Petrography and P-T estimates of burial stage of eclogites from Kreuzeck Massif, Eastern Alps, Austria

Martin Michálek

Univerzita Komenského v Bratislave, Prírodovedecká fakulta, katedra mineralógie a petrológie, Mlynská Dolina, 842 05, Bratislva, Slovensko; [email protected]

Introduction and aims of work In general, eclogites and eclogites facies rocks are interpreted as oceanic and/or continental crustal material buried to mantle depths. These rocks provide important information on the early stage of orogenic processes. Some cases allow us to determine according to the textures and microfabrics establish the eclogitic and post-eclogitic deformation history from burial by subduction to subsequent exhumation. The reviews of exhumation mechanism of high-pressure (HP) rocks from Alps have been proposed by several authors [1, 2, 3, 4, 5]. The purpose of this contribution is petrography and P-T estimates of the burial stage of eclogites from the Polinik structural complex in Kreuzeck massif, Eastern Alps in Austria. Abbreviations of are: andradite (Adr), almandine (Alm), (Am), anortithe (An), clinopyroxene (Cpx), (En), ferrosilite (Fs), grossulare (Grs), garnet (Grt), hornblenda (Hbl), jadeite (Jd), kyanite (Ky), phengite (Phg), plagioclase (Pl), pargasite (Prg), (Prp), quartz (Qtz), rutile (Rt), spessartine (Sps), staurolite (St), titanite (Ttn), zoisite (Zo), wollastonite (Wo) [6].

Geological settings The internal zones of the Eastern Alps are subdivided into two mega-units: (1) Austroalpine nappe complex, and (2) the Penninic nappe complex. AA complexes represent an orogenic wedge [7] of continental crust that formed during the Early Cretaceus collision [8, 9] following the closure of the Meliata-Halstatt ocean in the Late Jurassic. This event caused an extreme shortening of AA unit and adjacent structural complexes metamorphosed in eclogite to greenshist facies cropping out in the Kreuzeck Massif. The Polinic structural complex is located in the Kreuzeck massif S of the Tauern Window as a part of Austro-Alpine (AA) basement structural complexes. According to mapping work [10, 11, 12] there are at least 5 stacked AA structural complexes in the Kreuzeck Massif. They are (from north and tectonic bottom):

1244 1. the Ragga structural complex 2. the Polinik structural complex 3. the Strieden structural complex 4. the Hochkreuz structural complex 5. the Steinfeld structural complex The Polinik structural complex is restricted along main mylonite zone (MMZ) and around the Polinik Mt. peak [13]. The Polinik complex shows the strongest Alpine overprint [5]. This complex consist of Ky-St-Grt gneisses, eclogitic high-Na amphibolites and Bar- bearing amphibolites, granitic feldspar-gneisses to micashists, leucocrate granitic gneisses [5]. Recently has been found tourmaline bearing eclogite in this region [14].

Methodology Microscope study has been referred to observation of microstructure of methamorphic minerals in eclogite and for selection of proper pairs for geothemobarometry. Chemical composition of minerals and BSE images has been obtained by using a CAMECA SX-100 electron–microprobe at Geological Institute of Dionýz Štúr in Bratislava. P-T estimation was evaluated by conventional geothemobarometry using several calibrations [15, 16].

Petrography Eclogites are composed of garnet, omhacite, zoisite, amphibole and ±rutile, titanite

(Fig. 1). Idioblastic to xenoblastic garnets contains inclusion of Zo, Am and Qtz and Cpx1.

Clinopyroxene is preserved as an inclusion in garnets (Cpx1) and in matrix (Cpx2), as well.

Cpx2 in matrix is partially replaced by Cpx3+Pl symplectite or Cpx3+Am3+Pl symplectite.

Amphibole occurs as inclusions associated with Zo and quartz within garnets cores (Am1), in kelyphitic rims (Am2) around garnet, in symplectite (Am3) with plagioclase and clinopyroxene

(Cpx2) and as a porhyroblast in matrix (Am4). Plagioclase occurs around zoisite and garnet and mainly as a intergrowths in symplectites with Cpx3 and Am3. Zoisite is preserved as inclusions in garnet, and as a porhyroblast in the matrix.

1245

Fig. 1: Microtextures of eclogite. a, b) Grt with inclusions and Cpx2 (Omp), c, d) Grt, Cpx2(Omp), Am2 around garnet and in matrix (Am4), Zo in matrix, Symplectites after Cpx2 (Omp), Rt and Ttn.

Mineral chemistry

Porphyroblast of garnets (Alm44-50Prp12-24Grs30-38Sps0,5-3,3) show prograde garnet growth (Fig. 2, 3a). The range of Fe2+/(Fe2++Mg) is decreasing from core to rim (0,84-0,66) and Mg/(Mg+Fe2+) is increasing (0,16-0,34) (Fig. 2a). One inclusion of garnet was found in an amphibole inclusion in the porhyroblast of garnet. Chemical composition of this unique inclusion of garnet is Grs47Alm39Prp7Sps5 Adr2 (Fig. 2b). Amphibole inclusion shows zonation from core to rim (Mg-Hbl to Al-Fe-Prg). Clinopyroxene occurs as an inclusions

(Cpx1-En39-42Fs10-12Wo47-49 in garnet and in matrix (Cpx2-En40-42Fs8-10Wo48-49) with jadeite content of 20-24 mol %. Decompression breakdown of matrix Cpx1 produces symplectitic

Na-poor Cpx3 (Jd<10%) +Pl, some with intergrowth of amphibole (Fig.2a,b). According to classification of [15], amphibole inclusion (Am1) is Mg- Hbl to Alumino-ferro pargasite. Chemical composition of amphibole (Am2) around garnet is pargasitic. Matrix amphibole and in symplectites is of Mg-Hbl composition. Plagioclase occurs around zoisite where An content is 24 mol %, and in symplectites with Cpx3 and Am3 where An content is in

1246 range 19-26 mol %. Inclusions of zoisite are Fe poorer then matrix porphyroblasts.

