Tyddyn Gyrfer
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Precambrian · Rock Type: Gneiss
Description
Description Tyddyn Gyrfer is of special interest for its Precambrian geology and lies within the largest outcrop of gneisses in southern Britain. The site is located approximately 10 km north-west of Llangefni and consists of low-lying, rocky outcrops of metamorphic rock which were subjected to high temperatures and high pressure (high-grade). Tyddyn Gyrfer provides a small but informative exposure of interleaved paragneisses and amphibolites that are representative of the upper amphibolite facies Central Anglesey Gneisses in the late Neoproterozoic Coedana Complex. The dominant lithology comprises finely banded biotite-garnet-sillimanite gneiss together with retrogressed plagioclase-and hornblende-bearing amphibolite interpreted as thin sills or dykes intruded into the paragneissic protolith. Sm-Nd isotopic data from the amphibolites indicate a maximum age of 1000 (million years) Ma for the formation and metamorphism. Isotopic data from the paragneisses indicate crustal residency ages of 1800 Ma which suggests that the sedimentary protoliths contained detritus derived from an ancient cratonic terrane rather than from the nearby Avalonian magmatic arc terranes which typically have a maximum crustal residency age of 1300 Ma. These isotopic data, together with petrological information, also support a correlation with the Rosslare Complex of south-east Ireland. Remarks The site is a revision to the former Nature Conservancy Council’s ‘Geological Conservation Review’, a national survey and evaluation of sites of geological and geomorphological interest. The site is described in the GCR volume entitled ‘Precambrian Rocks of England and Wales’ (Carney et al., 2000). Radiometric dating is a technique used to date rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. At Tyddyn Gyrfer the Sm-Nd technique has been used to determine the age relationships of rocks based on the decay of a long-lived samarium (Sm) isotope to a neodymium (Nd) isotope.