Hostname: page-component-8448b6f56d-dnltx Total loading time: 0 Render date: 2024-04-18T11:50:56.328Z Has data issue: false hasContentIssue false
  • English
  • Français

A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

Published online by Cambridge University Press:  28 July 2015

BETHAN A. PHILLIPS ,
ANDREW C. KERR  and
RICHARD BEVINS
BETHAN A. PHILLIPS*
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, Wales
ANDREW C. KERR
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, Wales
RICHARD BEVINS
Affiliation:
Department of Natural Sciences, National Museum Cardiff, Cathays Park, Cardiff CF10 3NP, Wales
*
†Author for correspondence: phillipsba@cardiff.ac.uk
Get access Rights & Permissions [Opens in a new window]

Abstract

The Fishguard Volcanic Group represents an excellently preserved example of a volcanic sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis and proposed tectonic setting for the Llanvirn (467–458 Ma) Fishguard Volcanic Group, South Wales, UK. New major and trace element geochemical data and petrographic observations are used to re-evaluate the magma chamber processes, mantle melting and source region. The new data reveal that the Fishguard Volcanic Group represents a closely related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel lherzolite source which had been modified by subduction components. The rocks of the Fishguard Volcanic Group are co-genetic and the felsic members are related to the more primitive basalts mainly by low-pressure fractional crystallization. The geochemistry of the lavas was significantly influenced by subduction processes associated with a coeval arc, while significant amounts of assimilation of continental crust along with fractional crystallization appear to have contributed to the compositions of the most evolved lavas. The Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting but no true seafloor spreading had occurred.

