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Evidence for Partial Melting of Eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

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Evidence for Partial Melting of Eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic Journal of Mineralogical and Petrological Sciences, Volume 111, page 405–419, 2016 Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic Takahiro MIYAZAKI*, Daisuke NAKAMURA*, Akihiro TAMURA**, Martin SVOJTKA***, † Shoji ARAI** and Takao HIRAJIMA *Department of Earth Sciences, Faculty of Science, Okayama University, Okayama 700–8530, Japan **Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920–1192, Japan ***Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, 16502 Praha 6, Czech Republic †Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606–8502, Japan Eclogites from the Moldanubian Zone of the Bohemian Massif (Czech Republic) have experienced ultrahigh– pressure (UHP) and ultrahigh–temperature (UHT) metamorphic conditions (P > 4.0 GPa, T > 1000 °C). One eclogite sample collected from Nové Dvory, eastern part of Czech Republic, contains leucocratic pockets. The mineral assemblage of the melanocratic part of the eclogite is garnet + omphacite + rutile + later–stage minerals (biotite + amphibole + plagioclase). The leucocratic pockets mainly consist of plagioclase and include moderate amounts of garnet and clinopyroxene + small amounts of biotite and amphibole. Garnet and clinopyroxene grains in the leucocratic pockets display faceted shapes whereas those in the melanocratic part show irregular shapes. These are consistent with the idea that garnet and clinopyroxene grains in the leucocratic pockets had been surrounded by melt, as melt can provide free space for the formation of such faceted shapes. The major and trace element composition of garnet grains in the melanocratic part are the same as those in the leucocratic pockets. Clinopyroxene also shows the same major and trace element composition for irregular shape grains in the melanocratic part as faceted grains in the leucocratic pockets. If the melt had originated from the gneiss surrounding the peridotite body with eclogite lenses or layers, the composition of clinopyroxene in the leucocratic pockets would have been completely different from that in the melanocratic part. In addition, the trace element composition of melt estimated from clinopyroxene composition resembles that of Tonalite – Trondjhemite – Granodiorite (TTG), which can be produced by partial melting of eclogite. Thus, these obser- vational and analytical data suggest that the leucocratic pockets were originally melt that was internally pro- duced by partial melting of the eclogite. Keywords: Partial melting, Eclogite, Leucocratic pockets, Bohemian Massif, Moldanubian Zone INTRODUCTION for partial melting of natural eclogite has been rarely re- ported, except for a few examples (e.g., Konilov et al., Tonalite–Trondjhemite–Granodiorite (TTG) and adakitic 2011; Chen et al., 2012; Gao et al., 2012). Konilov et al. rocks can be derived from partial melting of subducting (2011) and Chen et al. (2012) reported the presence of oceanic basalt or gabbro (e.g., Defant and Drummond, felsic veins in eclogite and interpreted that the felsic veins 1990; Rapp et al., 2003). Thus, slab melting can generate were products of partial melting of eclogite. Konilov et andesitic melt that can be a source of continental crust, if al. (2011) showed that the trace element compositions of young and hot oceanic crust was subducted. Thereby, the the felsic veins were very similar to those of the average study of natural partially molten eclogite can give us pre- TTG and experimental melt of eclogite, whereas the cious information about the mechanism of continental trace–element compositions of the felsic veins reported crust formation and is important for the elucidation of by Chen et al. (2012) are quite different from the natural the evolution process of the Earth. However, evidence TTG compositions. Other works (e.g., Gao et al., 2012) treated the presence of multiphase solid (MS) inclusions doi:10.2465/jmps.151029c as evidence for partial melting of eclogite, although Gao D. Nakamura, [email protected]–u.ac.jp Corresponding author et al. (2012) did not estimate the chemical composition of 406 T. Miyazaki, D. Nakamura, A. Tamura, M. Svojtka, S. Arai and T. Hirajima the melt from the MS inclusions. Naemura et al., 2009). On the other hand, evidence for An eclogite sample with leucocratic pockets (<2.0 UHP metamorphism of felsic granulite and gneiss in the cm) was collected from Nové Dvory located in the east- Bohemian Massif is rare, but a few findings of microdia- ern part of Czech Republic. On the origin of the eclogite mond and coesite from such felsic rocks have been re- from Nové Dvory, two possible hypotheses have been cently reported (e.g., Kobayashi et al., 2008; Kotková et proposed: gabbroic origin at the plagioclase–stable low– al., 2011; Perraki and Faryad, 2014), indicating that some pressure conditions (Obata et al., 2006) and high–pres- garnet peridotite