Elsevier

Lithos

Volumes 302–303, March 2018, Pages 1-19
Lithos

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

https://doi.org/10.1016/j.lithos.2017.12.012Get rights and content

Highlights

  • New precise U–Pb zircon dates for this important terrane in the Caledonian-Appalachian orogenic belt.

  • Findings provide important new constraint to calibrating the Middle Ordovician timescale.

  • The young age, inherited zircons and Th/Yb data all favour formation on continental crust, likely as part of the complex Laurentian margin.

  • Deposit style and textures question the previous hypothesis for formation of the associated copper deposit.

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.

Introduction

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 (Herrington et al., 2017, Herrington and Brown, 2011, Rogers et al., 2007, van Staal, 2007). Orthomagmatic and volcanogenic massive sulfide (VMS) deposits may be preserved in accreted oceanic tracts or rifted island arcs (Herrington et al., 2005, Piercey, 2011), whereas mesothermal gold mineralization typically forms during the later stages of orogenesis associated with orogenic collapse and deep-seated crustal structures (Herrington and Brown, 2011, Kerrich et al., 2005). An integrated approach using detailed field mapping, whole rock geochemistry, U–Pb zircon geochronology and biostratigraphy forms a powerful tool for unraveling complex orogens, and may highlight the prospectivity of accreted terranes for different styles of mineralization.

The Caledonian-Appalachian orogenic belt records the opening of the Iapetus ocean during the late Proterozoic (c. 565 Ma: van staal et al., 2014; Fig. 1a) and the events associated with its closure during the early Paleozoic (Chew et al., 2010, Cooper et al., 2013, Dewey, 2005, Draut et al., 2004). The Grampian event preserves the first major phase of this closure in the British and Irish Caledonides, and was associated with the accretion of ophiolites, island arcs and microcontinental blocks to the Laurentian margin between the Late Cambrian and Middle Ordovician (Chew et al., 2008, Chew et al., 2010, Cooper et al., 2011, Dewey and Mange, 1999, Dewey and Shackleton, 1984, Draut et al., 2004, Hollis et al., 2012). The Grampian is broadly equivalent to the Taconic event of the Canadian Appalachians (van Staal, 2007; Fig. 1b-c), with VMS deposits that developed in oceanic and arc/backarc settings (Piercey, 2007) emplaced during three phases of arc/ophiolite accretion (van Staal, 2007, van staal et al., 2014).

Fig. 2 shows the broad evolution of the Grampian orogeny as it applies to the current region of study, based on the three equivalent and well-documented arc/ophiolite accretion events in the Newfoundland Appalachians (modified after van Staal, 2007, van staal et al., 2014, Chew et al., 2010, Hollis et al., 2012). In the British and Irish Caledonides, the accretion of early c. 510–495 Ma suprasubduction affinity oceanic crust (e.g. Deer Park Complex, Highland Boundary ophiolite, Chew et al., 2010) occurred shortly after its formation, most likely onto outboard blocks of Laurentian-affinity microcontinetal crust (Fig. 2a; see Chew et al., 2010). As ocean closure continued, subduction was associated with the development of the juvenile c. 490–477 Ma Lough Nafooey arc system (i.e. Lough Nafooey Group: Ryan et al., 1980, Draut et al., 2004, Chew et al., 2007, Ryan and Dewey, 2011, McConnell et al., 2009; Figs. 1a, 2b), with its accretion to the Laurentian margin constrained to c. 484–477 Ma (Draut et al., 2004, Hollis et al., 2013a). Syncollisional volcanism during the deposition of the c. 477–468 Tourmakeady Group (Fig. 2c) was contemporaneous with Grampian deformation and metamorphism (c. 475–465 Ma; Friedrich et al., 1999a, Friedrich et al., 1999b, Chew et al., 2008). Following a reversal in subduction polarity (from south to northward directed), magmatic activity in western Ireland is recorded by the eruption of c. 464 Ma ignimbrites of the Murrisk Group (Dewey and Mange, 1999) and the continued emplacement of granitic rocks across the Connemara terrane (Friedrich et al., 1999a; Figs. 1a, 2d).

The c. 484–470 Ma Tyrone and Ballantrae arc-ophiolite complexes most likely record the development of an arc system distinct to that of western Ireland (Hollis et al., 2012, Hollis et al., 2013a, Hollis et al., 2013b, Stone, 2014; Figs. 1a, 2b, d). In the west of Ireland, the Lough Nafooey arc developed from c. 490 Ma above a south-dipping subduction zone away from the Laurentian margin (Fig. 2b), whilst the Tyrone and Ballantrae arcs which are much younger, were accreted later (Fig. 2d; Hollis et al., 2013a), and show evidence for widespread arc-rifting and VMS-style mineralization (Hollis et al., 2014). The accretion of these younger arcs to outboard fragments of microcontinental crust (such as the Tyrone Central Inlier and Midland Valley block; prior to c. 470 Ma in Tyrone) was followed by the emplacement of continental arc intrusive rocks across the composite Laurentian margin until at least c. 464 Ma in Ireland (Cooper et al., 2011, Cooper and Mitchell, 2004, Flowerdew et al., 2005) and c. 457 in Scotland (Carty et al., 2013, Oliver et al., 2000, Oliver et al., 2008). Remnants of peri-Laurentian affinity island arcs also occur along the Iapetus Suture zone south of the Southern Uplands – Down-Longford accretionary prism, such as at Grangegeeth (McConnell et al., 2010) (Fig. 1a).