Fig. 2: BSE images of eclogitic metabasite. a)Garnet with inclusions of Cpx, Zo, and Am. b) Grt inclusion in Am inclusion in garnet. c) Partial decomposition of matrix omphacite to Cpx3-Am3-Pl symplectites. d) Matrix omphacite, zoisite, amphibole around garnet and Cpx3-Pl symplectites.

P-T estimates of the burial stage

The high-pressure mineral assemblage is represented by GrtMg-zone, CpxJd24, Zo, Rt, Qtz. Into calculation of the burial stage of metamorphism was taking into account garnet with maximum of Prp content (up to 24 mol. %) and clinopyroxene with maximum Jd content (up to 24 mol. %). Due to absence of plagioclase and phengite, we can calculate minimum pressure condition, where the intersection between Grt-Cpx geothemometer [15] and Jd geobarometer [16] is 579±41°C at minimum 12kbar±1.

Discussion and conclusions Estimated P-T conditions of burial stage is 579±41°C at minimum 12kbar±1. The maximum temperature of the burial stage of 530°C at minimum 11 kbar [5] has been determined. Recently has been studied tourmaline bearing eclogite [14] from the Kreuzeck

1247 Mts., with high pressure assemblage Grt, Omp, Phg , Zo, Ca- Am, Rt, Qtz. The peak of metamorphism yields P-T conditions of 16±1 kbar and 650±30°C. Due to absence of Phg in our sample, it was not possible to determine accurate pressure, just minimum, 12 kbar at 579±41°C. The results of temperature and pressure of burial stage corresponds to published results [5]. Tourmaline bearing eclogite contains Omp with Jd content up to 48 mol. % what differs from this study and samples studied before [5] because of different place of collection of the sample in the Polinik structural unit.

Acknowledgement Support from Comenius University Grant (No. UK/351/2009) and Slovak Research and Development Agency (No. APVV-0279-07) is greatly acknowledged.

Bibliography [1] Spalla M. I., Lardeaux J. M., Dal Piaz G. V., Gosso G., Messiga B. (1996) J. Geod., 21, p. 257 [2] Kurz W., Neubauer F., and Dachs E. (1998) Tectonophysics, 285. p.183 [3] Putiš M., Korikovsky S. P., Pushkarev Yu A. (2000) Jb. Geol. B.-A., 142, p.73 [4] Putiš M., Korikovsky S. P., Wallbrecher E., Unzog W., Olesen N. Oe., Fritz H. (2002) J. Struct. Geol., 24, p. 339 [5] Putiš M., Korikovsky S. P., Unzog W., Olesen N. O., (2002) Slovak Geol. Mag., 8(1), p. 65

[6] Siivola J, Schmid R. A. (2007) Systematic nomenclature for metamorphic rocks: 12. List of mineral abbreviations. Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks. Recommendations, web version of 01.02.2007.

http://www.bgs.ac.uk/scmr/products.html. [7] Platt J. P. (1993) Terra Nova 5, p. 119 [8] Neubauer F. (1994) Geowissenschaften 12, p. 136. [9] Dallmeyer R. D., Handler R., Neubauer F., Fritz H. (1998) J. Geol. 106, p. 71 [10] Putiš M., Bezák V., Janák M., Kohút M., Kováčik M., Madarás J., Marko F., Plašienka D. (1995) Geological map of the western part of the Kreuzeck Massif, Eastern Alps. Austrian Geological Survey, scale 1:25 000, Vienna.

1248 [11] Putiš M., Bezák V., Janák M., Kohút M., Kováčik M., Madarás J., Marko F., Plašienka D. (1996) Geological map of the central part of the Kreuzeck Massif, Eastern Alps. Austrian Geological Survey, scale 1:25 000, Vienna. [12] Putiš M., Bezák V., Kohút M., Kovácik M., Marko F., Plašienka D. (1997) Geological map of the eastern part of the Kreuzeck Massif, Eastern Alps. Austrian Geological Survey, scale 1:25 000, Vienna. [13] Hoke L., (1990) Jb. Geol. B.-A. 133, p. 5 [14] Konzett J., Hoinkes G., Hauzenberger Ch., Krenn K., Repolust T., Whitehouse M., (2009) 8th International Eclogite Conference, Abstracts of 8th International Eclogite Conference, Xining, Qinghai Province, China, p. 73 [15] Ravna E. J. K., (2000) J. Metamorph. Geol. 18, p. 211 [16] Holland T. J. B. (1980) Amer. Mineral.,65, p. 129

1249