Ötztal-Stubai crystalline
The Ötztal-Stubai-Kristallin is a basement , the rock of which has undergone several transformations (polymetamorphosis). It is part of the Eastern Alpine ceiling system that was pushed over the Pennine geological units during the Alpine mountain formation . The Ötztal-Stubai crystalline shows various similarities with the Silvretta cover, especially the formation of the rocks and the structures point to a similar and related tectonic "starting position". The bulging of the Engadine window in the Tertiary then led to its dismantling.
Limits
The boundaries of this geological unit are of a tectonic nature and can be drawn as follows:
- Eastern border: The Ötztal-Stubai-Kristallin (ÖSK for short) is delimited in the east by the Brenner line , which is roughly followed by the Wipptal . This geological fault runs north-south and dips to the west. The lateral offset is approx. 15–26 km. This large displacement also forms the outer boundary to the Tauern window (Penninic) and the Innsbruck quartz phyllites.
- Northern border: In the north, the ÖSK is bordered by the Inntallinie towards the Northern Limestone Alps .
- Western border: The western border is formed by the Engadine line .
- Southern boundary: The southern boundary is tectonically more complex. In the south-west the crystalline unit is formed by the Schlinig line . Along this, the ÖSK was pushed over the Sesvenna ceiling . Further to the east, a kilometer-thick shear zone was formed ; in the southeast, a continuation of the fault is no longer traceable. Another possible topic could be the Passeier - and Jaufenlinie represent.
Petrography and structures
The basis of the ÖSK, which is characterized by multiple metamorphoses, consists mainly of medium to high metamorphic para gneiss , which were originally formed from clay or sandy material. There are also mica slate , quartzite , orthogneiss , amphibolite , eclogite and, rarely, marbles . Various tectonic styles appear in the ÖSK. The northern part is characterized by east-west inclined fold axes , inserted orthogneiss, which arose from igneous rocks , are oriented in the same direction. A similar orientation cannot be observed in the southern part, here spacious folds with a steep fold axis ( loop tectonics) and micro- folds in the millimeter range characterize the picture. Loop tectonics probably originated in an early phase of the Alpid mountain formation.
mineralogy
Garnet, amphibole and plagioclase show, in accordance with the polymetamorphic character of these rocks, a discontinuous zoning (Eo-Alpine cores, Alpine margins). The zoned Ca amphiboles show a progressive transition from a Mg-rich core to an Fe-rich rim, a general Fe ↔ Mg exchange is therefore to be assumed. A similar development can be seen in the feldspars: the nuclei of the zoned plagioclase are almost pure albite (Ab96-97), while the edges gradually become more anorthite (Ab80-67). The most noticeable zoning in the garnet is represented by the elements magnesium (Mg) and manganese (Mn). The grain edges are clearly enriched in Mg, the nuclei in Mn.
Metamorphic evolution
The oldest rocks of the ÖK were discovered by uranium / lead isotope analyzes on zirconia in paragneiss. These resulted in a considerable educational age of> 1500 Ma (Grauert, 1969). Since the entire area has undergone at least three metamorphoses and has thus been overprinted polymetamorphically, the zircons in the metamorphic rocks are only present in detritic form.
Three different metamorphic phases could be discovered in the Ötztal-Stubai-Kristallin:
Prevariscan metamorphosis
This metamorphosis can still be found in the various migmatites (Winnebach-Migmatit, Verpeilmigmatit and the Nauderer Gaisloch). They were formed in the biotite - plagioclase gneiss of the middle ÖSK under amphibolite-granulite facial conditions at a temperature between 660 and 685 ° C and a pressure of over 4 kbar. Dating of zirconia from the Winnebach migmatite shows a minimum age of anatexis of 490 million years (Klötzli-Chowanetz et al., 2001). Thöni et al. (2008) determined an age of 430-450 million years for the formation of the migmatites based on uranium-thorium-lead-monazite dating.