masses and surrounding felsic rocks sure cumulate of garnet and clinopyroxene (e.g., Beard may have shared the same P–T history, although peak et al., 1992). In either case, the eclogite and garnet peri- P–T conditions of granulite in the Gföhl unit have been dotite from Nové Dvory have experienced high–pressure estimated to be approximately 1.6–2.0 GPa, 900–1100 °C (HP) and high–temperature (HT) conditions, with pres- (e.g., Carswell and O’Brien, 1993). sures above 4.0 GPa and temperatures as high as 1000 The study area, Nové Dvory, is located in the east- °C (Medaris et al., 1990; Nakamura et al., 2004), which ern part of the Bohemian Massif, where a garnet perido- are distinctly higher in temperature than the wet solidus tite mass, including several eclogite layers, with a size of of basaltic rocks (e.g., Peacock et al., 1994; Schmidt and approximately 2 km2 occurs and is surrounded by mig- Poli, 1998; Vielzeuf and Schmidt, 2001; Skjerlie and matitic gneiss (Medaris et al., 1990). The Nové Dvory Douce, 2002). This indicates that the studied eclogite peridotite has been interpreted as part of former mantle should have been partially molten if aqueous fluid was wedge material, and eclogite layers are believed to rep- present within the eclogite or some hydrous minerals resent high–P cumulates from silicate melt (Medaris et such as amphibole, epidote, or phengite (e.g., Hermann al., 2005). Obata et al. (2006) have proposed that kyan- and Green, 2001) were decomposed to release H2O near ite–quartz eclogite was originally low–P cumulus gabbro the peak P–T conditions. and then subjected to an increase in pressure up to mantle In this paper, we discuss whether or not the eclogite depths. The studied eclogite sample including leucocratic including the leucocratic pockets underwent partial melt- pockets was collected from the southeastern part of this ing, through the combination of detailed observations and peridotite mass (49°04′39′′N, 16°05′E). Three samples of analyses of major and trace element compositions of gar- eclogites from Nové Dvory yield Sm–Nd Grt–Cpx–WR net and clinopyroxene both in the matrix of the eclogite isochron ages of 332–342 Ma (Beard et al., 1992), which and in the leucocratic pockets. In addition, we estimated is approximately the same as the metamorphic age (~ 340 trace–element compositions of the melt by point analyses Ma) of the Gföhl granulites (e.g., Sláma et al., 2008), of clinopyroxene in the leucocratic pockets in conjunc- although eclogites from other areas, such as Biskupice tion with partition coefficient data between clinopyroxene and Bečváry, have yielded different Sm–Nd ages of and the melt. This study will give insight into the nature around 320 and 380 Ma, respectively (Beard et al., 1992). of the partial melting process of eclogite. PETROGRAPHY GEOLOGICAL OUTLINE Eclogite from the study area can be divided into kyanite– The Bohemian Massif is situated at the eastern end of the bearing and kyanite–free types. Most of the kyanite–bear- Variscan orogenic belt, which is divided into the northern ing eclogite samples include quartz (probably trans- Saxothuringian Zone and the southern Moldanubian formed from coesite), whereas many of kyanite–free Zone. The Moldanubian Zone is further subdivided into samples lack quartz and consist mainly of garnet and the structurally lower Monotonous and Varied units and clinopyroxene with small amounts of rutile, apatite and higher Gföhl unit (e.g., Matte et al., 1990). The latter opaque minerals as well as later–stage phases such as Gföhl unit mainly consists of felsic granulite and migma- plagioclase, amphibole and biotite. Hydrous minerals of titic gneiss which include sporadically distributed gar- the UHP stage such as epidote and phengite are absent net peridotite masses or layers with and without eclogite from both types of eclogite, which is probably due to the (e.g., Medaris et al., 1990; 2005). One eclogite layer extremely high–temperature conditions that these eclo- without peridotite occurs within the felsic Gföhl granulite gites have experienced. Detailed petrological characteris- in the central part of Czech Republic (near the village tics of kyanite–bearing eclogite from this locality are de- of Spačice) (Medaris et al., 1998). Geothermo–barometric scribed in Nakamura et al. (2004). calculations suggest that several eclogite and garnet peri- The sample selected for this study (ND110) is ky- dotite masses in the Gföhl unit experienced UHP condi- anite–free and quartz–free dark–green bi–mineralic eclo- tions (e.g., Nakamura et al., 2004; Medaris et al., 2006; gite with leucocratic pockets that mainly comprise large Partial melting of eclogite 407 Figure 1. Photographs of the studied eclogite from Nové Dvory includ- ing leucocratic pockets. (a) Scan- ned section of the studied sample ND110. White parts are the leuco- cratic pockets that mainly consist of plagioclase (Pl). (b) Scanned thin section (ND110A) made from the sample ND110. Pale–green mineral is clinopyroxene, dark– green one is amphibole and brown one is biotite.
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