Inverian

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The Inverian was an orogenetic phase that deformed and metamorphosed the Lewisian rocks in the Hebridean Terran at the beginning of the Paleoproterozoic Era about 2,490 million years ago .

etymology

The term Inverian is derived from its type locality Lochinver on the northwest coast of Scotland .

characterization

Lochinver with Suilven

In the basement of the Hebridean Terran, the Inverian is the second deformation phase. It followed the dry, granulite- faced Badcallian , but was closed before the mafic Scourie dykes entered . The Inverian was less intense than the Badcallian event and only the pressure-temperature conditions of the amphibolite facies were reached. It is a static, water-containing retromorphosis .

For the temperatures achieved during the retromorphosis, Zirkler et al. (2012) by means of equilibrium modeling of mineral paragenesis 520 to 550 ° C. Sills (1983) gave a slightly higher estimate of around 600 ° C.

Dynamically, the Inverian is characterized by broad, relatively steeply dipping, ductile shear zones in the kilometer range that shift diagonally to the right . Examples are the Canisp shear zone , the Gruinard shear zone and the Diabaig shear zone .

At the same time, planar and linear structures and folds were formed . Water penetrated into the shear zones, as a result of which pyroxenes were converted into water-bearing mineral phases such as biotite , hornblende , epidote and others.

Dating

At the beginning of the Paleoproterozoic, ages obtained from zirconia using the uranium-lead method indicate a metamorphosis that is around 2500 million years old, which the Inverian is likely to represent. At Badcall Point, Corfu et al. (1994) found an age of 2,490 million years BP using the same radiometric method. MacDonald et al. (2015) found 2482 million years BP. The deformation of pegmatites and microgranites that penetrated the basement after the Badcallian, dated to the period 2480 to 2420 million years BP , can probably be attributed to the Inverian. The penetration of the later Scourie dykes is dated to the interval 2418 to 2375 million years BP.

root cause

A repeated oblique collision of crustal blocks of the Hebridean Terran is believed to be the cause of the formation of the shear zones of the Inverian. A study by Goodenough et al. (2013) found that the two crustal blocks of the Rhiconich Terran and Assynt Terran along the Laxford Shear Zone docked with each other during the Inverian around 2,480 million years BP.

Individual evidence

  1. ^ A. Zirkler, TE Johnson, RW White, T. Zack: Polymetamorphism in themainland Lewisian complex, NW Scotland - phase equilibria and geochronological constraints from the Cnoc an t'Sidhean suite . In: J. Metamorph. Geol. Band 30 , no. 8 , 2012, p. 865-885 .
  2. ^ JD Sills: Mineralogical changes occurring during the retrogression of Archean gneisses from the Lewisian complex of NW Scotland . In: Lithos . tape 16 , no. 2 , 1983, p. 113-124 .
  3. ^ P. Attfield: The structural history of the Canisp shear zone . In: RG Park, J. Tarney (Ed.): Evolution of the Lewisian and Comparable Precambrian High-Grade Terrains . 1987.
  4. ^ A b Nigel Woodcock, Neil Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, 2000, ISBN 0-632-03656-7 .
  5. ↑ F. Corfu, LM Heaman, G. Rogers: Polymetamorphic evolution of the Lewisian Complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile . In: Contrib. Mineral. Petrol. tape 117 , no. 3 , 1994, p. 215-228 .
  6. ↑ John M. MacDonald, inter alia: Temperature-time evolution of the Assynt Terrane of the Lewisian Gneiss Complex of Northwest Scotland from zircon U-Pb dating and Ti thermometry . In: Precambrian Research . tape 260 , 2015, p. 55-75 .
  7. ↑ JHFL Davies, L. Heaman: New U-Pb baddeleyite and zircon ages for the Scourie dyke swarm: a long-lived large igneous province with implications for the Paleoproterozoic evolution of NW Scotland . In: Precambrian Res. Band 249 , 2014, pp. 180-198 .
  8. ↑ KM Goodenough, Q. Crowley, M. Krabbendam, SF Parry: New U-Pb ageconstraints for the Laxford Shear Zone, NW Scotland: evidence for tectono-magmatic processes associated with the formation of a Palaeoproterozoic supercontinent . In: Precambrian Res. Band 223 , 2013, p. 1-19 .