First find of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany

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1 Eur. J. Mineral. 2001, 13, First find of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany HANS-JOACHIM MASSONNE* Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D Stuttgart, Germany Abstract: In the ultrahigh-pressure metamorphic area of the Saidenbach reservoir, Erzgebirge, where microdiamondbearing gneiss occurs, coesite was found as inclusion in garnet and omphacite in an unusually well preserved eclogite. Most of the eclogite bodies of this area are thoroughly altered, with all omphacite transformed to plagioclase + amphibole symplectites. The coesite-bearing eclogite contains abundant dolomite in the matrix and as surprisingly large inclusions in garnet, which is another unusual feature compared to the many eclogites of the Saxonian Erzgebirge. Coesite is at least partly replaced by quartz showing a palisade texture. This pattern is accompanied by abundant radial cracks in the host minerals. Polycrystalline inclusions of equigranular quartz are also related to former coesite because some of them preserve minor coesite relics. In such cases, cracks in the host mineral are not more pronounced than around moderately transformed coesite. No K-feldspar was observed in inclusions consisting of partly replaced coesite. Thus, inclusions in omphacite of K-feldspar quartz symplectites with abundant cracks around, which occur as well in eclogites near the Saidenbach reservoir, cannot be coesite pseudomorphs. Key-words: coesite, ultrahigh-pressure metamorphism, eclogite, Erzgebirge, diamond. Introduction Although polycrystalline quartz inclusions in garnet and omphacite in Erzgebirge eclogite were interpreted as pseudomorphs after coesite (Schmädicke, 1991), this high-pressure silica polymorph had never been positively identified in the Erzgebirge. Its former occurrence there was even questionable, since the polycrystalline aggregates depicted in Schmädicke (1991) show striking resemblance to K-feldspar quartz inclusions reported in the same eclogites (Massonne et al., 2000) and in those of the Su-Lu region, China (Yang et al., 1998), which cannot be simply interpreted as pseudomorphs after coesite. In the central portion of the Variscan crystalline area of the Erzgebirge, that is part of the Bohemian Massif, Massonne (1999) detected microdiamonds in hardly foliated garnet-muscovite gneiss. These rocks occur near the Saidenbach reservoir, about 1.5 km northwest of the village of Forchheim, in the so-called Gneiss- Eclogite Unit (GEU) that dominates the central portion of the Erzgebirge (Fig. 1, inset). The true inclusion character of these microdiamonds that can reach diameters up to 30 µm was proven by micro-raman spectroscopy (Nasdala & Massonne, 2000). The quantification method of Massonne et al. (1998) applied to specifically polished rock thin-sections shows that microdiamonds occur as inclusions in garnet, kyanite and zircon with a maximum concentration of up to 150 grains exposed on the 5 cm 2 surface of a thin-section. Further field and petrological work in this area has led to the discovery of coesite in nearby eclog- * imima@po.uni-stuttgart.de DOI: / /2001/ /01/ $ E. Schweizerbart sche Verlagsbuchhandlung. D Stuttgart

Geological setting The oval-shaped metamorphic area of the Erzgebirge can be subdivided into four major units. Three of them are shown in the inset map of Fig. 1.

2 566 H.-J. Massonne Fig. 1. Geological map of the eastern section of the Saidenbach reservoir area. The inset, simplified after Krohe (1996), shows the position of this area and the major crystalline units of the Erzgebirge. ite, which is reported here for the first time from the Erzgebirge. Geological setting The oval-shaped metamorphic area of the Erzgebirge can be subdivided into four major units. Three of them are shown in the inset map of Fig. 1. These units of medium to high metamorphic grade are surrounded by Variscan low-grade metasediments and metavolcanics that form the fourth major metamorphic unit, but also by Late Variscan granites, Permian volcanics as well as by Mesozoic and Tertiary sediments and volcanics. Abundant eclogite occurrences are known from the Micaschist-Eclogite Unit and the GEU. The latter unit mainly consists of various types of paraand orthogneisses. According to Willner et al. (1997) and Kröner & Willner (1998), these gneisses have experienced high-pressure metamorphism in Early Carboniferous times. In the area around the Saidenbach reservoir where diamondiferous gneisses occur, eclogites are abundant as well. After several mapping campaigns with students, the author was able to draw the map of Fig. 1 showing the distribution of eclogite and diamondiferous gneiss, mainly on the Table 1. Electron microprobe analyses of garnet and included minerals (sample E99-24).

Coesite relics in central Erzgebirge 567 basis of loose blocks. Additional valuable information for the map of Fig.