Keywords

geochemistrymagmatismback-arc basinsubduction
Type
Original Articles
Information
Geological Magazine , Volume 153 , Issue 3 , May 2016 , pp. 410 - 425
Copyright
Copyright © Cambridge University Press 2015 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Bevins, R. E. 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal 17, 1–21.Google Scholar
Bevins, R. E., Kokelaar, B. P. & Dunkley, P. N. 1984. Petrology and geochemistry of lower to middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition. Proceedings of the Geologists’ Association 95, 337–47.Google Scholar
Bevins, R. E., Lees, G. J. & Roach, R. A. 1991. Ordovician bimodal volcanism in SW Wales: geochemical evidence for petrogenesis of the silicic rocks. Journal of the Geological Society, London 148, 719–29.CrossRefGoogle Scholar
Bevins, R. E. & Roach, R. A. 1979. Early Ordovician volcanism in Dyfed, SW Wales. In The Caledonides of the British Isles – Reviewed (eds Harris, A. L., Holland, C. H. & Leake, B. E.), pp. 603–9. Geological Society of London, Special Publication no. 8.Google Scholar
Bevins, R. E. & Robinson, D. 1988. Short paper: low grade metamorphism of the Welsh Basin Lower Palaeozoic succession: an example of diastathermal metamorphism? Journal of the Geological Society, London 145, 363–6.Google Scholar
Bevins, R. E. & Rowbotham, G. 1983. Low grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal 18, 141–68.Google Scholar
Boudreau, A. 1999. PELE—a version of the MELTS software program for the PC platform. Computers and Geosciences 25, 201–3.CrossRefGoogle Scholar
Cann, J. R. 1970. Rb, Sr, Y, Zr and Nb in some ocean floor basaltic rocks. Earth and Planetary Science Letters 10, 7–11.Google Scholar
Cocks, L. R. M. & Torsvik, T. H. 2006. European geography in a global context from the Vendian to the end of the Palaeozoic. In European Lithosphere Dynamics (eds Gee, D. G. & Stephenson, R. A.), pp.83–95. Geological Society of London, Memoirs no. 32.Google Scholar
Cocks, L. R. M. & Torsvik, T. H. 2011. The Palaeozoic geography of Laurentia and western Laurussia: a stable craton with mobile margins. Earth-Science Reviews 106, 1–51.Google Scholar
Cox, A. H. 1930. Preliminary note on the geological structure of Pen Caer and Strumble Head, Pembrokeshire. Proceedings of the Geologists’ Association 41, 274–89.Google Scholar
Davidson, J., Turner, S. & Plank, T. 2013. Dy/Dy*: variations arising from mantle sources and petrogenetic processes. Journal of Petrology 54, 525–37.Google Scholar
DePaolo, D. J. 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189–202.CrossRefGoogle Scholar
Fitton, J. G. & Hughes, D. J. 1970. Volcanism and plate tectonics in the British Ordovician. Earth and Planetary Science Letters 8, 223–8.CrossRefGoogle Scholar
Fitton, J. G., Thirlwall, M. F. & Hughes, D. J. 1982. Volcanism in the Caledonian orogenic belt of Britain. In Andesites (eds Thorpe, R. S.), pp. 611–36. London: Wiley.Google Scholar
Gribble, R. F., Stern, R. J., Newman, S., Blommer, S. H. & O’Hearn, T. 1998. Chemical and isotopic composition of lavas from the northern Mariana Trough: implications for magma genesis in back-arc basin. Journal of Petrology 39, 125–54.Google Scholar
Hastie, A. R., Kerr, A. C., Mitchell, S. F. & Millar, I. L. 2008. Geochemistry and petrogenesis of Cretaceous oceanic plateau lavas in eastern Jamaica. Lithos 101, 323–43.Google Scholar
Hastie, A. R., Kerr, A. C., Pearce, J. A. & Mitchell, S. F. 2007. Classification of altered volcanic island arc rocks using immobile trace elements: development of the Th–Co discrimination diagram. Journal of Petrology 48, 2341–57.CrossRefGoogle Scholar
Hastie, A. R., Mitchell, S. F., Treloar, P. J., Kerr, A. C., Neill, I. & Barfod, D. N. 2013. Geochemical components in a Cretaceous island arc: the Th/La–(Ce/Ce*)Nd diagram and implications for subduction initiation in the inter-American region. Lithos 162–163, 57–6.Google Scholar
Hickson, C. J. & Juras, S. J. 1986. Sample contamination by grinding. Canadian Mineralogist 24, 585–9.Google Scholar
Kokelaar, B. P. 1988. Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society, London 145, 759–75.Google Scholar
Kokelaar, B. P., Howells, M. F., Bevins, R. E. & Roach, R. A. 1984 a. Volcanic and associated sedimentary and tectonic processes in the Ordovician marginal basin of Wales: a field guide. In Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins (eds Kokelaar, B. P. & Howells, M. F.), pp. 291–322. Geological Society of London, Special Publication no. 16.Google Scholar
Kokelaar, B. P., Howells, M. F., Bevins, R. E., Roach, R. A. & Dunkley, P. N. 1984 b. The Ordovician marginal basin in Wales. In Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins (eds Kokelaar, B. P. & Howells, M. F.), pp. 245–69. Geological Society of London, Special Publication no. 16.Google Scholar
Langmuir, C. H., Klein, E. M. & Plank, T. 1992. Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges. In Mantle Flow and Melt Generation at Mid-Ocean Ridges (eds Phipps Morgan, J., Blackman, D. K. & Sinton, J. M.), pp. 183–280. American Geophysical Union, Monograph vol. 71. Washington, DC, USA.Google Scholar
Leat, P. T., Jackson, S. E., Thorpe, R. S. & Stillman, C. J. 1986. Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides. Journal of the Geological Society, London 143, 259–73.Google Scholar
