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Age and geochemistry of the Charlestown Group, Ireland: implications for the Grampian orogeny, its mineral potential and the Ordovician timescale

Age and geochemistry of the Charlestown Group, Ireland: implications for the Grampian orogeny, its mineral potential and the Ordovician timescale
Age and geochemistry of the Charlestown Group, Ireland: implications for the Grampian orogeny, its mineral potential and the Ordovician timescale

Accurately reconstructing the growth of continental margins during episodes of ocean closure has important implications for understanding the formation, preservation and location of mineral deposits in ancient orogens. The Charlestown Group of county Mayo, Ireland, forms an important yet understudied link in the Caledonian-Appalachian orogenic belt located between the well documented sectors of western Ireland and Northern Ireland. We have reassessed its role in the Ordovician Grampian orogeny, based on new fieldwork, high-resolution airborne geophysics, graptolite biostratigraphy, U–Pb zircon dating, whole rock geochemistry, and an examination of historic drillcore from across the volcanic inlier. The Charlestown Group can be divided into three formations: Horan, Carracastle, and Tawnyinah. The Horan Formation comprises a mixed sequence of tholeiitic to calc-alkaline basalt, crystal tuff and sedimentary rocks (e.g. black shale, chert), forming within an evolving peri-Laurentian affinity island arc. The presence of graptolites Pseudisograptus of the manubriatus group and the discovery of Exigraptus uniformis and Skiagraptus gnomonicus favour a latest Dapingian (i.e. Yapeenian Ya 2/late Arenig) age for the Horan Formation (equivalent to c. 471.2–470.5 Ma according to the timescale of Sadler et al., 2009). Together with three new U–Pb zircon ages of 471.95–470.82 Ma from enclosing felsic tuffs and volcanic breccias, this fauna provides an important new constraint for calibrating the Middle Ordovician timescale. Overlying deposits of the Carracastle and Tawnyinah formations are dominated by LILE- and LREE-enriched calc-alkaline andesitic tuffs and flows, coarse volcanic breccias and quartz-feldspar porphyritic intrusive rocks, overlain by more silicic tuffs and volcanic breccias with rare occurrences of sedimentary rocks. The relatively young age for the Charlestown Group in the Grampian orogeny, coupled with high Th/Yb and zircon inheritance (c. 2.7 Ga) in intrusive rocks indicate that the arc was founded upon continental crust (either composite Laurentian margin or microcontinental block). Regional correlation is best fitted to an association with the post-subduction flip volcanic/intrusive rocks of the Irish Caledonides, specifically the late-stage development of the Tyrone Igneous Complex, intrusive rocks of Connemara (western Ireland) and the Slishwood Division (Co. Sligo). Examination of breccia textures and mineralization across the volcanic inlier questions the previous porphyry hypothesis for the genesis of the Charlestown Cu deposit, which are more consistent with a volcanogenic massive sulfide (VMS) deposit.

Appalachian, Biostratigraphy, Caledonian, Grampian, Graptolite, Taconic, Timescale (calibration), U–Pb zircon, VMS
0024-4937
302-303
Herrington, Richard J.
118a02cb-b04d-4110-a96a-a227ae6ccb8f
Hollis, Steven P.
8a26f76c-3b3d-4767-b8d4-146e952af0d0
Cooper, Mark R.
68d782fa-c882-41e0-a21e-b1a88bf34f99
Stobbs, Iain
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Tapster, Simon
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Rushton, Adrian
fdb0908c-291b-499c-900c-7e07291db811
McConnell, Brian
446d2164-c8b0-4bab-847e-04ad4e890fd2
Jeffries, Teresa
d1661e6a-9c9b-437a-9a0c-cd187fbadf32
Herrington, Richard J.
118a02cb-b04d-4110-a96a-a227ae6ccb8f
Hollis, Steven P.
8a26f76c-3b3d-4767-b8d4-146e952af0d0
Cooper, Mark R.
68d782fa-c882-41e0-a21e-b1a88bf34f99
Stobbs, Iain
bd12fa30-0bdd-4d15-912c-b47de14e260c
Tapster, Simon
e452030a-530c-4c2c-8709-22c03caf1c97
Rushton, Adrian
fdb0908c-291b-499c-900c-7e07291db811
McConnell, Brian
446d2164-c8b0-4bab-847e-04ad4e890fd2
Jeffries, Teresa
d1661e6a-9c9b-437a-9a0c-cd187fbadf32