Haselbach valleys. The last mapping campaign in September 1999 has led to the detection of two new occurrences of diamondiferous gneiss in addition to the two lenses mentioned by Massonne (1999).

These eclogite occurrences were already noted on the old 1:25,000 geological maps. In the map of Fig.

Because the few true outcrops along the shore line of the Saidenbach reservoir show mainly eclogites, one gains the impression that at least the eclogite zones consist of many small eclogite lenses

3 Coesite relics in central Erzgebirge 567 basis of loose blocks. Additional valuable information for the map of Fig. 1 came from the 1:25,000 geological maps prepared in the late twenties (Reinisch, 1931) and more than hundred years ago (Hazard, 1886), as the reservoir had not yet flooded the Saidenbach and Haselbach valleys. The last mapping campaign in September 1999 has led to the detection of two new occurrences of diamondiferous gneiss in addition to the two lenses mentioned by Massonne (1999). During this campaign blocks of fresh eclogites (E99-21 to E99-25) were sampled at the northern strand of the Saidenbach reservoir, which is covered by water most time of the year. These eclogite occurrences were already noted on the old 1:25,000 geological maps. In the map of Fig. 1 areas are outlined where either abundant blocks of eclogite or those of diamondiferous gneiss, which grade into foliated diamond-free gneiss, were recognized. Because the few true outcrops along the shore line of the Saidenbach reservoir show mainly eclogites, one gains the impression that at least the eclogite zones consist of many small eclogite lenses once glued by molten felsic rocks that now form either schlieren or pegmatite-like rocks. There seems to be no direct contact between the diamondiferous gneisses and the zones rich in eclogites. Only the find of a few eclogite blocks within the diamondiferous gneiss zone at the southern shore of the northern branch in the eastern portion of the Saidenbach reservoir (Fig. 1) could be an indication for a direct relation of eclogite and diamondiferous gneiss. Petrography The many thin-sections of the eclogites that were sampled at or near the shore line of the southern branch in the eastern portion of the Saidenbach reservoir and along the slopes and hill tops north Fig. 2. Coesite inclusion in omphacite of eclogite sample E99-24 as seen in plane polarized light. The scale bar in the lower left corner of the photomicrograph is 20 µm. Only the outermost rim of the coesite crystal is replaced by quartz. Fig. 3. Coesite inclusion in garnet of eclogite sample E a. Photomicrograph, plane polarized light. The coesite crystal in the centre of the picture is partially replaced by a rim of quartz. The semi-opaque inclusion in coesite is rutile. Scale bar represents 50 µm. b. Backscattered-electron image of the same object, obtained with a CAMECA SX50 electron microprobe. c. Cathodoluminescence image of the same object.

