The Early-Alpine metamorphism in the Austroalpine basement of the Saualpe

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1 The Early-Alpine metamorphism in the Austroalpine basement of the Saualpe Felicitas Kaiser 1 1 Hainichener Straße 4, Freiberg Abstract. In the southeastern Austroalpine units type locality eclogites and highpressure metapelites are known from the Saualpe domain. These high-pressure rocks resulted from the burial of the thinned passive margin of the Meliata backarc basin. Isotopic data derived from metabasic high-pressure mineral assemblages of eclogites infer subduction and related high-pressure metamorphism around 90 Ma. Subsequent decompression / exhumation immediately followed the high-pressure event (extending to 70 Ma) and is documented by garnet data from recrystallised micaschists at the final stage. Introduction The Saualpe crystalline complex is a part of the Austroalpine nappe system, the highest tectonic unit in the Alps. The highgrade polymetamorphic rocks exhibit an overprint that ranges from lowest greenschist- to higher amphibolite- to eclogitefacies grade. Wide-ranging coherent areas of the amphibolite- to eclogite-facies metamorphic Austroalpine basement are exposed both west and east of the Penninic Tauern Window (THÖNI and JAGOUTZ, 1993) that mark a high-pressure zone recording an Early-Alpine (Cretaceous) subduction zone (THÖNI, 1994) since most eclogites are subduction/continental collision-related high-pressure metabasic rocks (MILLER et al., 1987). New geochronological data from the southeastern parts of the Austroalpine basement nappes are reviewed for the type-locality eclogites from the Saualpe and their host rocks in order to constrain the timing of eclogite metamorphism and their geotectonic setting thus providing essential information on the geodynamics in orogenic belts (MILLER et al., 2005).

2 Felicitas Kaiser Geological setting As already mentioned the Saualpe crystalline series, an area of about 600 km 2, situated in SE Austria, Carinthia (PILGER and WEIßENBACH, 1975) and present the

2 2 Felicitas Kaiser Geological setting As already mentioned the Saualpe crystalline series, an area of about 600 km 2, situated in SE Austria, Carinthia (PILGER and WEIßENBACH, 1975) and present the southeastern part of the large Austroalpine nappe system (Fig.1),. This crystalline complex consists predominantly of highly metamorphosed gneisses, rich in mica, and micaschists including locally clustered eclogites, amphibolites, metacarbonates, pegmatites and orthogneisses. The eclogite- and related metabasite-intercalations are located in different levels of the Saualpe high-grade series and form pods and lenses in the outcrop-scale, though they might have separated from originally vast coherent bodies (MILLER et al., 1987, 1990). The variously deformed nappes of the Saualpe series are segmented into a NNW-SSE trending system of faults mainly by tertiary tectonic activity. Similar rocks are located in the neighbouring Koralpe region which experienced a comparable deformation and metamorphism. The basic eclogites mentioned above are well preserved and first described in the particular area of the Saualpe, therefore the Saualpe is the type-locality for eclogite (THÖNI and JAGOUTZ, 1991, 1993). Fig.1 Simplified geological map of the Eastern Alps, showing the Austroalpine basement units and eclogite occurrences. SAU Saualpe. Modified after THÖNI, The nappes structure of the Saualpe and its lithological units (after PILGER and WEIßENBACH, 1975) The rock-sequences exposed in the Saualpe region are affected by metamorphism and shearing which erased biostratigraphical markers and re-arranged the original lithostratigraphical positions. In the following (Fig.2) all primary, i.e. pre-meta-

3 Felicitas Kaiser morphic, facial and tectonic facts are disregarded. The purpose is rather to depict the actual crystalline sequence of the Saualpe in vertical and horizontal distribution.

