Maen Gwyn
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Geology & landscape
Geological Period: Precambrian · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Description Maen Gwyn is of special interest because it records the intrusive relationship between two components of the Precambrian Coedana Complex, namely the Coedana Granite and its surrounding envelope of low-grade hornfels. The site is located approximately 7km north-north-west of Llangefni and consists of low, rocky outcrops of baked metamorphic rock. Maen Gwyn provides exposures of fine-grained and muscovite-rich metasedimentary hornfels which occur both as marginal country rock to the late Neoproterozoic (614 million years (Ma)) Coedana Granite and as xenolithic masses within the intrusion. The hornfels represent thermally metamorphosed quartzo-feldspathic sedimentary rocks and exhibit a composite fabric which records a pre-intrusion history involving two episodes, namely the formation of the original sedimentary layering (S ) and a sub-parallel tectonic foliation (S ). This composite fabric is cut 0 1 by non-porphyritic veins of the Coedana Granite which display a weak, post-magmatic ductile fabric that is equated with extensive brittle deformation seen in the hornfels. Dated at just under 600 Ma, the greenschist facies metamorphic grade of the hornfels contrasts with the upper amphibolite grades attained in the gneissic component of the Coedana Complex. The rocks exposed at this site therefore have an important bearing on the evolution of the Coedana Complex and, in particular, on the thermal and deformation history of the country rocks into which the Coedana Granite was emplaced. 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. The hornfels at Maen Gwyn has been dated using Rb-Sr (rubidium-strontium) and K-Ar (potassium-argon) methods, both giving an age of just under 600Ma.