Martinez, F., Taylor, B., Baker, E. T., Resing, J. A. & Walker, S. L. 2006. Opposing trends in crustal thickness and spreading rate along the back-arc Eastern Lau Spreading Center: implications for controls on ridge morphology, faulting, and hydrothermal activity. Earth and Planetary Science Letters 245, 655–72.Google Scholar
McConnell, B. J., Stillman, C. J. & Hertogen, J. 1991. An Ordovician basalt to peralkaline rhyolite fractionation series from Avoca, Ireland. Journal of the Geological Society, London 148, 711–8.Google Scholar
McDonald, I. & Viljoen, K. S. 2006. Platinum-group element geochemistry of mantle eclogites: a reconnaissance study of xenoliths from the Orapa kimberlite, Botswana. Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy: Section B 115, 81–93.Google Scholar
Merriman, R. J., Bevins, R. E. & Ball, T. K. 1986. Petrological and geochemical variations within the Tal y Fan intrusion: a study of element mobility during low-grade metamorphism with implications for petrotectonic modelling. Journal of Petrology 27, 1409–36.CrossRefGoogle Scholar
Miyashiro, A. 1975. Volcanic rock series and tectonic setting. Annual Review of Earth and Planetary Sciences 3, 251.CrossRefGoogle Scholar
Murphy, J. B., Hamilton, M. A. & LeBlanc, B. 2011. Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia. CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology. Contribution to International Geological Correlation Programme (IGCP) Project 497. Canadian Journal of Earth Sciences 49, 346–58.Google Scholar
Murphy, J. B. & Nance, R. D. 2008. The Pangea conundrum. Geology 36, 703–6.Google Scholar
Pearce, J. A. & Cann, J. R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters 19, 290–300.Google Scholar
Pearce, J. A. & Stern, R. J. 2006. Origin of back-arc basin magmas: trace element and isotope perspectives. In Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical Interactions (eds Christie, D. M., Fisher, C. R., Lee, S. M. & Givens, S.), pp. 63–86. American Geophysical Union, Monograph vol. 166. Washington, DC, USA.Google Scholar
Pearce, J. A., Stern, R. J., Bloomer, S. H. & Fryer, P. 2005. Geochemical mapping of the Mariana arc-basin system: implications for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems 6, Q07006. doi: 10.1029/2004GC000895.Google Scholar
Pearce, J. A. 2008. Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos 100, 14–48.Google Scholar
Phillips, W. E., Stillman, C. J. & Murphy, T. 1976. A Caledonian plate tectonic model. Journal of the Geological Society, London 132, 579–609.Google Scholar
Plank, T. & Langmuir, C. H. 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chemical Geology 145, 325–94.CrossRefGoogle Scholar
Reed, F. R. C. 1895. The geology of the country around Fishguard. Quarterly Journal of the Geological Society of London 51, 149–95.Google Scholar
Robinson, D. & Bevins, R. E. 1986. Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales. Journal of Metamorphic Geology 4, 101–3.Google Scholar
Robinson, D., Reverdatto, V. V., Bevins, R. E., Polyansky, O. P. & Sheplev, V. S. 1999. Thermal modeling of convergent and extensional tectonic settings for the development of low-grade metamorphism in the Welsh Basin. Journal of Geophysical Research 104, 23069–79.Google Scholar
Rollinson, H. R. 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation. Essex: Longman Scientific & Technical, 352 pp.Google Scholar
Rudnick, R. L. & Gao, S. 2003. Composition of the continental crust. Treatise on Geochemistry 3, 1–64.Google Scholar
Sdrolias, S. & Müller, R. D. 2006. Controls on back-arc basin formation. Geochemistry, Geophysics, Geosystems 7, Q04016. doi: 10.1029/2005GC001090.Google Scholar
Sinton, J. M., Ford, L., Chappell, B. & Mc Culloch, M. 2003. Magma genesis and mantle heterogeneity in the Manus back-arc basin, Papua New Guinea. Journal of Petrology 44, 159–95.Google Scholar
Stern, R. J. 2002. Subduction zones. Reviews of Geophysics, 40, 1–13.Google Scholar
Sun, S. S. & McDonough, W. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins (eds Saunders, A. D. & Norry, M. J.), pp. 313–45. Geological Society of London, Special Publication no. 42.Google Scholar
Tatsumi, Y. & Eggins, S. 1995. Subduction Zone Magmatism. Cambridge, Massachusetts: Blackwell.Google Scholar
Thomas, G. E. & Thomas, T. M. 1956. The volcanic rocks of the area between Fishguard and Strumble Head, Pembrokeshire. Quarterly Journal of the Geological Society of London 112, 291–314.Google Scholar
Thorpe, R. S., Leat, P. T., Mann, A. C., Howells, M. F., Reedman, A. J. & Campbell, S. D. G. 1993. Magmatic evolution of the Ordovician Snowdon volcanic centre, North Wales (UK). Journal of Petrology 34, 711–41.Google Scholar
Trench, A. & Torsvik, T. H. 1992. The closure of the Iapetus Ocean and Tornquist Sea: new palaeomagnetic constraints. Journal of the Geological Society, London 149, 867–87.Google Scholar
van Staal, C. R., Whalen, J. B., Valverde-Vaquero, P., Zagorevski, A. & Rogers, N. 2009. Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians. In Ancient Orogens and Modern Analogues (eds Murphy, J. B., Keppie, J. D. & Hynes, A. J.), pp. 271–316. Geological Society of London, Special Publication no. 327.Google Scholar
Wood, D. A., Joron, J. L. & Treuil, M. 1979. A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings. Earth and Planetary Science Letters 45, 326–36.Google Scholar
Supplementary material: File

Phillips supplementary material S1

Phillips supplementary material

Download Phillips supplementary material S1(File)
File 33.8 KB