CYNGOR CEFN GWLAD
CYMRU COUNTRYSIDE COUNCIL FOR WALES
SITE OF SPECIAL SCIENTIFIC INTEREST: CITATION
ISLE OF ANGLESEY
Date of Notification: 21 October 2010
National Grid Reference: SH 534716, SH 530723, SH 545742
OS Maps: 1:50,000 Sheet number: 114
1:10,000 Sheet number: SH57 SE
Site Area: 7.1 ha
The site comprises the Marquess of Anglesey’s Column, a roadside cutting on the A55 and a series of crags at Castellior Farm. The exposures around the base of the Marquess of Anglesey’s Column provided the first known examples of blue glaucophanic amphibole in the British Isles and these continue to be a key research locality. The selected road cutting on the A55 at Llanfair P.G. provides the only exposures of unweathered blueschists in Britain, whilst the crags at Castellior Farm show particularly fine exposures of intensely foliated and tightly folded dark blueschists. Collectively, these exposures provide an unrivalled opportunity to study blueschist mineralogy and are highly significant for interpreting the Precambrian tectonic history of the region.
This site is of international importance because it displays one of the oldest known assemblages of blueschist in the world. The rocks comprise fine-grained, intensely foliated and lineated, blue-grey schists in which the foliation is deformed by intrafolial folds. The protolith was a sequence of basalts and sediments, the former now represented by crossite- and glaucophane-bearing metabasites and the latter by phengite-mica schists and lawsonite schists. The metabasites display a Mid-Ocean Ridge Basalt geochemical signature confirming their origin as oceanic crustal rocks. These underwent ocean-floor metamorphism which has been dated at 590-580Ma, indicating a late Neoproterozoic age for oceanic crust generation, whilst mineral ages of 560-550Ma record cooling and uplift of the blueschists by oblique strike-slip faulting within the Menai Strait Fault System. The blueschist facies mineralogy reflects a pre-uplift phase of high-pressure, low-temperature subduction zone metamorphism, a feature which contrasts strongly with the arc-related magmatic and sedimentary environments identified in all other Neoproterozoic terranes in southern Britain. The preservation of blueschists is a globally rare phenomenon, particularly in pre-Mesozoic rocks, and their identification in the Precambrian basement of both Anglesey and Llyn provides an important constraint on the late Neoproterozoic plate tectonic evolution of Eastern Avalonia.
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).