Scourie dykes

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The Scourie dykes are a mafic gang that invaded the polymetamorphic basement of the Hebridean Terran during the Paleoproterozoic .

Etymology and first description

The Engl. The name Scourie dykes is derived from the eponymous type locality Scourie on the northwest coast of Scotland . The word dyke (in American English dike ) means gear. A first scientific description of two dolerite courses near Scourie was made by Teall in 1885.

characterization

Scourie dykes, now present as amphibolite schist, cut through gray greezes of the Scourian. Both units were tectonized during the Laxfordian and later interspersed with granite veins. Road outcrop on the A838 north of Laxford Bridge

In many continental blocks, mafic veins invaded the crustal area at the beginning of the Paleoproterozoic (during the period 2400 to 2000 million years BP , ie during the Siderium and the Rhyacian ). This rifting possibly documents the breaking up of a supercontinent that was consolidated in the late Archean (Nunavutia?). The Scourie dykes are to be placed in this context. Their intrusion into the Lewisian gneiss occurred after the Hebridean terran had undergone the two tectono-metamorphic phases of Badcallian (around 2,700 million years BP) and Inverian (around 2,500 million years BP). Sutton and Watson (1951) made use of this fact in order to be able to distinguish later deformations of the Laxfordian in the polymetamorphic basement from the early phases. In terranos in which the Scourie dykes are undeformed, no later deformations have occurred. Conversely, deformed Scourie dykes are a direct reference to the later deformations of the Laxfordian.

composition

Lithology

The Scourie dykes can be divided into four groups petrologically . By far the most common, with over 90% of the veins, are tholeiitic , iron-rich quartz dolerites . Magnesium-rich bronzite picrites , norite and olivine gabbros are much rarer . The dolerite and norite show chilled vein margins, whereas the ultramafic picrites and the olivine gabbros show a coarse mineral growth of orthopyroxene and augite on their margins .

mineralogy

The main minerals in the scourie dykes are orthopyroxene (up to 59 vol%), clinopyroxene (up to 35 vol%), plagioclase (up to 48 vol%) and olivine (up to 10 vol%). Quartz (up to 5% by volume), apatite , oxides (around 5% by volume), epidote and clinozoisite are subordinate . Track manner are sphene , garnet , biotite and calcite encountered. Orthopyroxene and clinopyroxene are often partially or completely replaced by pseudomorphic hornblende with actinolite edges. Plagioclase can Serizitisierung and Saussuritisierung suffer and Olvin Iddingsitisierung .

Occurrence

The Scourie dykes are most common in the Assynt (contains the type locality) and in the Gruinard Terran . However, they also occur sporadically in the Rhinconich Terran , Tarbert Terran and Uist Terran .

metamorphosis

Teall had already noticed the different amphibolitization of the 10 to 50 meter wide corridors, depending on the intensity of the metamorphosis . In the Rhinconich and Gruinard Terran the amphibolitization was complete, as both crust blocks were amphibolite facially overprinted during the Laxfordian in the interval 1900 to 1800 million years BP. In the Assynt Terran, however, the Laxfordian was limited to shear zones, so that in this terran not only completely amphibolitized tunnels but also tunnels with original mineral content and plutonic structure could survive.

Dating

The most recent dating of the Scourie dykes using the uranium-lead method on baddeleyite by Davies and Heaman (2014) gave the time interval 2418 to 2375 million years BP. The intrusion of the dikes thus took place over a period of around 40 million years.

Individual evidence

  1. ↑ Teall, JJH: The metamorphosis of dolerite into hornblende-schist . In: QJ Geol.Soc. London . tape 41 , 1885, p. 132-145 .
  2. ^ J. Tarney: The Scourie dyke suite and the nature of the Inverian event in Assynt . In: RG Park, J. Tarney (Ed.): The Early Precambrian of Scotland and Related Rocks of Greenland. Proceedings of a conference held at the Department of Geology, University of Keele, March 1971 . University of Keele, Newcastle 1973, OCLC 463223913 , p. 105-118 .
  3. ↑ J. Tarney, BL Weaver: Mineralogy, petrology and geochemistry of the Scourie Dykes. Petrogenesis and crystallization processes in dykes intruded at depth. In: Geological Society, London Special Publication . tape 27 , no. 1 , January 1987, pp. 217-233 , doi : 10.1144 / GSL.SP.1987.027.01.19 .
  4. ^ ZK Zhu, RK O'Nions, NS Belshaw, AJ Gibb: Lewisian crustal history from in situ SIMS mineral chronometry and related metamorphic textures . In: Chem. Geol. tape 136 , 1997, pp. 205-218 .
  5. ↑ 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 .