Herrington, Richard J., Hollis, Steven P., Cooper, Mark R., Stobbs, Iain, Tapster, Simon, Rushton, Adrian, McConnell, Brian and Jeffries, Teresa (2018) Age and geochemistry of the Charlestown Group, Ireland: implications for the Grampian orogeny, its mineral potential and the Ordovician timescale. Lithos, 302-303, 302-303. (doi:10.1016/j.lithos.2017.12.012).

Record type: Article

Abstract

Accurately reconstructing the growth of continental margins during episodes of ocean closure has important implications for understanding the formation, preservation and location of mineral deposits in ancient orogens. The Charlestown Group of county Mayo, Ireland, forms an important yet understudied link in the Caledonian-Appalachian orogenic belt located between the well documented sectors of western Ireland and Northern Ireland. We have reassessed its role in the Ordovician Grampian orogeny, based on new fieldwork, high-resolution airborne geophysics, graptolite biostratigraphy, U–Pb zircon dating, whole rock geochemistry, and an examination of historic drillcore from across the volcanic inlier. The Charlestown Group can be divided into three formations: Horan, Carracastle, and Tawnyinah. The Horan Formation comprises a mixed sequence of tholeiitic to calc-alkaline basalt, crystal tuff and sedimentary rocks (e.g. black shale, chert), forming within an evolving peri-Laurentian affinity island arc. The presence of graptolites Pseudisograptus of the manubriatus group and the discovery of Exigraptus uniformis and Skiagraptus gnomonicus favour a latest Dapingian (i.e. Yapeenian Ya 2/late Arenig) age for the Horan Formation (equivalent to c. 471.2–470.5 Ma according to the timescale of Sadler et al., 2009). Together with three new U–Pb zircon ages of 471.95–470.82 Ma from enclosing felsic tuffs and volcanic breccias, this fauna provides an important new constraint for calibrating the Middle Ordovician timescale. Overlying deposits of the Carracastle and Tawnyinah formations are dominated by LILE- and LREE-enriched calc-alkaline andesitic tuffs and flows, coarse volcanic breccias and quartz-feldspar porphyritic intrusive rocks, overlain by more silicic tuffs and volcanic breccias with rare occurrences of sedimentary rocks. The relatively young age for the Charlestown Group in the Grampian orogeny, coupled with high Th/Yb and zircon inheritance (c. 2.7 Ga) in intrusive rocks indicate that the arc was founded upon continental crust (either composite Laurentian margin or microcontinental block). Regional correlation is best fitted to an association with the post-subduction flip volcanic/intrusive rocks of the Irish Caledonides, specifically the late-stage development of the Tyrone Igneous Complex, intrusive rocks of Connemara (western Ireland) and the Slishwood Division (Co. Sligo). Examination of breccia textures and mineralization across the volcanic inlier questions the previous porphyry hypothesis for the genesis of the Charlestown Cu deposit, which are more consistent with a volcanogenic massive sulfide (VMS) deposit.

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More information

Accepted/In Press date: 18 December 2017
e-pub ahead of print date: 24 December 2017
Published date: 1 March 2018
Keywords: Appalachian, Biostratigraphy, Caledonian, Grampian, Graptolite, Taconic, Timescale (calibration), U–Pb zircon, VMS

Identifiers

Local EPrints ID: 420477
URI: http://eprints.soton.ac.uk/id/eprint/420477
ISSN: 0024-4937
PURE UUID: 34e01692-9507-4199-981b-730a99eb2b6d

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Date deposited: 08 May 2018 16:30
Last modified: 05 Mar 2019 17:31

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Contributors

Author: Richard J. Herrington
Author: Steven P. Hollis
Author: Mark R. Cooper
Author: Iain Stobbs
Author: Simon Tapster
Author: Adrian Rushton
Author: Brian McConnell
Author: Teresa Jeffries

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