3 3 Felicitas Kaiser morphic, facial and tectonic facts are disregarded. The purpose is rather to depict the actual crystalline sequence of the Saualpe in vertical and horizontal distribution. The Basement series of the Saualpe arise in the window of the crystalline Kliening, actually belonging to the Stubalpen crystalline. It is composed of gneisses, rich in mica, coarse micaschists, amphibolites as well as thin strata of marbles, calc silicate schists and quartzites. The rock-series, originating from the Phyllite Group of the Saualpe towards the footwall, are addressed to as High Cyrstalline. Regarding their protoliths they consist of metamorphic, sandy-clayish at least of psammitic-pelitic sediments that have been metamorphosed from the hanging wall to the footwall with increasing intensity of crystallisation. In the lower part of the High Crystalline they occur as foliated gneisses, in the upper part as micaschists. Accordingly, the High Crystalline can be discriminated from the footwall to the hanging wall into a Foliated Gneiss Group, Micaschist Group, respectively. The actual High Cyrstalline of the Saualpe ('catazone') begins with the Preims series. Marbles crop out in the lowermost layers which are underlain by finegrained and thin-bedded paragneisses, including the Plattengneiss. The superimposed rocks are formerly known as the Eclogite series. In the lower and upper part garnet-amphibolites, representing intercalated greenstone rocks, lie in transition to eclogites; eclogite itself is only found in the middle part. Increasing intensity downwards implicates retrograde metamorphism to amphibolites. In the Saualpe the rock-sequences of the Micaschist Group are dispersed over two spatially completely seperated areals. The northern area it is referred to as Plankogel series. The Micaschist Group ('mesozone') is composed of rocks which are in the metamorphic state of amphibolite facies occurring as micaschists, amphibolites and marbles. The Plankogel formation, constituting the deepest part of the Micaschist Group, comprises dark, clotty Fig.2 A compositional section through garnet-micaschists with garnets crystallised the High Crystalline of the Saualpe. in {211} oriented facets, quartzites and biotite-plagioclase-micaschists as well as Modified after PILGER and WEIßENBACH, Disthenflasergneiss whose most noticeable signs are small-grained, denselypacked as well as loose and streaky accumulations of kyanite that are pseudomor-

4 4 Felicitas Kaiser phoses replacing andalustite or staurolithe.the Micaschist Group is regarded as a repetition of the upper series in this higher metamorphised region of the Saualpe. Facies areas that were primarily in juxtaposition, today are found overthrusted, sheared and laminated due to tectonic evolution. The rocks of the Phyllite Group ('epizone') frame the Saualpe in the south and southeast. The grade of metamorphism decreases towards the hanging wall into regions of very low grade metamorphism ('anchizone'). All together the metamorphism is a gradual transition from kata- to mesozone ending in epi- and anchizone which is involved in a multistage process of crystallisation and deformation of the Saualpe. Isotopic constraints for high-pressure metamorphism in Austroalpine nappes The Saualpe region is the type-locality for a rock defined as eclogite, first described by the french mineralogist HAÜY in 1822 (THÖNI and JAGOUTZ, 1991). The spectacular Kupplerbrunn eclogite is composed of pink to dull red garnet, green omphacite ('green diallage'), abundant blue kyanite and also quartz, white to yellowish zoisite and dark amphibole (known as 'carinthine'). In general, eclogites are related to subduction/continental collision processes. Thus, reconstructing early evolution stages of an orogenic belt, additional data from analysing these high-pressure metamorphic rocks might be helpful (MILLER, 1990 MILLER et al., 2005, THÖNI, 2006). Derivation and evolution of the eclogite and metapelite protoliths The major eclogite minerals - garnet and omphacite - comprise rare-earth elements (REE), representing essential trace elements, from which Nd and Sm are separated. The Sm-Nd isotopic system on this metabasic high-pressure assemblage is said to bear a great potential for high-precision age dating, since the garnet structure strongly discriminates for Nd to Sm, resulting in Sm/Nd ratios applicable for geochronological purposes (THÖNI and MILLER, 1996). Nevertheless, the problems arise with interpreting the Sm-Nd data. In figure 3 (Fig.3) all Sm-Nd (WR = whole rock) data for the basic eclogites and metagabbros of the Saualpe basement are plotted (THÖNI and JAGOUTZ, 1993). The obvious low spread in Sm/Nd interdicts any time information to be determined for the eclogite protolith formation. However, an unaltered gabbro relic from the Koralpe at Bärofen, which is situated to the southeast of the Saualpe and also represents the Austroalpine basement, provides two Sm-Nd isochrons for plagioclase, clinopyroxene and whole rock of 275 ± 18 Ma and 216 ± 10 Ma, respectively (THÖNI and JAGOUTZ, 1993). These ages are interpreted to date the magmatic crystallisation of the eclogite protolith material.

