Cymru Terrane
| Proterozoic eon |
| Geologic timescale of the Proterozoic |
ImageSize = width:175 height:395 PlotArea = left:40 right:5 bottom:100 top:60 AlignBars = justify Period = from:542 till:2500 TimeAxis = orientation:vertical ScaleMajor = unit:year increment:500 start:1000 ScaleMinor = unit:year increment:100 start:600 Colors = id:eon value:rgb legend:Proterozoic_eon # blue id:era value:rgb legend:Era # aquamarine id:black value:black Legend = left:15 top:40 BarData = bar:eon bar:era PlotData= width:100 align:center textcolor:black mark: bar:eon color:eon from:542 till:2500 # text:Proterozoic~ bar:era color:era from:542 till:1000 text:Neoproterozoic~ from:1000 till:1600 text:Mesoproterozoic~ from:1600 till:2500 text:Paleoproterozoic~ TextData = tabs: pos: text:"Preceded by:" text:"Archaean eon" text:"and Neoarchean era" pos: text:"Followed by:" text: "Phanerozoic eon" text: "and Paleozoic era" |
The Cymru Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes are the Charnwood Terrane, Fenland Terrane, Wrekin Terrane and the Monian Composite Terrane. In this article the definition of terrane is that implying rocks associated with the composition of the Precambrian basement. The Cymru Terrane is bounded to the northwest by the Menai Strait Fault System and to the southeast by the Pontesford Lineament. The geological terrane to the west is the Monian Composite Terrane and to the east is Wrekin Terrane. The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.
The Proterozoic rocks of the Cymru Terrane are typified in North Wales by the Arfon Group, Sarn Complex and the Twt Hill Granite. The former is in the Bangor area with the latter two located on the Llyn Peninsula. The St Davids Granophyre and the Pebidian Supergroup are located on the coastline of St Davids Peninsula. Inferred Proterozoic volcanic deposits are noted in the Bryn-Teg Borehole.
The Padarn Tuff, which is thought to be contemporaneous with the Sarn Complex, is unconformable with the overlying Fachwen Formation and the unconformity has been constrained with dates noted of 604.7 Ma+/-1.6Ma from the tuffs and 572.5 Ma+/-1.2Ma from the Fachwen Formation.
Information on the basement is somewhat sparse with no zircons noted to be older than 617 to 638 Ma. However, there are outcrops of the Parwyd Gneiss which are retrogressed granitoid gneiss and garnet amphibolite that occur with the Llyn Shear Zone. This is conjectured to be one of two probabilities, either as an exotic sliver, or as a metamorphic slice from the Monian Composite Terrane. The Granitoid Gneiss has Sm-Nd ages of 1350 Ma which is in the same temporal region as the Sarn Granite
Intrusive geology
St David's Granophyre
In the southwest of Wales is the St David’s Granophyre the stratigraphical relationship of which was subject of heated debates towards the end of the 19th Century predominantly between Sir Archibald Geikie and Henry Hicks. Geikie insisted upon the Pebidian Supergroup and the high-level intrusion being placed in the Cambrian sequence with Hicks taking the opposing view and suggesting that the Granophyre which cross cuts the Pebidian Supergroup should be placed in the Precambrian and by consequence placing the Pebidian Supergroup in the Precambrian. The issue was finally resolved by Green by revealing, in an excavation, an unconformity in which an excavation of a Cambrian basal conglomerate is shown to cut the granophyre.The St David's Granophyre has historically been incorrectly assigned to be alaskite or trondhjemite due to low concentrations of potassium feldspar but this has been shown to be an alteration product from its original calc-alkaline granite which is noted to exhibit a volcanic arc signature. Correlation of the granophyric intrusion with the Arfon Group rather than the nearby Coomb Volcanic Formation has been suggested with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.
Sarn Complex
The largest plutonic body in the terrane has limited outcrop and is sheared by the Llyn Shear Zone in the west and covered by later sediments to the east. Altered to Greenschist facies the pluton contains a bimodal suite of gabbro-diorite, monzogranite and granodiorite.The Sarn Granite is leucocratic and covers an expanse of c.6 km^2 in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component has been confirmed as having a Neoproterozoic age of 614Ma+/-2Ma using U-Pb Zircon dating. So therefore, the shearing of the Llyn is also temporally constrained by the date.
Twt Hill Granite
This is a small fine-grained, leucocratic intrusive body that cuts the Padarn Tuff at the western end of the Bangor-Caernarfon Ridge.Volcano-sedimentary geology
The thick sequences of volcano-sedimentary facies are present in North Wales and South Wales and are generally considered to be coeval but are geochemically distinct, showing acidic and basic qualities respectively. Both are cut by minor granitic intrusions.The Pebidian Supergroup is interbedded basic lavas and acid tuffs, is cut by the St David's Granophyre. There is evidence presented that subaerial and subaqueous deposition has occurred in a Welsh Basin that is likely to have been undergoing varying rates of subsidence thus promoting aerial emergence. Greenschist alteration is in keeping with the regional metamorphism that is characteristic of Welsh Neoproterozoic rocks.
In the Pebidian Supergroup the basal unit is largely basaltic and exhibits columnar jointing, autobrecciated lavas, scoriaceous and fine grained tuffs. The volcano-sediments have an increased acid component until the uppermost beds of the unit exhibit the basaltic influence once again. Such information is preserved in the Rhosson Group as autobrecciated lava, tuffs and scoria deposits.
The Arfon Group
This volcano-sedimentary group incorporates over 4000m of deposits and were previously considered to be of Cambrian age. U-Pb data suggests that the whole succession is indeed Precambrian Neoproterozoic age. Exposure of the lower unit is exhibited on a ridge between Bangor and Caernarfon and also on a ridge near Llyn Padarn This is noted to be a thick sequence of acid ash flow tuffs and exhibits welding and are thought to be rapid deposition of thin air-fall tuffs and rhyolite flows placed in a bounded half-graben .Isotope data shows a U-Pb zircon from the lower part of the succession as 614+/-2Ma and 604.7+/-1.6Ma confirms a Neoproterozoic age.