4 568 H.-J. Massonne of the reservoir (see Fig. 1) commonly show that all the matrix omphacite was transformed to symplectites mainly consisting of plagioclase and amphibole, while some omphacite was preserved as inclusion in garnet. The garnet in these eclogites was only marginally altered, offering promising prospects for the search for microdiamonds applying the method of Massonne et al. (1998), but no microdiamond could be detected. After the discovery of fresh eclogite blocks further work was undertaken on the Saidenbach eclogites. Again no microdiamond was identified in polished rock thin-sections but the petrogenetic relations in the high-pressure matrix could be established. The fresh eclogites range from fine-grained foliated rocks to hardly or non-foliated coarsergrained rocks with average grain sizes in the mm range. Sample E99-24 (Fig. 1) belongs to the latter type and is also conspicuous because of the many relatively large inclusions that appear especially in garnet. Among these inclusions rutile, kyanite, phengite, clinozoisite, calcite and abundant dolomite were observed (Table 1). These minerals also occur in the matrix with quartz. In addition to these, another inclusion mineral was recognized, coesite. Figures 2 and 3 display the best preserved coesite inclusions in omphacite and garnet, respectively, in sample E Nevertheless, there too, coesite is partially replaced by a phase with clearly lower index of refraction, quartz. With the aid of a CAMECA SX50 electron microprobe the composition of both phases were determined to be identical, as demonstrated by the backscattered electron image of Fig. 3b, and virtually pure SiO 2 (Table 1). Further indications for this identification come from the palisade texture of quartz surrounding coesite and from the abundant radial cracks in the host minerals around the inclusions. Such features were already reported from other ultrahigh-pressure metamorphic areas, for instance, by Chopin (1984) from the coesite type locality in the Dora-Maira Massif, Western Alps, by Smith (1984) and Wain et al. (2000) from Norway, by Wang et al. (1989) and Hirajima et al. (1990) from the Dabie-Sulu terrain in Eastern China and by Caby (1994) from Mali. The cathodoluminescence images obtained with the electron microprobe (Fig. 3c) show that quartz with rather bluish luminescence colour surrounds weakly luminescent coesite. This is in contrast to other reports of bluish luminescent coesite and reddish-violet (Schertl & Okay, 1994) or orange luminescence colours for palisade quartz. Coesite and its pseudomorphs in the central Erzgebirge Several stages from relatively well preserved to almost completely transformed coesite were observed in sample E Surprisingly, in cases of strongly transformed coesite, cracks in the host mineral are not more pronounced than around inclusions of hardly transformed coesite. From the observation of various replacement stages of coesite, the assumption is made that polycrystalline inclusions of equigranular quartz observed in garnet represent former coesite, as well. At least one inclusion consisting of minor coesite relics and equigranular quartz grains points to that (see also Wain et al., 2000; Parkinson, 2000). Under these circumstances, the polycrystalline quartz inclusions reported by Schmädicke (1991) and Schmädicke et al. (1992) from other eclogite localities within the GEU could be actual pseudomorphs after coesite, although definite evidence is still lacking for them. The additional argument for former coesite that radial cracks occur around such inclusions can be misleading, as was shown by Wendt et al. (1993): such radial cracks can occur upon decompression and cooling simply because of contrasting elastic properties of the host and inclusion minerals. This is the most likely explanation for the radial cracks occurring around the dolomite inclusions in garnet in sample E The microscopic observations made on sample E99-24 confirm that the transformation of coesite leads to pure quartz (Fig. 3). Thus, any polycrystalline quartz inclusion with additional K-feldspar in eclogitic garnet and omphacite, as reported by Yang et al. (1998) from the Su-Lu UHP terrane, China, should not be simply the result of a transformation from coesite. Many such inclusions with conspicuous radial cracks in the host minerals exist in eclogites from the GEU, for instance in those occurring in a small abandoned quarry 2.5 km SE of the village of Eppendorf and 4.5 km N of the coesite locality described here. These polycrystalline K-feldspar quartz aggregates show striking resemblance to the aggregates described as pseudomorphs after coesite by Schmädicke (1991) and Schmädicke et al. (1992), in part from the same localities. These inclusions consisting of K-feldspar quartz symplectites were interpreted by Massonne et al. (2000) as former K-cymrite. The detection of coesite in diamond-free eclogites from the Saidenbach reservoir could be a hint to a P-T evolution similar to that of the near-

