Laxfordian

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The Laxfordian was a tectonothermal event that deformed and metamorphosed the polymetamorphic basement of the Hebridean Terran in the Paleoproterozoic .

Etymology and first description

The Loch Laxford type locality

The Laxfordian derives from hole Laxford , his eponymous, on the northwest coast of Scotland located type locality from. The term was first introduced into the literature in 1951 by Sutton and Watson.

introduction

The period 2000 to 1600 million years BP was characterized worldwide by a convergence of continental blocks and the development of active continental margins . At the end of the Paleoproterozoic, a forced growth of the continental margins occurred through orogenes . This orogenic activity is represented in north-west Scotland by the Laxfordian. When the individual terran blocks, consisting of archaic gneisses of the Lewisian , in particular the Rhiconich, Assynt and Gruinard terran, came closer together, deformation and metamorphosis of the basement occurred .

characterization

The Laxfordian occurred after the Scourie dykes invaded . The metamorphosis generally reached the grade of the amphibolite facies , on the Outer Hebrides even the granulite facies . Regional folding and crust thickening occurred . Furthermore, shear zones shifting to the right formed , which reveal a transpressive tectonic regime. Many of these shear zones, such as the Laxford shear zone or the Diabaig shear zone , are linked to steep weak zones of the previous Inverian . In the Outer Hebrides, the shear zones are flat.

Occurrence and effects

The Tektono metamorphosis of the Laxfordian mainly affected the Rhiconich and Assynt terran . It changed the Scourie Dykes, the Loch Maree Group , the 1855 million year old Rubha Ruadh granites in the Rhiconich Terran (which were pulled apart and boudinated into ptygmatic folds) and generally overprinted the rocks of the Lewisian .

The effects of the Laxfordian were dependent on the terran. The Assynt Terran was only affected to a lesser extent, whereas in the Rhiconich Terran the changes were of a continuous nature. In the Assynt Terran, gneiss foliation is predominantly flat, apart from local folds with structures from Badcallian and Inverian . The Scourie dykes are undeformed. The deformations of the Laxfordian here were concentrated in discrete, predominantly northwest-southeast trending shear zones. They show a steep dip and are only several meters wide. Under amphibolite facial conditions, planar and linear structures developed and original pyroxenes were regressed to hornblende (static overprinting of granulite facial structures).

The kinematic effects on the Rhiconich Terran were much more significant. The result was a continuous, relatively flat, North-Vergent fold structure, which culminates in the Strath-Diamond anticlinory. The Scourie dykes were rotated into the foliation, folded (their fold structure also shows North Vergence), elongated, sheared and boudinated. Migmatite occurrences and isolated granite veins indicate anatexis . After crossing the Rubha-Ruadh-Ganitz zone, the fold structure in the contact area with the Assynt terran becomes steeper. An oblique, transpressive docking of the granulite facial gneiss of the Assynt Terran from the south-south-east is assumed to explain the tectonic structures.

Physical conditions

Sills (1983) determined the metamorphosis temperatures of > 500 ° C in shear zones using muscovites . Droop et al. (1999) found values ​​of 530 to 630 ° C for the peak area of ​​the metamorphosis; they carried out their measurements on metapelites of the Loch Maree Group .

Dating

The radiometric age of the Laxfordian could be determined on the basis of new titanite formations in the amphibolite structure at 1740 million years BP . Dallmeyer et al. (2001) found a Hornblende cooling age of 1705 million years BP. Kinny et al. (2005) determined the interval 1790 to 1670 million years BP with the uranium-lead method on zirconium and titanite.

Note: Older studies indicate four deformation phases (D1 to D4) for the Laxfordian, which extend over the period 1900 to 1150 million years BP. According to this, the terranean docking did not take place until around 1600 million years BP during phase D2 (lower amphibolite facies; right-hand transpression with north-verging upheavals and steep folds). Phase D3 (around 1400 million years BP) already took place under the physical conditions of the upper green schist facies and with D4 (1400 to 1150 million years BP) the ductile area of ​​the middle earth crust was finally abandoned; the deformation took place in the upper crust by means of left-sided lateral shifts and under the conditions of the lower green schist facies.

Individual evidence

  1. ↑ Sutton, J. & Watson, J .: The pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the North-West Highlands and its bearing on the chronological classification of the Lewisian . In: Quarterly Journal of the Geological Society of London . tape 106 , 1951, pp. 241-296 .
  2. ↑ MacDonald, John M .: 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 .
  3. ^ Beach, A., Coward, MP and Graham, RH: An interpretation of the structural evolution of the Laxford front, north-west Scotland . In: Scottish Journal of Geology . tape 9 , 1974, p. 297-308 .
  4. ^ Watson, JV: Lewisian . In: GY Craig (ed.): Geology of Scotland, 2nd edn. 1983, p. 23-47 .
  5. ^ Sills, JD: Mineralogical changes occurring during the retrogression of Archean gneisses from the Lewisian complex of NW Scotland . In: Lithos . tape 16 (2) , 198, pp. 113-124 .
  6. ^ Droop, GTR, Fernandes, LAD, Shaw, S .: Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland . In: Scot. J. Geol. Volume 35 , 1999, pp. 31-50 .
  7. ↑ 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: Contributions to Mineralogy and Petrology . tape 117 , 1994, pp. 215-228 .
  8. ↑ Kinny, PD & Friend, CRL: U – Pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian Complex of Northwest Scotland . In: Contributions to Mineralogy and Petrology . tape 129 , 1997, pp. 326-340 .
  9. ↑ Dallmeyer, RD, Strachan, RA, Rogers, G., Watt, GR & Friend, CRL: Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40Ar / 39Ar and Rb-Sr chronology . In: Journal of the Geological Society . tape 158 . London 2001, p. 501-512 .
  10. ↑ Kinny, PD, Friend, CRL, Love, GJ: Proposal for a terrane-based nomenclature for the Lewisian gneiss complex of northwest Scotland . In: J. Geol. Soc. tape 162 , 2005, pp. 175-186 .
  11. ↑ Beacom, LE: The Kinematic Evolution of Reactivated and Non-Reactivated Faults in Basement Rocks, NW Scotland . Queen's University of Belfast, PhD thesis, 1999.