South Connemara Research Articles

Result of an epidemiological study of sea lice infestation in South Connemara, West of Ireland

Sea lice infestation patterns within a bay in Connemara, Co Galway were investigated to elucidate sources of infestation pressure and dynamics of lateral and vertical transmission of infective stages of sea lice within and between sites in a series of complex embayments. Direct measurement of sea lice infestation using sentinel cages and 3D modelling of particle distribution were used to analyse sea lice distribution. The results indicate the potential value of sentinel cages for investigating sea lice transfer between locations within a bay system and shows the potential usefulness of hydrographic modelling in informing choices of site location and sea lice management strategies.

Long-lived granite-related molybdenite mineralization at Connemara, western Irish Caledonides

AbstractNew Re–Os age determinations from the Galway Granite (samples: KMG = 402.2 ± 1.1 Ma, LLG = 399.5 ± 1.7 Ma and GBM = 383.3 ± 1.1 Ma) show that in south Connemara, late Caledonian granite-related molybdenite mineralization extended from c. 423 Ma to c. 380 Ma. These events overlap and are in excellent agreement with the published granite emplacement history determined by U–Pb zircon geochronology. The spatial distribution of the late-Caledonian Connemara granites indicates that initial emplacement and molybdenite mineralization occurred at c. 420 Ma (that is, the Omey Granite and probably the Inish, Leterfrack and Roundstone granites) to the N and NW of the Skird Rocks Fault, an extension of the orogen-parallel Southern Uplands Fault in western Ireland. A generally southern and eastward progression of granite emplacement (and molybdenite mineralization) sited along the Skird Rocks Fault then followed, at c. 410 Ma (Roundstone Murvey and Carna granites), at c. 400 Ma (Errisbeg Townland Granite, Megacrystic Granite, Mingling Mixing Zone Granodiorite, Lough Lurgan Granite and Kilkieran Murvey Granite) and at c. 380 Ma (Costelloe Murvey Granite, Shannapheasteen and Knock granites). The duration of granite magmatism and mineralization in Connemara is similar to other sectors of the Appalachian–Caledonian orogeny and several tectonic processes (e.g. slab-breakoff, asthenospheric flow, transtension and decompression) may account for the duration and variety of granite magmatism of the western Irish Caledonides.

Caledonian strike-slip terrane accretion in W. Ireland: insights from very low-grade metamorphism (illite–chlorite crystallinity and b0parameter)

AbstractAnalysis of pelites with detrital white-micas in the Clew Bay–Galway Bay segment of the Irish Caledonides indicates that b0data from whole-rock and < 2 μm fractions generally show differences smaller than the errors of the method, irrespective of (001) illite crystallinity values, probably due to metamorphic recrystallization. Intermediate pressure metamorphism of the Ordovician–Silurian Clew Bay Group indicates slow subduction, allowing partial thermal re-equilibration before exhumation. In contrast, the Croagh Patrick Group Laurentian shelf-sediments underwent high-pressure alteration, suggesting rapid subduction/exhumation, synchronous with strike-slip faulting. The Murrisk Group, which underwent high-intermediate pressure metamorphism in an Ordovician back-arc, forms a separate terrane to the Croagh Patrick Group to the north and also to the Ordovician Lough Nafooey and Tourmakeady groups and Rosroe Formation in the south, in which low-intermediate pressure alteration occurred. These, together with the Silurian North Galway Group, may have undergone heating due to movement over or deposition on the hot Gowlaun Detachment as the Connemara Dalradian was exhumed. The South Connemara Group also underwent a high-pressure alteration, consistent with its inferred subduction environment. Evidence of contact alteration, due to known or inferred buried late- to post-Caledonian granitoid plutons, has been found in the Clew Bay, Louisburg–Clare Island, Croagh Patrick, Murrisk and South Connemara groups. These show evidence of lower-pressure alteration than the surrounding country-rocks.