5 Coesite relics in central Erzgebirge 569 by diamond-bearing gneisses. However, such a conclusion might be premature and awaits completion of a combination of detailed microscopic observations, extensive electron microprobe work and reliable geothermobarometry. Actually, a similar problem arises in the only other UHPM area where the occurrence of metamorphic microdiamond was indisputably established, i.e. the Kokchetav Massif, Kazakhstan (Sobolev & Shatsky, 1990). Evidence for UHP conditions was reported from various rock types there (Shatsky et al., 1995; Parkinson, 2000), but it remained unclear whether all the various eclogite lenses, in particular those occurring close to the diamondiferous gneisses at lake Kumdy-Kol, had experienced UHPM conditions. There, coesite inclusions are extremely rare (Korsakov et al., 1998). The UHPM area of the Saidenbach reservoir is therefore one more place on Earth where it may become possible to understand the mechanisms of the uplift of rocks from depths of 100 km or more. Acknowledgements: My thanks are due to Andreas Brandelik for the digital drawing of the geological map. Furthermore, I would like to thank all the students (Ute Grosch, Ingo Klemm, Ingo Müller, Uschi Nowlan, Marcia Rivas- Kozlowski, Anja Stachowiak) that mapped a part of the relevant area during a mapping course in Heinz-Jürgen Bernhardt strongly supported the electron microprobe work at Ruhr-Universität Bochum. The quality of the manuscript benefited from the review work of Christian Chopin. References Caby, R. (1994): Precambrian coesite from northern Mali: First record and implications for plate tectonics in the trans-saharan segment of the Pan-African belt. Eur. J. Mineral., 6, Chopin, C. (1984): Coesite and pure pyrope in high grade blueschists of the Western Alps: a first record and some consequences. Contrib. Mineral. Petrol., 86, Hazard, J. (1886): Geologische Specialkarte des Königreichs Sachsen No Section Pockau- Lengefeld. Königliches Finanz-Ministerium (ed.) under the supervision of Credner, H. Hirajima, T., Ishiwatari, A., Cong, B., Zhang, R., Banno, S., Nozaka, T. (1990): Coesite from Mengzhong eclogite at Dhonghai county, northeastern Jiangsu province, China. Mineral. Mag., 54, Korsakov, A.V., Shatsky, V.S., Sobolev, N.V. (1998): The first finding of coesite in the eclogites of the Kokchetav Massif. Doklady Earth Sciences, 360, (transl. from Doklady Akad. Nauk, 360, No. 1, 77-81); Krohe, A. (1996): Variscan tectonics of central Europe: Postaccretionary intraplate deformation of weak continental lithosphere. Tectonics, 15, Kröner, A. & Willner, A.P. (1998): Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge, Saxony, Germany. Contrib. Mineral. Petrol., 132, Massonne, H.-J. (1999): A new occurrence of microdiamonds in quartzofeldspathic rocks of the Saxonian Erzgebirge, Germany, and their metamorphic evolution. Proceed. 7th Int. Kimberlite Conf., Cape Town 1998, 2, Massonne, H.-J., Bernhardt, H.-J., Dettmar, D., Kessler, E., Medenbach, O., Westphal, T. (1998): Simple identification and quantification of microdiamonds in rock thin-sections. Eur. J. Mineral., 10, Massonne, H.-J., Dobrzhinetskaya, L., Green, H.W. II (2000): Quartz - K-feldspar intergrowths enclosed in eclogitic garnet and omphacite. Are they pseudomorphs after coesite? Ext. Abstract, 31 st Int. Geol. Congr. Rio de Janeiro, Brazil, 4 p. (on CD, search for Massone). Nasdala, L. & Massonne, H.-J. (2000): Microdiamonds from the Saxonian Erzgebirge, Germany: in situ micro-raman characterisation. Eur. J. Mineral., 12, Parkinson, C.D. (2000): Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav Massif, Kazakhstan: a record of progressive UHP metamorphism. Lithos, 52, Reinisch, R. (1931): Geologische Karte von Sachsen im Maßstab 1:25000 Nr Blatt Lengefeld. Finanzministerium (ed.) under the supervision of Kossmat, F., with contributions of Endler, L., Gärtner, G., and Härtel, J. Schertl, H.-P. & Okay, A.I. (1994): A coesite inclusion in dolomite in Dabie Shan, China: petrological and rheological significance. Eur. J. Mineral., 6, Schmädicke, E. (1991): Quartz pseudomorphs after coesite in eclogites from the Saxonian Erzgebirge. Eur. J. Mineral., 3, Schmädicke, E., Okrusch, M., Schmidt, W. (1992): Eclogite-facies rocks in the Saxonian Erzgebirge, Germany: high pressure metamorphism under contrasting P-T-conditions. Contrib. Mineral. Petrol., 110, Shatsky, V.S., Sobolev, N.V., Vavilov, M.A. (1995): Diamond-bearing metamorphic rocks of the Kokchetav Massif (Northern Kazakhstan). In: Coleman R.G. & Wang X. (eds.) Ultrahigh pressure metamorphism, , Cambridge Univ. Press.

6 570 H.-J. Massonne Smith, D.C. (1984): Coesite in clinopyroxene in the Caledonides and its implications for geodynamics. Nature, 310, Sobolev, N.V. & Shatsky, V.S. (1990): Diamond inclusions in garnets from metamorphic rocks: a new environment for diamond formation. Nature, 343, Wain, A., Waters, D., Jephcoat, A., Olijynk, H. (2000): The high-pressure to ultrahigh-pressure eclogite transition in the Western Gneiss Region, Norway. Eur. J. Mineral., 12, Wang, X., Liou, J.G., Mao, H.K. (1989): Coesite-bearing eclogite from the Dabie Mountains in central China. Geology, 17, Wendt, A., D Arco, P., Goffé, B., Oberhänsli, R. (1993): Radial cracks around α-quartz inclusions in almandine: constraints on the metamorphic history of the Oman mountains. Earth planet. Sci. Lett., 114, Willner, A.P., Rötzler, K., Maresch, W.V. (1997): Pressure-temperature and fluid evolution of quartzofeldspathic metamorphic rocks with a relic highpressure, granulite-facies history from the Central Erzgebirge (Saxony, Germany). J. Petrol., 38, Yang, J., Godard, G., Smith, D.C. (1998): K-feldsparbearing coesite pseudomorphs in an eclogite from Lanshantou (Eastern China). Eur. J. Mineral., 10, Received 6 December 1999 Modified version received 28 November 2000 Accepted 19 December 2000

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