Ehrwaldite

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The Ehrwaldit is an extremely rare, primitive, alkaline ultrabasic gangue of the Northern Limestone Alps , which the lamprophyres is expected. Its geochemical composition is that of a nepheline basanite . Its age is around 100 million years BP , the rock thus intruded in the Upper Albium on the Lower Cretaceous / Upper Cretaceous border.

Etymology and first description

Ehrwaldite is named after its type locality near Ehrwald in Tyrol . The gangue was first scientifically described by Adolf Pichler in 1866, who then gave it its current name in 1875.

Discovery and occurrence

Ehrwaldite was discovered on August 28, 1865 by Adolf Pichler in the Lehnbachgraben at around 1700 meters above sea level to the west below the Schneeferner head . In addition to the type locality, which is also known as the so-called torrent stage , other occurrences are known south of Imst , south of the Zugspitze (and especially south of the Wettererschrofen ) in the Puitental zone ( young layers zone ) and east of the Birkkarspitze . All of these occurrences belong tectonically to the Lechtal Nappe . The east-west extension of the deposits is at least 50 kilometers. Strikingly, they are all not far from the thrust front of the Inntal Nappe .

description

The black, augit porphyry-like Ehrwaldit tunnels are between 1 and 2 meters wide. The rock appears lamprophyric and resembles a monchiquitic melaphyre. The dikes are mostly concordant to the stratification of the intruded Mesozoic sediments, but can also be discordant. The sediments were metamorphically changed by the intrusion - mostly no more than 1 to 2 centimeters, but locally also up to 1 meter. At the type locality the Ehrwaldite intruded limestone and cherts of the Malm (upper Jura), in the Karwendel Triassic to Jura and in the southern Wetterstein marls and siltstones of the Lower Cretaceous . Stratigraphically, the maximum age of the Ehrwaldite is therefore Lower Cretaceous. After their penetration, the corridors were deformed brittle in the course of the mountain formation of the Alps (recognizable by fractures provided with armor ), but they were not subject to any regional metamorphosis. The host sediments were not heated to more than 130 ° C ( diagenesis range ), recognizable by the illite crystallinity and the vitinite reflection ( degree of coalification ), the R max of which is 0.7.

Petrology

Petrography

The Ehrwaldite gangue consists of up to 30% of a fine-grained, devitrified base, which makes a clear petrological classification difficult. As phenocrysts act olivine (often serpentinized), clinopyroxene and aluminous spinel . Foreign crystals (xenocrystals) and foreign rock inclusions (xenolites) are present in abundance with 5 to 10%. The semi-crystalline matrix is ​​dominated by augite prisms (occasionally arranged radially), usually serpentinized olivines of the composition oil 87-91 , kaersutite ( amphibole ) and biotite, and subordinate apatite and titanomagnetite . The devitrified part of the matrix contains micro- to cryptocrystalline, radial- rayed chlorite , gusset-filling analcime and rare natrolite . Sometimes magnetite , alkali feldspar , biotite and zeolite are also present in the matrix . The amygdals in almond varieties are filled with zeolites such as analcime, natrolite and heulandite as well as with calcite and chlorite.

Geochemical composition

In terms of their normative rock composition , the Ehrwaldite dikes are Nepehelin normative with a nepheline content of 11 to 17 percent by weight and olivine normative with an olivine content of 13 to 17 percent by weight (however, nepheline does not occur in the actual mineral stock). Their magnesium number is relatively high and ranges between 74 and 78. Ehrwaldites are rocks with an emphasis on sodium and a Na 2 O / K 2 O ratio of 1.7 to 3.2. With an SiO 2 content of 38 to 41 percent by weight, they can be classified as typical nepheline basanites.

The following geochemical analyzes from the Lehnbachgraben should illustrate its composition:

Oxide
wt.%		 Lehnbachgraben
EJ 6		 Lehnbachgraben
EJ 3		 Lehnbachgraben
EJ 10 - rich in almonds		 Trace elements
ppm		 EJ 6 EJ 3 EJ 10
SiO 2 38.20 39.25 40.75 F. 1351 1044 1406
TiO 2 3.19 3.20 3.58 Ba 614 650 725
Al 2 O 3 11.14 11.90 13.90 Rb 20th 23 26th
Fe 2 O 3 5.07 3.49 5.80 Sr 757 997 871
FeO 6.25 7.77 5.71 Nb 79 89 105
MnO 0.20 0.19 0.17 La 60 64 86
MgO 12.64 12.44 8.17 Ce 111 123 136
CaO 12.99 12.75 10.76 Zr 306 318 326
Na 2 O 1.70 2.30 3.80 V 326 337 319
K 2 O 0.98 0.93 1.32 Cr 446 434 119
P 2 O 5 0.86 0.77 0.88 Ni 295 325 123
H 2 O + 5.59 4.29 4.03 Cu 67 56 36
CO 2 0.33 0.27 0.16 Zn 104 102 104