The Occurrence of <i>Anguillicola crassus</i> (Kuwahar, Nimi, And Hagaki, 1974), an Introduced Nematode, in an Unexploited Western Irish Eel Population

The presence of Anguillicola crassus, an introduced Asian parasite of eels, into south Connemara is reported. European eels Anguilla anguilla were found to be infected in the three basins of varying salinity of Lough Ahalia, but eels sampled in the nearby marine Camus Bay were uninfected. Variations in infection parameters were analysed. It was concluded that the parasite is a recent

The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland

The Caledonian geology of western Ireland records the collision of two arc complexes with the Laurentian Margin during the closure of the Iapetus Ocean. An earlier complex collided with this hitherto passive margin in the mid-Ordovician during the Grampian Orogeny. Subsequently, arc magmatism developed along the Laurentian margin and continued until the late Silurian collision between Laurentian and Avalonia. The Ordovician volcanic and sedimentary rocks comprising the South Connemara Group lie along the Southern Uplands Fault, the terrane boundary separating these two arc complexes. Palaeontological dating indicates an Arenig-Llanvirn age for part of this complex (Williams, Armstrong and Harper, 1988), making it contemporaneous with the earlier arcs. However, most authors correlate this complex with the northern belt of the Southern Uplands (Morris, 1983; Williams, D.M., 1984. The stratigraphy and sedimentology of the Ordovician Party Group, south-eastern Murrisk, Ireland. Geological Journal, 19, 173–186; Williams et al., 1988), associated with post-Grampian subduction of north directed polarity. We present new field evidence that the South Connemara Group is tectonically disrupted by bedding parallel shear zones and that contacts previously interpreted as conformable are marked by units of tectonic mélange. We present structural and provenance arguments consistent with the mélanges forming above a north-dipping subduction zone after 463Ma. This Group is reinterpreted as occurring within a subduction–accretion complex that was generated by the accretion of early Ordovician mafic seamounts into a post-Grampian trench, thus reconciling the age of the Group with its generally accepted tectonic setting. We discuss the regional significance of this finding with respect to the Caledonide-Appalachian orogeny and argue that this is the site along which the Iapetus Ocean closed.

Siluro-Devonian palaeomagnetism, terrane emplacement and rotation in the Caledonides of western Ireland

SUMMARY The palaeomagnetism of a Silurian succession (Lough Mask Formation lavas and sediments) and intrusive rocks (Kilbride dolerites and andesites and microgranodiorites, Killary Harbour area) in the Connemara-Mayo segment of the Irish Caledonides is described. Magnetizations are related to the regional tectonic history with the aid of fold, conglomerate and contact tests. They describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this Caledonian inlier. The shallow components appear to pre-date Siluro-Devonian folding while the intermediate ones post-date this episode. The change in palaeofield inclination is identical to the transition described by contemporaneous deep level (Silurian) to shallow and surface level (Lower Devonian) bodies in the British and Scandinavian Caledonides but declinations are rotated consistently to the west. The Lower-Middle Silurian vector is rotated clockwise by c. 100. This difference is progressively reduced to c. 50 by the time of the Siluro-Devonian D2 folding, showing that this fold generation (now oriented E-W) developed in parallelism with contemporaneous folds on the NE strike continuation of the Caledonides. Post-folding remanences continue to deviate in a clockwise sense from the remainder of the Caledonides showing that rotation continued into Middle-Upper Devonian times and finally ceased during Carboniferous times. The integrated effect of this rotation is recognized in Ordovician magnetizations from the Dalradian metamorphic terrane of south Connemara. It can be explained by block rotations within a zone of distributed deformation according to the McKenzie-Jackson model and illustrated by many neovolcanic zones. Palaeomagnetism defines a tectonic regime commencing with rotations of c. 3Myr-' and lasting from mid-Silurian to Carboniferous times. Western Ireland is sited within a closure gap along the Iapetus suture extending from Ireland to Newfoundland, and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the Caledonian orogen.