5 5 Felicitas Kaiser Due to corresponding geochemical signatures of the metabasites, the Bärofen is set up as a provisional model for the eclogites of the whole study area, limiting the time for eclogite metamorphism (cf. the 275 Ma figure). Considering this presumption, the age of the metamorphic assemblages of all the eclogites has to be younger than 275 Ma, since their data points plot explicitly on the right hand site of the gabbro reference line. Another trend line constricts this upper time limit for the Early-Alpine pressure peak, i.e. the high-p maximum age of about 150 Ma, referring to Sm-Nd analysis of a gabbro-eclogite sample of the Koralpe, meaning the metamorphism is even younger than 150 Ma. The high-p minimum age of about 95 Ma brackets this Early-Alpine metamorphic evolution towards a lower time limit. Recent multi-isotopic studies on Sm-Nd, Lu-Hf and U-Pb methods including both Ky-free and Ky-bearing eclogites yield a much better time constraint for eclogite facies metamorphism at Kupplerbrunn in the Saualpe region (THÖNI, Fig.3 Isochrone plot of whole rock (WR) and mineral Sm-Nd data for eclogites and metagabbros from the Saualpe and Koralpe basement. Modified after THÖNI and JAGOUTZ, ). Ages between 94 and 88 Ma are acquired and suggest a short-lived high-pressure event. A rapid exhumation between Ma followed (THÖNI, 2006). These results are consistent with the 90 Ma Sm-Nd data for the garnets from the eclogite host rocks at Kupplerbrunn (cf. below). In contrast to previous models, these data infer an eclogite metamorphism related to the Alpine orogeny; memories of older eclogite events (pre-alpine) obviously do not exist (after THÖNI and JAGOUTZ, 1993)! Additional Sm-Nd analyses on garnets from the metapelitic host rocks of eclogite, shown in figure 4 (Fig.4), yield a corresponding age of 90 ± 3 Ma, dating the thermal peak that followed the Early-Alpine high-pressure event. The cooling of Early-Alpine metamorphism, some Ma ago, is deduced by THÖNI and JAGOUTZ (1993), according to Rb-Sr Fig.4 Summary of isotopic mineral ages from the central Saualpe area. Modified after THÖNI and JAGOUTZ, 1993.

