, Mand GW. Geologische Karte von Oberösterreich 1:200000. Erläuterungen. Geologische Bundesanstalt: Wien; 2011. p. 255 - 12.
Vellmer C, Wedepohl KH. Geochemical characterization and origin of granitoids from the South Bohemian Batholith in Lower Austria. Contributions to Mineralogy and Petrology. 1994; 118 :13-32. DOI: 10.1007/BF00310608 - 13.
Gerdes A, Wörner G, Henk A. Post-collisional granite generation and HT/LP metamorphism by radiogenic heating: The Variscan South Bohemian Batholith. Journal of the Geological Society of London. 2000; 157 :577-587. DOI: 10.1144/jgs.157.3.577 - 14.
Gerdes A, Friedl G, Parrish RR, Finger F. High-resolution geochronology of the Variscan granite emplacement—The South Bohemian Batholith. Journal of the Czech Geological Society. 2003; 48 :53-54 - 15.
Finger F, Gerdes A, René M, Riegler G. The Saxo-Danubian granite belt: Magmatic response to post-collisional delamination of mantle lithosphere below the south-western sector of the Bohemian Massif (Variscan orogeny). Geologica Carpathica. 2009; 60 :205-212. DOI: 10.2478/v10096-009-0014-3 - 16.
Verner K, René M, Žák J, Janoušek V. A brief introduction in the geology of the Moldanubian batholith. In: Janoušek V, Žák J, editors. Eurogranites 2015: Variscan Plutons of the Bohemian Massif. Post-conference Field Trip Following the 26th IUGG General Assembly in Prague. Prague: Czech Geological Survey; 2015. pp. 103-109 - 17.
Franzen Ch. Zur Petrologie und Geochemie des Fürstenstein-Intrusivkomplexes (Bayerischer Wald) [thesis]. Heidelberg: Ruprecht-Karls-Universität; 1999. p. 154 - 18.
Pouchou JL, Pichoir F. “PAP” (φ-ρ–Z) procedure for improved quantitative microanalysis. In: Armstrong JT, editor. Microbeam Analysis. San Francisco: San Francisco Press; 1985. pp. 104-106 - 19.
Harlov D, Tropper P, Seifert W, Nijland T, Förster HJ. Formation of Al-rich titanite (CaTiSiO4O–Ca AlSiO4OH) reaction rims on ilmenite in metamorphic rocks as a function of f H2O andf O2. Lithos. 2006;88 :72-84. DOI: 10.1006/J.Lithos.2005.08.00 - 20.
Wones DR. Significance of the assemblage titanite + magnetite + quartz in granitic rocks. American Mineralogist. 1996; 74 :744-749 - 21.
Piccoli P, Candela P, Rivers M. Interpreting magmatic processes from accessory phases: Titanite—A small-scale recorder of large-scale processes. Transactions of the Royal Society of Edinburgh: Earth Sciences. 2000; 91 :257-267. DOI: 10.1017/s026359330007422 - 22.
Xirouchakis D, Lindsley DH, Andersen DJ. Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part I. Theory. American Mineralogist. 2001; 86 :247-253 - 23.
Xirouchakis D, Lindsley DH, Frost BR. Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part II. Application. American Mineralogist. 2001; 86 :254-264 - 24.
Troitzsch U, Ellis DJ. Thermodynamic properties and stability of AlF-bearing titanite CaTiSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology. 2002; 142 :543-563. DOI: 10.1007/s004100100309 - 25.
Tropper P, Manning CE, Essene EJ. The substitution of Al and Fe in titanite in high pressure and temperature: Experimental constraints on phase relations and solid solution properties. Journal of Petrology. 2002; 43 :1787-1814. DOI: 10.1093/petrology/43.10.1787 - 26.
Oberti R, Smith DC, Rossi G, Caucia F The crystal chemistry of high-aluminium titanites. European Journal of Mineralogy. 1991; 3 :777-792. DOI: 10.1127/ejm/3/5/0777 - 27.
Enami M, Suzuki K, Liou JG, Bird DK. Al–Fe3+ and F-OH substitutions in titanite and constraints on their P–T dependence. European Journal of Mineralogy. 1993; 5 :219-231. DOI: 10.1127/ejm/5/2/0219 - 28.
Broska I, Harlov D, Tropper P, Siman P. Formation of magmatic titanite and ilmenite–titanite phase relations during granite alteration in the Třibeč Mountains, Western Carpathians, Slovakia. Lithos. 2007; 95 :58-71. DOI: 10.1016/j.lithos.2006.07.012 - 29.
Fleischer M. Relation of relative concentrations of lanthanides in titanite to type of host rocks. American Mineralogist. 1978; 63 :869-873 - 30.
Seifert W. REE-, Zr-, and Th-rich titanite and associated accessory minerals from a kersantite in the Frankenwal, Germany. Mineralogy and Petrology. 2005; 84 :129-146. DOI: 10.1007/s00710-005-0076-6 - 31.
Russel JK, Groat LA, Halleran AD. LREE-rich niobian titanite from Mount Bisson, British Columbia: Chemistry and exchange mechanisms. The Canadian Mineralogist. 1994; 32 :575-587