End-Silurian modifications of Ordovician terranes in western Ireland

The Ordovician elements in the Caledonides of western Ireland may be divided into the Clew Bay, South Mayo, North Connemara and South Connemara sub-zones. Evidence suggests the presence of a number of proximal suspect terranes, and the remnants of one, or possibly two, arc complexes. Palaeontological and sedimentary evidence suggests an allochthonous relationship between the Silurian rocks of Croagh Patrick and North Galway. These successions may therefore not represent a single sedimentary overlap to the Ordovician suspect terranes. End-Silurian deformation was the result of transpression, which was responsible in part for the present configuration of the area’s geology. Deep seated shear zones, with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting, northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north. The end-Silurian event is intermediate between the high grade metamorphism in parts of the Appalachians and the non-metamorphic Silurian sequences of Scotland.

A geological and tectonic cross-section of the Caledonides of western Ireland

Geological and geophysical criteria allow division of the Caledonides of western Ireland into four principal zones. Zone 1, in the north, contains Grenvillian basement with late Proterozoic supra-crustals. Zone 2 contains the Westport Complex, the Deer Park Complex and the South Mayo Trough. Connemara and the Delany Dome Formation occupy Zone 3. Zone 4 contains the South Connemara Group and mid- to late-Ordovician and Silurian sediments and volcanic rocks. The various tectonic models proposed for these zones are discussed and a ‘minimum’ tectonic model is presented, which involves early Ordovician collision of the northern part of Zone 2 with the Laurentian margin (Zone 1) followed by subduction flip (northern part of Zone 4). The site of early collision between Zones 1 and 2 subsequently underwent orogenic collapse and strike-slip dismemberment leading to the sinistral transpressive emplacement of Zone 3. Assemblages in Zone 4 were accreted to the now south-facing arc. Northwards stress transfer consequent on suturing deformed the thinned crust and overlying Silurian sequences in Zones 2 and 3.

The age of the South Connemara Group, Ireland, and its relationship to the Southern Uplands Zone of Scotland and Ireland

Synopsis The South Connemara Group lies to the south of the Dalradian rocks of Connemara. It consists of approximately 3 km of interbedded pillow lavas, cherts, sandstones and conglomerates with significant amphibolite intrusives. Hitherto its age has been unknown although it has been contact metamorphosed by the Devonian Galway Granite. New palaeontological evidence suggests an early Ordovician age for the sequence. The petrofacies of the clastic units within the group, its structural style, age and tectonic position suggest similarities with parts of the Southern Uplands of Scotland and its equivalents in Ireland. The South Connemara Group bears greatest similarities to Tract 2 of the Southern Uplands. The age of the group supports the apparent SW to NE diachronism of clastic sedimentation in this part of the Caledonides. The proposed correlation imposes a constraint on the position of the Southern Upland Fault in Ireland.

Pre‐Caledonian basement in Ireland and its cover relationships

AbstractPre‐Caledonian basement is exposed in three areas within the Irish orthotectonic Caledonides: 1. In northwest County Mayo the Erris Complex comprises the Annagh Division, which is largely Grenvillian but includes older gneisses, and the Inishkea Division, which is probably equivalent to the older Moines of Scotland. 2. In the northeast Ox Mountains, Rosses Point and Lough Derg inliers there is a granulite fades, dominantly metasedimentary basement which is probably late Grenville in age. 3. Laxfordian and probably older gneisses are seen in the Inishtrahull Platform northeast of Malin Head, County Donegal.In addition, some gneisses within the belt of ‘Connemara migmatites’ in south Connemara may be fragments of allochthonous basement and some high grade metamorphic rocks in the Tyrone Central inlier have been referred to as pre‐Caledonian basement, but there is at present no conclusive support for these suggestions. The Ox Mountains succession seen in the Clew Bay region and the southwest and central Ox Mountains is probably not pre‐Caledonian basement.Basement in the paratectonic Caledonides is confined to the Cadomian and possibly pre‐Cadomian Rosslare Complex (older than 625 Ma) and the nearby late Precambrian (to early Cambrian ?) Cuilenstown Formation in southeast County Wexford. In addition, an unseen block of basement defined by magnetics immediately to the north of the Dingle peninsula of County Kerry is probably of Rosslare Complex affinity. Granulite facies xenoiiths recovered from volcanic rocks of Carboniferous age about 70 km to the west of Dublin, indicate the character of basement there, though its age is not proven.All contacts between basement and Lower Palaeozoic Caledonian cover in the orthotectonic Caledonides are synmetamorphic slide zones and faults and in the paratectonic Caledonides the contacts are either faults, which are often rooted in older mylonites, or are unexposed and presumed to be major structural breaks.