6 6 Felicitas Kaiser biotite ages from the metapelitic host rocks. P-T estimates In geothermometry the Fe 2+ -Mg exchange between garnet and clinopyroxene can be applied to approach the temperature of eclogite formation. It indicates equilibration conditions near C and 18 kbar with reference to additional information on the equilibration pressure that were obtained from experimental work on the fields of stability of eclogite phases (MILLER, 1990). Peak metamorphic conditions at 90 Ma were close to 680 C and 20 kbar according to phengite geothermobarometry from eclogites, i.e. within the eclogite facies (THÖNI and JAGOUTZ, 1993). Summarising the information, gained by analysing and interpreting isotopic data from the Saualpe eclogites and their host rocks, a) the eclogite metamorphism in the classic eclogite type-locality of the Saualpe region is of Early-Alpine age; b) the upper time limit for peak pressures is at about 150 Ma, the lowermost time limit at about 90 Ma; c) the Early-Alpine pressure and temperature peak was reached close to 90 Ma; d) regional cooling below greenschist facies was terminated at about 70 to 60 Ma (THÖNI and JAGOUTZ, 1993). The micaschist host rocks from the central Saualpe (after a case study by THÖNI and JAGOUTZ, 1996) The samples of THÖNI s and JAGOUTZ study were taken in the central Saualpe, being part of the 'catazone' (cf. above) as metapelitic host rocks of the basic eclogites at their type-locality of Kupplerbrunn. The micaschists contain the assemblage muscovite + quartz + garnet + biotite + staurolithe + kyanite + plagioclase. P-T estimates Anhedral garnet grains, 0,1 to > 1 mm in size, containing inclusions of rutile, quartz, opaques and mica, generally show some degree of chemical zoning. Element profiles are shown in figure 5 (Fig. 5), marking decreasing MgO and increasing FeO towards the rims. This significant increase of Fe/Mg towards the rim, linked with an increase of Mn, is compatible with a minor amount of local retrograde re-equilibration. Therefore, garnet growth took place whilst rapid exhumation of the hot rock mass at the final stages of the high-pressure event, caused by decompression. The

7 7 Felicitas Kaiser partly re-equilibration occurred only at the outermost garnet rims with the adjacent matrix biotite. Thus, the P-T estimates for the terminal metamorphic stage of the Kupplerbrunn micaschists base on the intersection of the garnet-biotite Fe-Mg exchange equilibrium. Calculations with the equilibria of garnet-kyanite-plagioclasequartz record temperatures of about C and pressures of about 7 kbar for the final stage metamorphism. Due to very little amounts of muscovite, rutile and quartz in garnet cores, the geothermometer, based on the distribution of Fe 2+ and Mg between garnet and phengite, is applicable for recording an earlier stage of the Alpine cycle recorded Fig.5 Element profiles showing decrease of MgO and increase of FeO towards the rims. Modified after MILLER and THÖNI, by high pyrope contents in garnet cores and rare phengitic cores in white mica, yielding intersections of the equilibria at 685 C. Pressures of close to 20.4 kbar were calculated using a barometer that considers the Ti content of white mica in coexistence with quartz and rutile. Regarding these results it is obvious that the metapelitic host rocks of the Saualpe experienced a similar Early-Alpine high-pres sure history, compared to the eclogite (cf. above). Chronological estimates Based on Sm-Nd geochronology the Saualpe metapelite host rock material was analysed isotopically. The high Sm/Nd ratios for metapelite garnet give rather precise isochrons. The results are compatible with analytical errors, giving ages between 88.5 ± 1.7 and ± 0.7 Ma recording peak metamorphic conditions (garnet growth event) in the Saualpe. The whole rock (WR) data show strongly diverging, negative 0 values Nd which are typical for old crustal material enriched in LIL (large ion lithophile; such as Sr, Rb, K, Rb, Ba, Th) elements. They result in t Nd DM age for the protolith material (THÖNI and JAGOUTZ, 1993, 1996). ages between 1.4 and 1.9 Ga, pointing to a Proterozoic Discussion of the isotopic data Minding the intimately intercalated high-pressure assemblages (eclogites) and rocks partly (re)-crystallised at immensely lower pressures (eclogite host rocks = micaschists) initiates the issue of whether both these rock types have been subject-

