Rhinns complex

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Located on the west coast of Scotland , the Rhinns Complex consists of paleoproterozoic igneous rocks that intruded during the early Statherian period and subsequently metamorphosed and deformed. They now form the basement of the metasedimentary Colonsay Group .

Etymology and Occurrence

Rocks of the Rhinns Complex at Claddach Bay on the southern tip of the Rhinns of Islay

The Rhinns Complex was named after its largest occurrence on the Rhinns of Islay ( Scottish Gaelic Na Roinn Ìleach ). In addition to the type locality on Islay, there are other occurrences of the magmatite complex on Colonsay and Inishtrahull Island . With dredges similar on the seabed in the Malin Sea between Islay and Inishtrahull igneous rocks have been brought to light which have been additionally detected with geophysical methods. The extent of the Rhinns Complex, which is part of the Colonsay-West Islay crustal block, is very limited on the surface, but it is believed that it is located southeast of the Great Glenn Fault also under the Central Highlands Terran .

introduction

In the period 2000 to 1600 million years BP, numerous continental collisions created the supercontinent Columbia . On its southern edge, between 1900 and 1600 million years BP, a subduction zone dipping to the north developed on an active continental edge of the Andean type. This former subduction zone is now represented by the Ketilid Belt of Greenland and the Svekofennid Belt of Scandinavia . Both belts are characterized by an HT / LP metamorphosis (high temperature / low pressure metamorphism) and calcareous magmatism, both features of a volcanic arc . The rocks are predominantly freshly differentiated melting products of the upper earth mantle with a very small proportion of assimilated, archaic earth crust . The Rhinns Complex and the Annagh Gneiss in Ireland are believed to be the link between these two belts.

Lithology

Geological map of Islay with the Rhinns Complex in the southwest of the Rhinns Peninsula

The Rhinns complex consists of alkaline igneous rocks , predominantly syenites and subordinate mafites , into which the gabbro layers penetrated. On Islay and Inishtrahull these magmatites are medium to coarse-grained, syenitic gneiss , on Colonsay as very strongly retrograded , granodioritic orthogneiss .

Whether the occurrence of Colonsay can still be counted as part of the Rhinns complex has meanwhile been questioned on the basis of age determinations and hafnium isotope analyzes (see below).

stratigraphy

The Rhinns complex is discordantly overlaid by the Colonsay Group or the Dalradian Supergroup .

metamorphosis

The metamorphosis of the Rhinns complex reached the pressure-temperature conditions of the amphibolite facies . The rocks were then subjected to severe retromorphism .

Dating

According to Daly (2009), samarium neodymium model ages for the Rhinns complex were 1978 to 1912 million years BP. Intrusion ages determined with the uranium-lead method on zirconia resulted in ± 5 million years BP for Syenite von Islay 1782 and ± 3 million years BP for Inishtrahull 1779. The age of 1710 million years BP, which was obtained on a metagabbro from Inishtrahull using the argon method on hornblende , documents the cooling of the rock after the metamorphosis (cooling age). For the metamorphosis itself, Loewy et al. (2003) found BP on zircons on Islay from 1729 to 1725 million years. This age can be correlated with the early Laxfordian in the Hebridean Terran and with the high pressure metamorphosis in the Glennelg-Attadale Inlier .

The Colonsay orthogneiss was dated 1889 ± 7 million years BP. It is therefore older than the Rhinns complex and was already intruded in the Orosirium . Its multiple deformed pegmatites gave BP in 1797 ± 7 and 1794 ± 7 million years. At 1890 million years old, the orthogneiss shows hafnium values ​​of εHf = +8 and thus has an extremely juvenile character. The Rhinns complex, on the other hand, only has εHf = +5 at 1800 million years BP and could therefore also represent a postorogenic granitoid.

Individual evidence

  1. ↑ a b Daly, JS; Flowerdew, MJ, McAteer, C., Horstwood, MSA, Whitehouse, MJ & Chew, DM: Palaeoproterozoic orthogneiss on Colonsay, SW Scotland . In: 52nd Irish Geological Research Meeting . 2009, p. 28 .
  2. ↑ a b Daly, JS: Precambrian . In: CH Holland & IS Sanders (Eds.): The Geology of Ireland, 2nd edition . Dunedin Academic Press, Edinburgh 2009, pp. 7-42 .
  3. ↑ Marcantonio, F. et al .: A 1,800-million-year-old Proterozoic gneiss terrane in Islay with implications for the crustal structure and evolution of Britain . In: Nature . tape 335 , 1988, pp. 62-4 .
  4. ↑ Daly, JS, Aitcheson, SJ, Cliff, RA, Gayer, RA & Rice, AHN: Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway . In: Journal of the Geological Society, London . tape 148 , 1991, pp. 29-40 .
  5. ^ Nigel Woodcock and Rob Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, Oxford 2000, ISBN 0-632-03656-7 .
  6. ↑ Loewy, SL et al.: Eastern Laurentia in Rodinia: constraints from whole-rock Pb and U / Pb geochronology . In: Tectonophysics . tape 375 , 2003, p. 169-97 .