Loch Maree Group

from Wikipedia, the free encyclopedia

The Loch Maree Group is a supracrustal series of volcanic and sedimentary rocks that were deposited in the Hebridean Terran during the Paleoproterozoic and deformed and metamorphosed during the Laxfordian .

etymology

Loch Maree with its many islands, seen from the north. In the foreground there are rocks from the Letterewe Synform.

The name Loch Maree Group, abbreviated to LMG, is derived from its eponymous type locality , Loch Maree ( Scottish Gaelic Loch Ma-ruibhe ) on the north-west coast of Scotland , on whose north-east bank almost half of the Loch Maree Group stands.

introduction

The period 2400 to 2000 million years BP ( Siderium and Rhyacium ) was characterized worldwide by crustal expansion and erupting continental margins. During this rift phase the Scourie dykes invaded the Lewisian of Scotland . By 2000 million years BP, oceanic crust had formed between each terran . In the interval from 2000 to 1900 million years BP ( Orosirium ) this oceanic crust formed between the southern area ( Rona Terran ) and the central area ( Gruinard and Gairloch Terran ) was subducted under the central area together with sediments of continental origin . The Loch Maree Group was born. During the early Laxfordian, the two crustal areas collided and around 1870 million years ago BP, the Loch Maree Group was pushed over the basement gneiss of the southern area, with the two rock bodies interlocking and folding .

Occurrence

Geological map of the Hebridean Terran. The Loch Maree supracrustal group is shown in olive green.

The Loch Maree Group occurrence is spread over two larger, separate areas. As already mentioned, the first occurrence at the type locality runs in a north-west-south-east direction parallel to the north-east bank of Loch Maree, which it follows for almost its entire length over 15 kilometers. The occurrence forms an approximately 5 km wide Syncline , which Letterewe Synform .

The second, further southwest located deposit is about 20 kilometers long and 3 kilometers wide; it strikes in a north-north-west-south-south-east direction east of Loch Gairloch . It lies on the southwest flank of the Tollie anticlinory , shows a steep dip and is separated from the anticlinory by the steep, brittle-ductile Creag-Bhan shear zone .

A smaller, third occurrence is the Gruinard Belt , a band barely 500 meters wide that crosses halfway between Gruinard Bay and Flonn Loch over a distance of around 10 kilometers with a strike direction northwest-southeast.

Lithology

The Loch Maree Group consists of rocks of both oceanic and continental origin. The oceanic rocks are former plateau or primitive island arch basalts, associated, abyssal sediments, iron-rich hydrothermal deposits ( band iron ores ) and shelf carbonates . The continental sediments are essentially metamorphosed Grauwacken , which are interpreted as delta beds .

The following stratigraphy exists for the second occurrence near Gairloch (from hanging to lying):

  • Lewisian Shieldaig Gneiss
  • Cloiche belt
  • Ard gneiss
  • Charlestown ski
  • Kerrysdale Basit
  • Flowerdale Schist
  • Aundrary basit
  • Creag Bhan belt
  • Lewisian Buainichian Gneiss

The gneisses of the Ialltaig Terran are pressed in between the Ard gneiss and the Cloiche belt as tectonic shear chips .

The geochemical signature of the metasediments excludes a derivation from the archaic gneisses of the Lewisian . This is also indicated by the zircon population in the Flowerdale Schist, the majority of which were formed between 2200 and 2000 million years BP.

metamorphosis

Due to the subduction processes, all of these rocks suffered the physical conditions of the amphibolite facies , with Droop u. a. (1999) were able to determine temperatures of 530 to 630 ° C using calcite-dolomite and garnet-biotite geothermometers and pressures of 6.5 kilobar and 0.65 gigapascal using garnet-plagioclase geobarometer . The plateau basalts were metamorphosed into amphibolites . The Grauwacken became quartz - biotite slates and the shelf carbonates became marble . It also emerged graphite schists, chlorite schists and other shales .

Stable mineral phases in the amphibolites are oligoclase and aluminum-rich hornblende , in the quartz-biotite slate oligoclase and garnet , in the calcium silicate marbles tremolite or diopside and in the iron ores garnet / grunerite . In younger, ductile shear zones , retromorphism took place under the conditions of the green schist facies . Garnet, feldspar and Al hornblende became unstable and turned into biotite, epidote , albite , muscovite and actinolite .

Development over time

  • The formation of the Loch Maree Group parent rocks cannot have occurred before 2200 to 2000 million years BP, as the youngest reclaimed zircon population is at that age. Archaic zircon ages show that in addition to crust sections from the Paleoproterozoic, also those from the Archaic were reconditioned. The formation process of the protoliths should have been completed around 2000 million years BP, as can be seen from the samarium - neodymium model age and detritic zirconia age.
  • Accretion-induced subduction, which has its modern equivalent in the Shimanto belt of Japan , in the Rhodope Mountains of Greece or in the Caribbean Andes of Colombia , was already in full swing around 1900 million years BP, as the age of the granodioritic and tonalitic Ard gneiss suggests. The Ard gneiss (an ortho-eye gneiss) was dated to 1903 + 3 / -2 million years and represents a typical, subduction-related TTG complex . Its primitive, geochemical fingerprint excludes subducted sediments as a melting source, but points to much more mafic parent rocks .
  • Park u. a. (2001) assume that the collision of the terrane blocks was completed by 1870 million years BP and that the thickened crustal area experienced an orogenetically induced collapse by 1860 million years BP.

Deformations

Loch Maree as seen from Tollie Farm, looking southeast. On the opposite bank are the rocks of the Letterewe Synform behind the Torridonian Supergroup (front left) and the Lewisian gneiss (center).