8 8 Felicitas Kaiser ed to a common high-pressure evolution. The contrasting equilibration state might have been caused by varying dehydration/hydration and continuous deformation processes during pressure release, following immediately the high-pressure event. As THÖNI and JAGOUTZ (1993) stated, during unloading and as a consequence of a different hydration and ongoing deformation-induced recrystallisation, the metapelitic eclogite host rocks were able to re-equilibrate towards amphibolite facies conditions, due to the strong competency contrasts between metabasic eclogites and metapelites, therefore, the more resistant basic eclogites partly kept their high-pressure memories. The Early-Alpine subduction Fig.6 A palaeogeographic reconstruction for Late Jurassic times. W Vienna, G Genève. Modified after SCHMID et al., Between the two supercontinents Gondwana and Laurasia a wedgeshaped ocean, called Tethys developed from late Palaeozoic times onward. Several crustal slices of Gondwana began to rift towards north from the southern passive margin in order to be accreted to Laurasia. Belonging to frontal slices of the Adriatic- Apulian microplate, the Austroalpine nappes originally formed the very northern part of the African continent (THÖNI and JAGOUTZ, 1993). The metabasic rocks of the Saualpe bear the geochemical environment of their precursors. Most eclogite protoliths derive from a N-MORB-type source, proving production of juvenile oceanic crust material due to rifting processes during Permo-Triassic time (cf. Bärofen gabbro relic) and therefore preceding the breakup of Meliata Ocean (THÖNI and JAGOUTZ, 1993; JANÁK et al. 2005). Reasoning this, thinning of consolidated pre-alpine basement along a rift took place, as well as crustal fragmentation and nucleation of a young ocean (after THÖNI and JAGOUTZ, 1993). Regarding their findings, THÖNI and JAGOUTZ, among others, developed firstly a model where metamorphism inferred for the study area of the Saualpe is related to the closure of the Meliata-Hallstatt ocean (or the Vardar ophiolite belt) (Fig.6) up to depths of km (MILLER, 1990). Its closure commenced in the Upper Jurassic and was completed during the Cretaceous (THÖNI and JAGOUTZ, 1993; THÖNI and MILLER, 1996). That subduction process caused the accretion of the Austroalpine domain to Europe (MILLER et al., 2005). The Bärofen gabbro could be part of the Vardar ophiolite belt as detrital chromian spinel was observed in the Lower Cretaceous series within the Northern Calcareous Alps that were fed essentially from source areas south of the Austroalpine region (according to sedimentological and palaeo-

9 9 Felicitas Kaiser current studies plus analyses of composition and distribution of heavy mineral spectra). Thus, the ophiolitic sequences could have been exposed for erosion along the southern margin of the Austroalpine domain some 100 Ma ago (THÖNI and JAGOUTZ, 1993). Secondly, the above authors stated that the high-pressure assemblages crystallised during this event are estimated to be younger than roughly 88 Ma. The metapelite data (cf. Fig.4) represent a fairly consistent chronological pattern demonstrating that the onset of regional cooling to less than 300 C was close to 90 Ma and lasted some 10 to 20 Ma (THÖNI and JAGOUTZ, 1993). Thus, the 90 Ma figure marks an interuption of prograde metamorphism. Geodynamically interpreted, that date refers to initial stacking of crustal fragments in the southeastern Austroalpine domain causing effective obduction of the eclogite facies rocks by tectonic underplating along the northern Adriatic continental margin which lead to rapid uplift and exhumation (THÖNI and JAGOUTZ, 1993). Conclusions The isotopic signatures for gabbroid rocks clearly indicate that the precursor rocks originated from a depleted mantle material close to the southeastern border of the Austroalpine basement in Early Permian times (THÖNI and JAGOUTZ, 1993; MILLER et al., 2005). A N-MORB type reservoir is likely to have been produced along a narrow rift zone, i.e. a branch of the western Tethys. Though a word of caution is required, as the N-MORB chemistry doest not necessarily reflect production at a MOR; it could document magmatism within an ocean-continent transition zone or in a back-arc setting as well (MILLER et al., 2005). Due to the closure of an ocean during the Late Jurassic to Early Cretaceous times subduction processes were initiated causing high-pressure metamorphism of the dislocated oceanic and continental basement slices (THÖNI and JAGOUTZ, 1993). The exact age of the peak-pressure conditions of this Early-Alpine subduction event in the SE Austroalpine units is estimated to be as young as 90 ± 3 Ma. Strong fractionation of Sm to Nd in garnets of the metapelites crystallised at this stage was utilised. In any case, it is younger than 150 Ma (THÖNI and JAGOUTZ, 1993). Hence, the 90 Ma date can be interpreted as the onset and therefore an uppermost limit for decompression during an early exhumation in the Late Cretaceous (MILLER et al., 2005). This Early-Alpine metamorphism of eclogite to amphibolite facies grade affected the Saualpe basement units. Though geothermometry of eclogites is not precise enough yet to uniquely define pressure and temperatures, overall P-T conditions are estimated around 20 kbar / 680 C for the peak and 7 kbar / C for the terminal stage of metamorphism (THÖNI and MILLER, 1996; MILLER et al., 2005; MILLER and KONZETT, 2005). The thermal peak was most likely reached during a documented decompressional event. The final stage of cooling within the

