Badcallian

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The Badcallian was an orogenetic phase that deformed and metamorphosed the Lewisian rocks at the beginning of the Neo-Archean Era about 2760 million years ago in the Hebridean Terran .

Etymology and first description

The name Badcallian is derived from its eponymous type locality Badcall , a small fishing village near Scourie on the north-west coast of Scotland . Scientifically, the term was first used by Park in 1970.

characterization

View from Handa to Scourie and the type region. In the background the Quinag

In the basement of the Hebridean Terran, the Badcallian is the oldest deformation phase. The original rocks of the Scourian , predominantly tonalites , which had formed in a subduction zone between 3030 and 2960 million years BP , were isoclinally folded and sheared, so that flat ribbon gneiss emerged (gneissification). However, there are no pronounced lineations . Between Scourie and Gruinard Bay , the accompanying metamorphosis reached the granulite facies and anhydrous mineral paragenesis of the orthopyroxene - quartz - feldspar type formed . The temperatures ranged between 950 and 1000 ° C during the metamorphosis and the pressures fluctuated between 11 and 15 kilobars or 1.1 to 1.5 gigapascals . These pT conditions prevail in the lower crust at depths between 35 and 50 kilometers. (Due to the ultra high temperatures engl. Ultra-High Temperatures or abbreviated UHT ) during the metamorphosis was places partial melting realized.

Dating

A radiometric dating using the Pb -Pb method on monazite inclusions in early entstandenem grenade - a reference to high-grade metamorphic conditions - yielded 2.76 billion years BP. Corfu et al. (1994) found BPs with the micro-sample to be 2710 million years old. However, ages around 2500 million years BP for the Badcallian were also determined. Which of these two age groups (around 2,700 million years BP or around 2,500 million years BP) ultimately applies to the high-grade metamorphosis is still controversial. It is possible that two granulite facial events occurred one after the other (but this is not in line with the terrain findings) or staggered due to the terrain.

causes

Due to the delimitation of the outcrop area, the tectonic position of Badcallian is not clear. One cause of this tectono-metamorphic event could be the collision of this crustal area with a volcanic arc terran , which occurred at a subduction zone.

Individual evidence

  1. ^ Park, RG: Observations on Lewisian chronology . In: Scot. J. Geol. Volume 6 (4) , 1970, pp. 379-399 .
  2. ^ A b Nigel Woodcock and Neil Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, 2000, ISBN 0-632-03656-7 .
  3. ↑ MacDonald, John M. et al.: Temperature-time evolution of the Assynt Terrane of the Lewisian Gneiss Complex of Northwest Scotland from zircon U-Pb dating and tithermometry . In: Precambrian Research . tape 260 , 2015, p. 55-75 .
  4. ^ Rollinson, H., Gravestock, P .: The trace element geochemistry of clinopyrox-enes from pyroxenites in the Lewisian of NW Scotland: insights into light rare earth element mobility during granulite-facies metamorphism . In: Contrib. Mineral. Petrol. tape 163 , 2012, p. 319-335 .
  5. ↑ Corfu, F., Heaman, LM, Rogers, G .: 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 (3) , 1994, pp. 215-228 .
  6. ↑ Crowley, QG, Key, R., Noble, SR: High-precision U – Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach . In: Gondwana Res. 2014.
  7. ^ Park, RJ and Tarney, J .: The Lewisian complex: a typical Precambrian high-grade terrain? In: Evolution of the Lewisian and Comparable Precambrian High Grade Terrains, Special Publication . tape 27 . Geological Society, London 1987, pp. 13-25 .