Friedrich Koller^* 1, Volker Höck ²

The Penninic realm, widely distributed in the Western and Central Alps can be followed along a series of windows across the whole range of the Eastern Alps. These are, from the W to the E the Lower Engadin Window (LEW), the Tauern Window (TW) and a group of small windows at the eastern margin of the Alps called the Rechnitz Window Group (RWG). Pre-Mesozoic rocks are restricted to the TW and to small fragments at the basement of the Tasna nappe in the LEW. Mesozoic rocks, such as Triassic quarzites, marbles and dolomites as well as Jurassic and Cretaceous phyllites, micaschists, calcmicaschists and other metasediments, ophiolites and non-ophiolitic volcanics occur throughout all Penninic windows.

Lower Engadine Window

The HP/LT event is relatively poorly documented. The remnants of this event can be found mainly in metabasites and they are documented by preserved lawsonite from several localities (Leimser and Purtscheller, 1980), by rare occurrences of blue amphiboles with compositions ranging from glaucophane to crossite (Oberhänsli et al., 1995; Leimser and Purtscheller, 1980), by widespread occurrences of pumpellyite (Koller and Höck, 1990). Further by preserved Fe-Mg-carpholite in metasediments below the Piz Mundin ophiolite (Oberhänsli et al., 1995) and high pressure phengites (Si = 3.1-3.45 pfu) from various metasediments (Stöckhert et al., 1990). So far relative few PT data exist for the conditions of this event. All of them range around 300-350 °C and 3 kbar (Koller et al., 1996). Only for the Piz Mundin area pressures are estimated at 6 kbar based on the carpholite composition (Oberhänsli, 1994).

Tauern Window

Apart from the pre-Mesozoic metamorphism three episodes of metamorphic events were recognized: an eclogite event, a blueschist metamorphism, and the final greenschist to amphibolite facies metamorphism called "Tauernkristallisation". The eclogitisation affects only a relatively small strip mainly at the southern escarpment of the TW, the blueschist metamorphism is more widely distributed but restricted to the ophiolites, their immediate cover and the areas tectonically below. Minerals formed in the blueschist event survived only rarely. The most conspicuous relicts from this stage are pseudomorphs after lawsonite reported from various lithologies within the TW. Some jadeite poor omphacites coexisting with albite rich plagicolase and occasionally blue amphiboles such as glaucophane and/or crossite are preserved and probably barroisitic amphiboles. Associated with this stage and possibly also with the eclogite event are high Si phengite with Si 3.30 - 3.40 pfu. In extreme cases the Si content may reach 3,65 pfu (Zimmermann et al., 1994). Frank et al. (1987) estimated the conditions of blueschist formation as T = 400-450 °C and P around 9 kbar. Zimmermann et al. (1994) calculated 10 kbar at 400 °C.

Rechnitz Window group

Within the ophiolitic sequence remnants of a LT/HP event with 330-370°C and 6 kbar is widespread (Koller, 1985). Typical minerals are alkali pyroxenes, glaucophane or crossite, rare pseudomorphs of lawsonite, high Si-phengite (up to 3.5 pfu), Mg-rich pumpellyite or epidote, stilpnomelane, hematite and rutile. Only few data from the sedimentary rocks exist.

Lower Austroalpine Nappe

The Lower Austroalpine Nappe covers in the NW (Tarntal mountains) and NE (Radstädter Tauern) the Penninic rocks of the Tauern Window. In the S this complex is represented by the zone of Matrei. The Mesozoic Hippold and Reckner nappe are built up by a huge variety of partly fossil bearing sediment sequences from Scythian to Malm ages (Enzenberg-Prähauser, 1976). The top of the Reckner nappe is formed by serpentinites and blueschists of the Reckner complex. For both, Reckner and Hippold nappe a Tertiary HP/LT event in the range of ~350 °C and ~10 kbar has been reported by Dingeldey et al. (1997). Typical minerals of this event are alkali pyroxenes (<Jd₃₇Ac₅₀), Mg-rich pumpellyite, stilpnomelane, and high Si phengite (<3.65). The to the Hippold nappe adjoined Penninic metasediments exhibit only intermediate pressure conditions (6-7 kbar).

Conclusion

Two metamorphic events are recognizable in all windows and in a restricted part of the Lower Austroalpine nappe where the older is regarded as a HP/LT metamorphism and the younger of Barrovian type (Hoinkes et al., 1999). Only in the TW an earlier eclogite metamorphism is recorded with an retrograde evolution path entirely different from the rest of the Penninic metamorphics. The blueschist event in the Eastern Alps is not well constraint in respect to age dating. Only for the TW the data by Zimmermann et al. (1994) suggest an age of late Eocene – Early Oligocene for the blueschist assemblages. Furthermore the Lower Austroalpine Reckner and Hippold nappe suggest also a Tertiary age for the blueschist event (Dingeldey et al., 1997). The HP/LT remnants in the LEW and RWG record somewhat lower pressure and lower temperature compared with the TW but indicate also an subduction zone event. In the LEW mostly the deeper parts (North Penninic metasediments) are metamorphosed in blueschist facies In the RWG the ophiolites indicate clearly the blueschist event, the metasediments below and above the ophiolites are not well investigated. Therefore, the question whether the blueschist event is synchronous throughout the Penninic realm in the Eastern Alps and its range is an unsolved problem. The various types, similarities, equivalences, and differences within typical profiles as well as the style of LT/HP rocks of the Penninic zone and of the Lower Austroalpine unit will be discussed in detail.