10 10 Felicitas Kaiser Austroalpine nappes down to 300 C was achieved in the Late Cretaceous, i.e. about 70 ± 10 Ma ago. The present tectonic situation, i.e. the nappe structure in its latest stage (cf. Fig1), developed after the high-pressure metamorphic event. References JANÁK, M., et al. (2005): First evidence for ultrahigh-pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep con- tinental subduction in the Eastern Alps. Tectonics, vol. 23,no.5 MILLER, C. (1990): Petrology of the type locality eclogites from the Koralpe and Saualpe (Eastern Alps), Austria. Schweizerische Mineralogische und Petro graphische Mitteilungen, vol. 70, pp MILLER, C. and J. KONZETT (2005): Comment on First evidence for ultrahigh-pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the Eastern Alps by Marian Janák et al.. Tectonics, vol. 24,TC6010 MILLER, C., et al. (1987): Geochemistry and origin of eclogites from the type locality Koralpe and Saualpe, Eastern Alps, Austria. Chemical Geology, vol. 67, no. 1-2, pp MILLER, C., et al. (2005): Refining the timing of eclogite metamorphism: a geoc hemical, petrological, Sm-Nd and U-Pb case study from the Pohorje Mountains, Slovenia (Eastern Alps). Contributions to Mineralogy and Petrology, vol. 150, no. 1, pp PILGER, A. and N. WEIßENBACH (1975): Geologie der Saualpe. Clausthaler Geologische Abhandlungen. SCHUSTER, R. and W.KURZ (2005): Eclogites in the Eastern Alps: highpressure metamorphism in the context of the Alpine orogeny. Mitteilungen der Österreichischen Mineralogischen Gesellschaft, vol. 150, pp THÖNI, M. (1994): Chronological constraints for Variscan vs Alpine eclogite fa cies metamorphism in the basement units of the Eastern Alps. Mineralogical Magazine, vol. 58A, no. L-Z, pp THÖNI, M. (2006): Dating eclogite-facies metamorphism in the Eastern Alps approaches, results, interpretations: a review. Mineralogy and Petrology, vol. 88, pp DOI /S THÖNI, M. and E. JAGOUTZ (1991): Some new aspects of dating orogenic belts: Sm-Nd, Rb-Sr, and Pb-Pb isotopic results from the Austroalpine Saualpe and Koralpe type locality (Carinthia/Styria, southeasternaustria). Geochimica et Cosmochimica Acta, vol. 56, pp THÖNI, M. and E. JAGOUTZ (1993): Isotopic constraints for eo-alpine high-p metamorphism in the Austroalpine nappes of the Eastern Alps: bear ing on Alpine orogenesis. Schweizerische Mineralogische und Petrographische Mitteilungen, vol. 73 (2), pp THÖNI, M. and C. MILLER (1996): Garnet Sm-Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P-T constraints on the thermal and tectonic history. Journal of metamorphic geology, vol. 14, pp

Metamorphic fluids, Naxos, Greece

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