The subduction and eventual collision had led to severe deformations in the two affected terrans , which are now summarized under the generic term of Laxfordian . A total of four deformation phases (D1 to D4) can be distinguished, which can be assigned to approximately the period 1900 to 1400 million years BP.

The first two deformation phases D1 and D2, known as the early Laxfordian , are usually difficult to tell apart. They are characterized by ductile deformations in the area of ​​the amphibolite facies. The late Laxfordian , which was only affected by the conditions of the green schist facies, consists of the deformation phases D3 and D4.

The penetration of the Tollie pegmatite , dated 1694 ± 5 ​​million years BP, provides a minimum age for D1 / D2 and at the same time a maximum age for D3. The last deformation phase D4 should then have occurred in the interval 1500 to 1400 million years BP.

Structures

The structures of the ductile, combined deformation phase D1 / D2, which are very likely to be attributed to the subduction process, were originally formed in a significant, shallow, northeast-dipping, mylonitic shear zone. The resulting thrust movements generally took place with hanging walls to the west-northwest. The high-grade D1 / D2 structures are LS tectonites with continuous elongation lines that dip slightly to the southeast. The direction of the extension linear is also regarded as the original direction of movement in the networked, flat-lying shear zones.

The postorogenic collapse is probably represented by the right-sided shear sense, in which the prone block is raised to the southwest - in contrast to the normally observed left-sided shear sense, in which the northeast block is raised.

The now steep, generally northwest-southeast trending structures (including the once flat shear zones) in the slate belt of the Loch Maree Group all go back to the deformation phase D3, which occurred almost 200 million years after the collision. She is responsible for the regional formation of wrinkles (Tollie-Antiklinorium, Letterewe-Synklinorium, Carnmore-Antiklinorium). The D3 phase can possibly be linked to the Labrador orogeny that took place around 1700 million years BP .

The last deformation phase D4 in the lower green schist facies only produced small, isolated, steep folds in the centimeter / meter range, which are often designed as open folds and are accompanied by cataclasite belts. This indicates that the Loch Maree Group was surfacing at the time and was already deformed under upper crustal conditions.

Paleogeographical location and connections

The Loch Maree Group had previously been interpreted as a sequence that was deposited in an expanded continental basin. It has consequently been associated with the North Atlantic craton of Greenland and Labrador . Your reinterpretation by Park u. a. (2001) as an accretion complex, however, now enables a correlation with other orrogens, also created by subduction. The following orogen belts come into question, all of which accreted BP Terrane between 1900 and 1830 million years:

It is very likely that the Loch Maree Group emerged as an extension of the latter two orrogens, Park being at the kink between the Nagssugtoqidian and the Lapland-Kola orogeny. There is also a possible connection with the Lofoten-Vesterålen-Terran and the Troms-Terran in Northern Norway .

Individual evidence

  1. ↑ a b c d R. G. Park u. a .: The Loch Maree Group: Paleoproterozoic subduction-accretion complex in the Lewisian of NW Scotland . In: Precambrian Research . tape 105 , 2001, p. 205-226 .
  2. ^ Nigel Woodcock, Rob Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, 2000, ISBN 0-632-03656-7 .
  3. ^ GTR Droop, LAD Fernandes, S. Shaw: Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland . In: Scott. J. Geol. Volume 35 , 1999, pp. 31-50 .
  4. ^ RK O'Nions, PJ Hamilton, PJ Hooker: A Nd isotope investigation of sediments related to crustal development in the British Isles . In: Earth Planet. Sci. Lett. tape 63 , 1983, pp. 229-240 .
  5. ^ MJ Whitehouse, D. Bridgwater, RG Park: Detrital zircon ages from the Loch Maree Group, Lewisian Complex, NW Scotland: confirmation of a Palaeoproterozoic Laurentia - Fennoscandia connection . In: Terra Nova . tape 9 , 1997, pp. 260-263 .
  6. ↑ S. Moorbath, RG Park: The Lewisian chronology of the southern region of the Scottish mainland . In: Scott. J. Geol. Volume 8 , 1971, p. 51-74 .
  7. ^ MP Coward, RG Park: The role of mid-crustal shear zones in the early Proterozoic evolution of the Lewisian. Evolution of the Lewisian and Comparable Precambrian High Grade Terrains . In: RG Park, J. Tarney (Ed.): Geol. Soc. Lond. Spec. Publ. Volume 27 , 1987, pp. 127-138 .
  8. ^ J. Wheeler, BF Windley, FB Davies: Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland. Evolution of the Lewisian and Comparable High Grade Terrains . In: RG Park, J. Tarney (Ed.): Geol. Soc. Lond. Spec. Publ. Volume 27 , 1987, pp. 153-164 .
  9. ^ YA Johnson, RG Park, JA Winchester: Geochemistry, petrogenesis and tectonic significance of the early Proterozoic Loch Maree Group amphibolites of the Lewisian Complex, NW Scotland. Geochemistry and Mineralization of Proterozoic Volcanic Suites . In: TC Pharaoh, RD Beckinsale, D. Rickard (Eds.): Geol. Soc. Lond. Spec. Publ. Volume 33 , 1987, pp. 255-269 .
  10. ↑ SG Bergh u. a .: Chapter 11. Was the Precambrian Basement of Western Troms and Lofoten-Vesterålen in Northern Norway Linked to the Lewisian of Scotland? A Comparison of Crustal Components, Tectonic Evolution and Amalgamation History . In: Tectonics - Recent Advances . 2012, p. 283-330 .