In the case of trace elements , the high concentrations of chromium (up to 450 ppm) and nickel (up to 325 ppm) confirm the primitive character of Ehrwaldite. They are also enriched in (compared to the mineralogy of mantle rocks) incompatible elements such as barium (600 to 725 ppm) and strontium (760 to 1050 ppm). The light rare earths (LREE) as well as niobium (79 to 105 ppm), vanadium (312 to 340 ppm) and zirconium (295 to 326 ppm) also have increased values . Compatible elements such as the heavy rare earths (HREE) and yttrium (16 to 23 ppm), on the other hand, are low in concentration.

The distribution pattern of the Ehrwaldite rare earths shows an excellent match with other basanites and olivine nephelinites in Europe. There are also great similarities to the olivine nephelinites and melilithites of south-east Australia and the leucitites of New South Wales , whereby the magmatites of south-east Australia are practically identical to the nephelinites and melilithites of Hawaii , which show a very narrow distribution curve .

Similar to the Ehrwaldites, basanites of the same time can also be found in the Krížna Nappe of the Western Carpathians ( Stražov Mountains ). It is also worth mentioning that Teschenites penetrated almost simultaneously (end of Aptian ) in the Silesian of the Northern Carpathians and in the Arosa zone in the southern Penninic of the Rätikon .

Isotope geochemistry

Sr-Nd isotope diagram of the Ehrwaldites. Noteworthy is their close relationship to the Basanites of the Massif Central, Melilithites of the Hegaus, as well as to the Basanites of the Siebengebirge and the Kaiserstuhl.

The following isotope ratios of Ehrwaldites are known:

  • 87 Sr / 86 Sr: 0.703313 to 0.703543 or ε Sr = - 12.4 to - 3.3
  • 143 Nd / 144 Nd: 0.512783 to 0.512847 or ε Nd = + 3.9 to + 5.2

They are therefore in or in the immediate vicinity of the mantle array , which comprises magmatites such as MORB and OIB formed from mantle rocks . It is therefore not surprising that the oceanic island basalts of Samoas , the Marquesas , the Society Islands , the Azores and the Kerguelen have very similar Nd-Sr isotope ratios. In addition, also joined by the flood basalts Dekkans (Ambenali series) and the Columbia River .

The Nd-Sr isotope ratios of the Ehrwaldites are quite comparable with other Basanites of the Central European Volcanic Province , especially with the Basanites of the French Massif Central , the Siebengebirge and the Kaiserstuhl . The Melilithites of the Hegau also reveal very similar isotope ratios. The basanites of the Vulkaneifel are also in the mantle array, but have significantly lower ε Nd values.

The subcontinental mantle below the northern Limestone Alps is thus practically identical to other regions in Central and Western Europe and has changed only slightly in its composition over the past 100 million years. In contrast to this are the alpine ophiolites - remnants of the Penninic oceanic crust - whose isotope ratios reveal a typically depleted sub-oceanic mantle.

Educational conditions

Ehrwaldite, like basanite, is a magma of very low viscosity that, according to melting experiments, was formed at a depth of around 80 kilometers and at a temperature of 1250 ° C in the subcontinental mantle. Its parent rock was lherzolite (pyrolite), which secreted the Ehrwaldite magma with a melting rate of 4 to 7% and an enrichment factor of 7 to 9 for LREE and 2 to 5 for HREE. In the course of crystallization, olivine, clinopyroxene and spinel were fractionated.

Dating

Trommsdorff (1990) examined four samples (three from the Lehnbachgraben and one from the southern edge of the Wetterstein) for their age using the potassium-argon method . The samples varied between 98.8 and 100.1 million years BP with a mean value of 99.4 million years BP. The associated very well-defined isochrones (regression line) resulted in 102 ± 2 million years BP. The Ehrwaldit Gang had penetrated towards the end of the Albium directly at the turn of the Lower Cretaceous / Upper Cretaceous.

meaning

The formation of the Ehrwaldite dikes must have taken place in the distensive area (with crust expansion), probably in a rift or rift zone. This assumption is corroborated by the analogous nepheline basanites of the neo-genic Central European volcanic province, all of which originated along expansion zones : Limagne Graben in the case of the Massif Central, Upper Rhine Graben in the case of the Kaiserstuhl and the Hegau (intersection of two fracture zones). Furthermore, this confirms very similar isotope ratios in the rift rocks of East Africa and the Rio Grande .