The sedimentology of the South Connemara Group, western Ireland – a possible Ordovician trench-fill sequence

AbstractThe stratigraphy of the ?Ordovician South Connemara Group is described. The group is approximately 3 km thick and includes sedimentary and volcanic rocks. The sedimentary elements of the succession include conglomerates, sandstones, shales, cherts and minor limestone breccias. The Lettermullen Formation exhibits the thickest continuous sedimentary sequence in the group. It is divided into five facies: a conglomerate facies, thick sandstone facies, medium sandstone facies, sandstone/siltstone facies, and chert facies. The sedimentary features of these rocks include channelling and grading and are described in terms of Bouma units where applicable. The coarse elements of the formation are thought to represent deposition in, or proximal to, submarine channels feeding a more distal depositional site represented by the finer elements. Igneous rocks of the group include pillowed and non-pillowed lavas, massive amphibolites and minor tuffs often in conformable contact with the sediments. Petrography of conglomerate clasts and sandstones indicates derivation from a continental source area. Palaeocurrent analysis reveals a primary easterly mode with a subsidiary northerly mode. It is suggested that the group exhibits some characteristics of trench-fill sequences and the succession may be related to similar environments within the Southern Uplands accretionary wedge.

The petrochemistry of the basic volcanic rocks of the South Connemara Group (Ordovician), western Ireland

Summary16 samples of Ordovician basic volcanic rocks of the South Connemara Group, which abut the southern side of the metamorphic rocks of the Connemara massif in western Ireland, have been analysed for both major and trace elements. Although subject to low grade regional metamorphism and subsequently hornfelsed by the Galway Granite (400 Ma), their immobile element contents do not appear to be significantly disturbed. These elements characterise the metabasites of the South Connemara Group as ocean floor basalts having their origins in a marginal basin. The Skird Rocks Fault, separating the South Connemara Group from high grade metamorphic rocks of the Connemara massif, is consequently regarded as the northern margin of the vestiges of the lapetus Ocean which can be traced into, and along, the Southern Uplands Fault.

Palaeozoic volcanism in Great Britain and Ireland

IT IS APPARENT that the study of Palaeozoic volcanism attracts a multi-disciplinary approach involving sedimentologists as closely as volcanologists, and utilizes techniques ranging from field mapping to the statistical handling of geochemical data. This interdependence was revealed by the list of contributions and even more by the discussion stimulated by the papers read at the September 1976 meeting of the Volcanic Studies Group, held in the Geology Department, Trinity College, Dublin. It is also clear that for a fair proportion of workers in both Caledonian and Hercynian volcanism, plate-tectonic models are now being invoked both to explain the petrogenesis of volcanic suites and to account for regional variations in petrochemistry. Speakers who eschewed this 'armchair' approach were concerned with regional field aspects and emphasized the need for detailed field mapping as a basis for any interpretation. Dr Max described work in progress on the correlation, distribution and deformation of the South Connemara Group volcanics, including the extension of land-based studies by surveys of islands in Galway Bay. Dr Howells demonstrated the relationship between the Bedded Pyroclastic Formation of central Snowdonia, and the Middle Crafnant Volcanic Formation of eastern Snowdonia, and showed that, whilst synchronous, two very different volcanic facies derived their material from different sources––the Bedded Pyroclastics from the Snowdon area and the Middle Crafnants from sources east of Betwsy Coed. In the region of Dolwyddelan a rising submarine ridge is invoked to provide separation between the shallow-water, bedded basaltic tuff sequence of the former and the acid

Stratigraphy, structure and metamorphism of Dalradian rocks of the Maumturk Mountains, Connemara, Ireland