Variscan metamorphosis
This phase took place under eclogite to amphibolite facial conditions:
- Eclogite facies : Found in the metabasites and ultrametabasites of the central part of the ÖSK. The pressure was about 27 kbar at a temperature of 730 ° C. The age is 360-350 million years.
- Amphibolite facies : This metamorphic facies can be found in the widespread metapelites (aluminosilicates +/- staurolite). The distribution of the Al 2 SiO 5 modifications is decisive : sillimanite in the central part, kyanite in the northern and southern part, andalusite in a small area in the sillimanite zone. However, all three modifications can also occur here. The pressure was 7 kbar at a temperature of 650 ° C. The age is 343-331 million years (Thöni 1993).
Alpidian metamorphosis
This metamorphic event took place under green slate facial conditions in the east up to epidote / amphibolite / eclogite facial conditions in the southwest and led to the formation of early alpine societies along the NE-SW inclined isograde. Where the conditions of this metamorphic phase were weaker than the Variscan, one can observe a collapse and decay of the second: B. staurolite , chloritoid ; Staurolite, paragonite + chlorite . The highest pressure-temperature conditions of the event in the early days of the alpine mountain formation were reached in the southwest of the ÖSK. Rocks in this area have experienced amphibolite to eclogite facial temperatures and pressures. This led to the formation of staurolite and thistle in the metapelites. The typical socialization is thus: garnet + biotite + muscovite + plagioclase + quartz as well as more or less paragonite / staurolite / thistle.
- Epidote amphibolite facies: This metamorphic facies can be found in the Schneeberg complex : the pressure was five to six kilobars, the temperature around 600 ° C. This complex stretches over a width of about 5 kilometers from Sterzing to the Texel group .
- Eclogite facies: This facies area can be found south of the Schneeberg complex: the pressure during metamorphosis exceeded 11-12 kbar in this area; the temperature ranged between 500 and 550 ° C.
The early alpine overprinting naturally led to a rejuvenation of the Variscan age measured with rubidium / strontium and potassium / argon isotope analyzes. This rejuvenation can be seen very well in the terrain, in the northwest the overprinting was only weak, the Variscan ages were preserved; increasing early alpine pressure-temperature conditions are indicated with decreasing age of the rocks towards the southwest. In the area with the highest pressures and temperatures, the geological clock was then reset .
Cooling down and exposure of the ÖSK
The cooling of the ÖSK started immediately after the climax of the early Halpidic metamorphosis, ie about 90-100 million years ago, and ended after about 30 million years at surface temperatures. According to Elias (1998), the drop in temperature did not take place regularly, but with various peaks and times with almost no cooling. In the eastern part of the ÖSK, indications of exposure of the rock through erosion ( chalk / tertiary) could be found (Fügenschuh, 2000). The average rate of exposure of the rock was determined to be one millimeter per year.
Geothermobarometry
The thermobarometric calculations give an average temperature of 580 ° C and a pressure of 0.90 GPa for the metabasites and 600 ° C, and 1.10 GPa for the metapelite. Thermometry using trace elements (Zr in rutile) confirms the calculated temperatures of approx. 550 ° C.
literature
- Egon Bernabè: Petrological and thermobarometric investigations on the Pflersch metabasite complex (Pflerschtal, South Tyrol - Italy). University of Innsbruck, 2009
- The Eo-alpine metamorphic evolution of amphibolites from the southern ötztal complex (Pflersch Valley, South-Tyrol, Italy) - Communications of the Austrian Mineralogical Society, No. 153 (2007) (2009)
- Tropper, P. and Recheis, A. (2003): Communications of the Austrian Mineralogical Society, No. 94
Individual evidence
- ↑ Fridolin Purtscheller: Ötztal and Stubai Alps . Geological Guide Collection, Volume 53, Verlag Borntraeger, 2nd edition, Berlin - Stuttgart 1978, page 96.