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Cymru Terrane

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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.[1] 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 Cymru Terrane and to the east is Charnwood Terrane.[1] The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.[2][3]

The Proterozoic rocks of the Cymru Terrane are typified in North Wales by the Arfon Group, Sarn Complex and the Twt Hill Granite.[1] The former is in the Bangor area (S & SW) 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 (Trawsfynydd, Gwynedd).[1][4]

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 604.7Ma+/-1.6Ma from the tuffs and 572.5Ma+/-1.2Ma from the Fachwen Formation. [5] Information on the basement geology is somewhat sparse with no zircons noted to be older than 617 to 638Ma. [5] However, there are outcrops of the Parwyd Gneiss which are retrogressed granitoid gneiss and garnet amphibolite that occur with the Llyn Shear Zone.[1] 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 1350Ma which is in the same temporal region as the Sarn Granite [1][5]

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 (then director of the Geological Survey) insisted upon the Pebidian Supergroup and the high-level intrusion being placed in the Cambrian sequence with Hicks [6] [7] 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[8] by revealing, in an excavation, an unconformity in which an excavation of a Cambrian basal conglomerate is shown to cut the granophyre.[8][1]

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.[9] Correlation of the granophyric intrusion with the Arfon Group rather than the nearby Coomb Volcanic Formation has been suggested[10] with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.[1][11]

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 (Arenig) sediments to the east.[1] Altered to Greenschist facies the pluton contains a bimodal suite of gabbro-diorite, monzogranite (Sarn Granite) and granodiorite.[11]

The Sarn Granite is leucocratic and covers an expanse of c.6km^2 in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component of the has been confirmed as having a Neoproterozoic age of 614Ma+/-2Ma using U-Pb Zircon dating.[11] So therefore, the shearing of the Llyn is also temporally constrained by the date.

Volcano-Sedimentary Geology

The thick sequences of volcano-sedimentary facies are present in North Wales (as the Arfon Group) and South Wales (as the Pebidian Supergroup) and are generally considered to be coeval but are geochemically distinct, showing acidic and basic qualities respectively.[11][10] Both ar cut by minor granitic intrusions.

The Pebidian Supergroup is interbedded basic lavas and acid tuffs, is cut by the St David's Granophyre.[1] 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.[10]

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.[3] Such infromation is preserved in the Rhosson Group as autobrecciated lava, tuffs and scoria deposits. [1][3]

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. [12] Exposure of the lower unit (Padarn Tuff) is exhibited on a ridge between Bangor and Caernarfon and also on a ridge near Llyn Padarn[1] 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 (or graben) . [12] Isotope data shows a U-Pb zircon from the lower part of the succession as 614+/-2Ma[13] and 604.7+/-1.6Ma[5] confirms a Neoproterozoic age.[1]

Palaeontology

No Ediacara fauna has been found in the Welsh Terrane to date. The age of the Bryn-teg Volcanic Formation is constrained by Lower Cambrian foraminifera in overlying beds. The Playsolenites cooperi are only otherwise known in the Placentian of SE Newfoundland.[14]

References

  1. ^ a b c d e f g h i j k l m P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
  2. ^ Pharaoh, T.C., Gibbons, W., Precambrian Rocks in England and Wales south of the Menai Strait Fault System, 1987 A Revised Correlation of the Precambrian Rocks in the British Isles
  3. ^ a b c J. N. Carney, J. M. Horak, et al., Precambrian Rocks of England and Wales, Joint Nature Conservation Committee. Geological Conservation Review Series 20 Cite error: The named reference "multiple3" was defined multiple times with different content (see the help page).
  4. ^ P.M. Allen, A.A. Jackson Bryn-Teg Borehole, North Wales., 1978, Bulletin of the Geological Survey of Great Britain, 61 1-52
  5. ^ a b c d W. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339.
  6. ^ H. Hicks On the Precambrian (Dimentian and Pebidian) rocks of St David’s, 1877, Quarterly Journal of the Geological Society of London, 33, 199-241
  7. ^ H. Hicks On the Precambrian rocks of Pembrokeshire with especial reference to the St David’s district., 1878, Journal of the Geological Society of London. 40 507-560
  8. ^ a b J. F. N. Green On the Geological Structure of the St David’s area, Pembrokeshire., 1908, Journal of the Geological Society of London. 64 363-383
  9. ^ T. W. Bloxham & M. H. J. Dirk The petrology and geochemistry of the St David’s granophyre and Cwm Bach Rhyolite, Pembrokeshire, Dyfed., 1988, Mineralogical Magazine, 52 563-575
  10. ^ a b c R. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, Geochemical Character of Neoproterozoic rocks in southwest Wales., 1995, Geological Magazine. 132 330-349
  11. ^ a b c d P. J. Patchett & J. Jocelyn U-Pb zircon ages for late Precambrian igneous rocks in South Wales., 1979, Journal of the Geological Society of London. 136 13-19
  12. ^ a b A. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321
  13. ^ R. D. Tucker & T. C. Pharaoh, U-Pb Zircon ages for late-Precambrian rocks in southern Britain., 1991, Journal of the Geological Society of London, 148, 435-433
  14. ^ D. McIlroy, O. R. Green, M. D. Brasier, Palaeobiology and evolution of the earliest agglutinated Foraminifera: Platysolenites, Spirosolenites and related forms., 2001, Lethaia, 34, 13-29

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