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| The St David's Granophyre has historically been incorrectly assigned to be ] or ] due to low concentrations of ] but this has been shown to be an alteration product from its original calc-alkaline ] which is noted to exhibit a ] signature.<ref name="multiple12">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</ref> Correlation of the ] intrusion with the ] rather than the nearby ] has been suggested<ref name="multiple13">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</ref> with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.<ref name="multiple">P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed</ref><ref name="multiple14">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</ref> | The St David's Granophyre has historically been incorrectly assigned to be ] or ] due to low concentrations of ] but this has been shown to be an alteration product from its original calc-alkaline ] which is noted to exhibit a ] signature.<ref name="multiple12">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</ref> Correlation of the ] intrusion with the ] rather than the nearby ] has been suggested<ref name="multiple13">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</ref> with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.<ref name="multiple">P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed</ref><ref name="multiple14">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</ref> | ||
| ===Sarn Complex=== | |||
| The largest plutonic body in the terrane has limited outcrop and is sheared by the ] in the west and covered by later (]) sediments to the east.<ref name="multiple">P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed</ref> Altered to ] facies the ] contains a bimodal suite of ]-], ] (Sarn Granite) and ]. | |||
| The ] is ] 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. | |||
| ==Sedimentary Geology== | ==Sedimentary Geology== | ||
Revision as of 15:41, 29 May 2012
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. The Cymru Terrane is bounded to the northwest by the Menai Strait Fault System and to the southeast by the Ponteford Lineament. The geological terrane to the west is the Cymru Terrane and to the east is Charnwood 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 Group and the Twt Hill Granite. 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).
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. Information on the basement geology is somewhat sparse with no zircons noted to be older than 617 to 638Ma. 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 1350Ma 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 (then director of the Geological Survey) 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 (Arenig) sediments to the east. Altered to Greenschist facies the pluton contains a bimodal suite of gabbro-diorite, monzogranite (Sarn Granite) and granodiorite.
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.
Sedimentary Geology
Palaeontology
References
- ^ P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
- 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
- J. N. Carney, J. M. Horak, et al., Precambrian Rocks of England and Wales, Joint Nature Conservation Committee. Geological Conservation Review Series 20
- P.M. Allen, A.A. Jackson Bryn-Teg Borehole, North Wales., 1978, Bulletin of the Geological Survey of Great Britain, 61 1-52
- ^ 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.
- 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
- 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
- ^ 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
- 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
- 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
- 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
External links
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