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Articles |
Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany
* e-mail: imima{at}po.uni-stuttgart.de
| Abstract |
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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-feldsparquartz 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 |
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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 cm2 surface of a thin-section.
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| Geological setting |
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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 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 (E9921 to E9925) 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 |
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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 E9924 (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.
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| Coesite and its pseudomorphs in the central Erzgebirge |
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The microscopic observations made on sample E9924 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-feldsparquartz 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-feldsparquartz 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 nearby 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 |
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Received 6 December 1999
Modified version received 28 November 2000
Accepted 19 December 2000
| References |
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