Appin (lower) and Argyll (middle) Dalradian metasediments and associated igneous rocks are described and a formal stratigraphy proposed. The deformation history has four main phases, D 1 to D 4 . Major "nappe" structures, formed during D 1 and D 2 , were refolded by two phases of approximately coaxial crossfolding developed during D 3 and D 4 . Two of the major F 4 cross-folds, the Connemara Antiform and Joyce's Synform, superficially dominate the structure of the area. The main penetrative fabric in the rocks, an axial planar schistosity, was developed during D 2 . The deformations were accompanied by metamorphism that reached its peak (in the Maumturks) between D 2 and D 3 (MP 2 ). The rocks were metamorphosed in the amphibolite facies and show an increase in grade from north to south across the area. It is suggested that the prograde MP 2 metamorphism, with coeval growth of staurolite and sillimanite, was partly caused by heat emanating from the complex of basic, ultrabasic and migmatitic rocks located during MP 2 in the upper limb of the F 1 Lissoughter Anticline, structurally above the present day outcrop in the Maumturks and now occurring mainly in south Connemara.

Deformation and metamorphism of Dalradian rocks and the evolution of the Connemara Cordillera

Dalradian metasediments in the Cur district of north-east Connemara have had a complex history of deformation and metamorphism. The earliest schistosity present formed during biotite-grade metamorphism and may be mimetic after a compaction fabric. Subsequently, the rocks attained garnet grade and major D 2 folds developed. This deformation involved an initial buckling followed by a coaxial homogeneous flattening and the structures produced were probably initially upright. After D 2 , staurolite, and locally kyanite, grew under moderately high pressure conditions. Continued increase in metamorphic grade was, however, apparently accompanied by regional uplift and erosion, for staurolite breakdown occurred at lower pressures than those required to form kyanite. Furthermore, the highest grade metamorphism was accompanied by a steepening of the thermal gradient, since breakdown of the assemblage staurolite + muscovite + quartz produced andalusite at high structural levels, but sillimanite deeper down. D 3 deformation began after the initial uplift at the peak of metamorphism and produced two major northward-facing nappes thrust over a basal fold-nappe. The nappes root to the south of the Corcogemore Mountains but continued uplift in south Connemara faulted out the root zones as later nappes developed. After cooling, broad open D 4 folds were formed and the Connemara Schists were thrust up and to the south over lower grade rocks. Uplift of the Connemara region may have been complementary to the subsidence of the Mayo Trough to the north, in which case the oldest Ordovician rocks in South Mayo may have been deposited at the same time that the peak of metamorphism was attained in Connemara.

New Light on the Dawros Peridotite, Connemara, Ireland

AbstractIt is established that the Dawros peridotite of north Connemara contains basic plagioclase mainly in a gabbroic layer in the peridotite but occasionally as rare crystals in the peridotite itself. Carlsbadalbite twin extinction angles indicate that the composition of this feldspar is similar to that in the adjoining Currywongaun-Doughruagh intrusion and that in the ultrabasic rocks of the Cashel-Lough Wheelaun intrusion and the Roundstone intrusion of south Connemara. As all these feldspars are extremely heavily saussuritized because of metamorphism it is difficult to determine their precise composition and so an exceptionally fresh plagioclase from an anorthosite in the Currywongaun-Doughruagh intrusion has been chemically analysed. It is An91·5Ab6·8Or1·7. This is in close agreement with the composition given by measuring 2θ(131)–2θ(131), the refractive indices and 2V while the extinction angles of Carlsbadalbite twins give only a slightly too anorthitic result (An95). The particularly calcic nature of the plagioclase is strong evidence that all the ultrabasic bodies in Connemara are genetically related. Some new structural ideas on the Dawros body are suggested which integrate the structure, petrography and cryptic variation.

The Geology of the Islands of South Connemara

AbstractThe exposed part of the metamorphic envelope on the south side of the Galway granite is described. It is characterized by the presence of pillow lavas and a thick greywacke series containing intraforma-tional conglomerates. This South Connemara Series is tentatively assigned to the Ordovician; such a correlation would require the Galway granite to be a laconic intrusion.