The Charlestown Group, exposed across approximately 45 km2 of Co. Mayo, Ireland (Fig. 3), forms an important link between the well documented Grampian rocks of western Ireland (Clift and Ryan, 1994, Dewey and Mange, 1999, Draut et al., 2002, Draut et al., 2004) and Northern Ireland (Chew et al., 2008, Cooper et al., 2008, Cooper et al., 2011, Draut et al., 2009, Hollis et al., 2012, Hutton et al., 1985). Previous work has largely been restricted to field mapping and graptolite biostratigraphy (e.g. Cummins, 1954, Dewey et al., 1970, O'Connor, 1987), and consequently it was not clear whether the Charlestown Group formed during the syncollisional stage of the Lough Nafooey arc system (broadly correlating with the Tourmakeady Group; see review by Chew, 2009), or as part of the younger Tyrone arc system (Hollis et al., 2013a). The latter is considered prospective for VMS mineralization (Clifford et al., 1992, Hollis et al., 2014, Hollis et al., 2016, Peatfield, 2003) and its accretion to the Laurentian margin has been implicated in subsequent mesothermal Au mineralization in the overlying Dalradian Supergroup at Curraghinalt (Earls et al., 1996, Herrington and Brown, 2011, Parnell et al., 2000, Rice et al., 2016).

Here we present the results of new fieldwork, whole rock geochemistry, airborne geophysics from the Tellus Border project, and the first U–Pb zircon ages for the Charlestown Group. In addition, we have refined biostratigraphic age constraints based on a new graptolite locality and a reexamination of a fauna collected by Cummins (1954). This work has important implications for understanding the evolution of the Grampian orogeny and the calibration of the Middle Ordovician timescale. Implications for base metal mineralization in County Mayo are discussed based on examination of drillcore from across the volcanic inlier.

Section snippets

Previous work

Although the Charlestown Group forms an integral part of the Irish Caledonides, published and unpublished research is limited to a handful of studies (Charlesworth, 1960, Cummins, 1954, Dewey et al., 1970, Long et al., 2005, O'Connor, 1987, O'Connor and Poustie, 1986). The Charlestown Group is unconformably overlain to the east by Silurian cover sequences, which together form the Charlestown Inlier, and bounded to the south, north and west by Carboniferous rocks (Fig. 3a). Cummins (1954)

Sampling and methods

Major exposures described by O'Connor (1987) were visited during 2012 and 2013, with additional traverses made across the well exposed Knock Airport section of the pyroxene diorite and uppermost Horan Formation (Fig. 3a, locality C). The stratigraphy through this sequence (from 146991E 296428N to 146949E 296469N Irish Grid) is described below and shown in Fig. 5. Samples were collected from this section and elsewhere for petrography, whole rock geochemistry (Fig. 2b), U–Pb zircon CA-ID-TIMS

Geology of the Charlestown Group

The geology of the Charlestown Group has been comprehensively described by O'Connor (1987), as summarized in Section 2 above. Here we supplement that account with new observations from Tellus Border geophysics (Fig. 3b), the well exposed Knock Airport section of the Horan Formation (Fig. 5), diamond drillcore, and key outcrops in the Carracastle and Tawnyinah formations. These results have significant implications for understanding the genesis of the Charlestown Cu deposit.

Evolution of the Charlestown Group and implications for regional correlations

In Fig. 2 our current understanding of the Grampian event as it applies to the northern British and Irish Caledonides was presented. Early formation of the Deer Park and Highland Border ophiolites (between the outriding blocks and the Laurentian margin; Fig. 2a) was followed by obduction of these ophiolites (Chew et al., 2010), subduction reversal and the formation of the Lough Nafooey arc system from c. 490 Ma (Draut et al., 2004; Fig. 2b). Using the timescale of Sadler et al. (2009), ‘hard’

Conclusions

We have reassessed the role of the Charlestown Group in the context of the c. 474–465 Ma 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 has been divided into three formations: Horan, Carracastle, and Tawnyinah. The Horan Formation comprises a mixed sequence of tholeiitic to calc-alkaline basalt,

Acknowledgements

The authors would like thank Quentin Crowley (Trinity College Dublin) for zircon separation, Emma Williams and Stanislav Strekopytov (NHM) for helping with whole rock geochemistry, and Anton Kearsley and Will Brownscombe (NHM) for SEM assistance. Tom Mcintyre is also thanked for access to drillcore at the GSI. Part of this work formed Iain Stobbs' MSci thesis at Imperial College London. Stephen Daly and John Dewey are thanked for many thoughtful discussions on the geology of the Charlestown

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