Alexander Tollmann's assumption that a subduction zone (with compression tectonics) already existed at the time under the deposit area of ​​the rocks of the Lech Valley blanket is therefore questionable. At this point in time, ceiling systems in the area of ​​the current Lechtal ceiling were probably not yet in place. However , it cannot be ruled out that further north in the area of ​​the current Bajuvarikum or Helvetikum subduction was already underway.

Individual evidence

  1. ^ Pichler, A .: Contributions to the Geognosie of Tyrol . In: Jb. D. KK Reichsanstalt Vienna . tape 16, 4 , 1866, pp. 501-504 .
  2. ^ Pichler, A .: Contributions to the Geognosie of Tyrol . In: New Jb. Mineral. Geol. Palaeont. Born in 1875, p. 926-936 .
  3. ↑ Ammpferer, O .: Over the southern edge of the Lechtal Alps between Arlberg and Ötztal . In: Yearbook of the Federal Geological Institute Vienna . tape 80 , 1930, pp. 438 .
  4. ↑ Volkmar Trommsdorff et al: Mid-Cretaceous alkaline magmatism in the Northern Calcareous Alps . In: Geologische Rundschau . tape 79/1 , 1990, pp. 85-97 .
  5. ↑ Krumm, H. et al .: From diagenesis to anchimetamorphism, upper Austroalpine sedimentary cover in Bavaria and Tyrol . In: Geodis. Acta . tape 2, 1 , 1988, pp. 33-47 .
  6. ↑ Trommsdorff, V .: About lamprophyre from the northern limestone Alps (Ehrwaldit) . In: Tscherm. mineral. petrol. Mitt. Band 8.2 , 1962, pp. 281-325 .
  7. ↑ Edgar, AD: The genesis of alkaline magmas with emphasis on their source regions: inferences from experimental studies . In: Fitton, JG and Upton, BGJ Alkaline Igneous Rocks (eds.): Geol. Soc. Spec. Publ.-Volume = No. 30 . 1987, p. 29-52 .
  8. ↑ Wedepohl, KH: Origin of the Tertiary basaltic volcanism in the northern Hessian depression . In: Contrib. Mineral. Petrol. tape 89, 2/3 , 1985, pp. 122-143 .
  9. ↑ a b Frey, FA et al .: Integrated models of basalt petrogenesis: A study of quartz tholeiites to olivine melilitites from soth eastern Australia utilizing geochemical and experimental petrological data . In: Journal of Petrology . tape 19, 3 , 1978, pp. 463-513 .
  10. ↑ Kay, RW and Gast, PW: The rare earth content and origin of alkali rich basalts . In: Journal of Geology . tape 81 , 1973, pp. 653-682 .
  11. ↑ Hovorka, D. and Spišiak, J .: Mesozoic volcanism in the western Carpathian section of the Tethys: Differences in space and time . In: Jb. Geol. B.-A. tape 136 . Vienna 1993, p. 769-782 .
  12. ↑ Chauvel, C. and Jahn, BM: Nd-Sr isotope and REE geochemistry of alkali basalts from the Massif Central, France . In: Geochim. Cosmochim. Acta . tape 48 , 1984, pp. 93-110 .
  13. ↑ Stille, P. et al .: Nd isotopic composition of Jurassic Tethys seawater and the genesis of alpine Mn-deposits: evidence from Sr-Nd isotopic data . In: Geochim. Cosmochim. Acta . tape 53 , 1989, pp. 5 .
  14. ↑ Ulmer, P. et al.: The genesis of Cretaceous basanites from the Calcareous Alps (Austria): Experimental, Geochemical and Field Constraints . In: IAVCEI Abs. New Mexico Bureau of Mines and Mineral Resources Bull. Volume 131 , 1989, pp. 274 .
  15. ^ Tollmann, A .: The Alpidic evolution of the Eastern Alps . In: Flügel, H. and Faupl, W. (Eds.): Geodynamics of the Eastern Alps . Deutike, Vienna 1987, p. 361-378 .