D. M. Williams

A basin model for the Silurian of the Midland Valley of Scotland and Ireland

Identification of terranes is frequently based on the apparent disparity of source area for sediment supply, palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural. As an example of such superficially unrelated stratigraphies, the Silurian successions of the Midland Valley of Scotland and the northern part of Ireland are compared. A diachronous late Llandovery (early-middle Telychian) transgressive episode, which is earlier in the Midland Valley inliers, is common to many of the successions. The transgression resulted in the development of comparable sedimentary facies and fossil assemblages. The clast compositions of Llandovery and Wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement. Areas of active volcanism appear to have migrated in a north-westerly direction with time; a similar sequence is evident in Canada. It is suggested that the common features of the Scottish-Irish successions may be accommodated in a unified basin model for Silurian sedimentation and volcanism. Such a basin may have formed in an intra-arc environment and been controlled by oblique-slip fault mechanisms although evidence for the presence of a Silurian arc is equivocal.

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.

Short Paper: Stratigraphical correlations adjacent to the Highland Boundary fault in the west of Ireland

An Ordovician age for at least part of the Clew Bay Supercomplex, a dismembered ophiolite together with contiguous deep-water sediments and volcanics occurring adjacent to the Highland Boundary fault in western Ireland, supplement comparisons with the Highland Border Complex of Scotland based on lithological similarities. An early phase of terrane development associated with ophiolite emplacement is represented in western Ireland whilst the recognition of similar intra-terrane magnafacies patterns permit a more complete analysis of the evolution of the Highland Border terrane.

A late Caledonian melange in Ireland: implications for tectonic models

The Clew Bay area in western Ireland contains the remnants of a Caledonian terrane which separates the Dalradian of North Mayo from the Ordovician of South Mayo to the south. Rocks of the area have been variously interpreted as representing part of the Dalradian succession, as a Cambrian to Ordovician rifted margin to subduction related basin, and as a shear carpet derived from an accretionary wedge overridden by an ophiolite in the early Ordovician. The lithologies present include discontinuous outcrops of ultrabasic and basic rocks, together with quartzose and semipelitic schists on the south shore of Clew Bay. These are in contact to the north with a sedimentary and volcanic sequence exposed both on the mainland and on Clare Island for which a detailed stratigraphy had previously been established. We reinterpret this latter sequence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics. Although microfossil evidence had previously shown that a chert block on Clare Island was of Late Llanvirn age, new fossil control shows that this date must be extended upwards. Microfossils extracted from a variety of lithologies within the melange show that the cherts range from Middle Ordovician to at least Caradoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the Wenlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effects of a significant tectonic episode in the late Silurian of the British and Irish Caledonides.

Silurian turbidite provenance and the closure of Iapetus

Subduction of Iapetus crust during the Caledonian orogeny has been seen by some authors as terminating at the end of Ordovician times and by others as continuing through the Silurian. We present petrological and chemical data which support a coeval arc origin for detritus in the Lettergesh Formation (Silurian), western Ireland. The arc, based on continental crust, lay to the north of the present outcrops of this formation, along the Laurentian margin of Iapetus, implying active oceanic subduction in the Silurian. This subduction may have been followed by some continental underplating around the middle of the Wenlock giving rise to a successor basin in Ireland and Scotland.

The onset of the Permo-Carboniferous glaciation: reconciling global stratigraphic evidence with biogenic apatite δ18O records in the late Visean

The δ18O values of phosphatic microfossils recovered from NW Ireland are used to determine the timing and magnitude of cooling associated with the onset of the Carboniferous glaciation. Microfossil fish δ18Oapatite demonstrates a +2.4‰ (V-SMOW) shift, which, once corrected for δ18Oseawater changes owing to evolving ice volumes, equates to an approximate 4.5 °C reduction in equatorial sea surface temperature between the earliest Asbian and the mid-Brigantian (late Visean). Both conodont and microfossil fish δ18Oapatite indicate stabilization of an ‘icehouse’ climate during the Brigantian and into the Serpukhovian. Substantial late Visean cooling identified herein is in good agreement with global glacioeustatic records. Supplementary material: Further information on the palaeogeography, lithostratigraphy and palaeoenvironments of the sections examined as well as the sample composition and analytical methods of oxygen isotope analyses is available at http://www.geolsoc.org.uk/SUP18511.

Implications of new microfloral evidence from the Clew Bay Complex for Silurian relationships in the western Irish Caledonides

The supposed Laurentian margin in western Ireland is separated from an Ordovician arc-related basin by the Clew Bay Complex and two Silurian successions, informally termed the Croagh Patrick Silurian and the Louisburgh Silurian, whose relationships are enigmatic. Palynomorphs, tubular structures and sheets of cuticle from turbidites within the Clew Bay Complex indicate a Silurian (Wenlock) age. In the light of these new data and reappraisal of contacts, the Croagh Patrick Silurian is now considered to be in tectonic contact with the Clew Bay Complex. The Louisburgh Silurian may also be in tectonic contact with parts of the Clew Bay Complex and unconformable on other parts, but is probably younger than previously thought. The Silurian age of elements of the Clew Bay Complex, and the relationship of the complex with other Silurian successions in western Ireland, indicate that significant deformation occurred in this part of the Caledonides during mid to end Silurian times, and imply that significant Silurian-Devonian terrane movement took place.

Umbers, ocean crust and the Irish Caledonides: terrane transpression and the morphology of the Laurentian margin

The Clew Bay Complex of western Ireland contains a variety of rock types. These include schists possibly of Dalradian association, amphibolites and serpentinites of unknown age, cherts, volcanic rocks and graded gabbros of mid to upper Ordovician age and shales and sandstones of Silurian age. Intercalated within the cherts of the complex on Clare Island are chocolate-brown horizons of a porous rock, geochemically comparable to hydrothermal mid-ocean ridge metalliferous sediments (e.g. umbers). These cherts overlie volcanic rocks of MORB affinities with an island arc influence. The association exposed on Clare island of graded gabbros, MORB volcanic rocks, discontinuous carbonaceous horizons, cherts, umbers and deep water shales suggest the presence of a mid- to late Ordovician spreading centre now preserved as a dismembered ophiolite. The Clew Bay and Highland Border Complexes, both being parts of the Border Terrane, have many similarities including evidence of mid- to late Ordovician spreading, and it is suggested they formed in the same basin. However, they are dissimilar to both the South Mayo and Midland Valley Ordovician and Silurian rock associations, from which they are separated by a terrane boundary. The geochemical data and field associations do not indicate back-arc or intra-continental rifting. We suggest, therefore, that elements of older ocean crust became trapped inboard of a collided early Ordovician arc at re-entrants along the Laurentian margin. This trapped crust became the site of mid- to late Ordovician spreading. We also suggest that the site of this entrapment was to the northeast of the present positions of the Clew Bay and Highland Border Complexes and that they were emplaced into their present sites by end-Silurian transpression. This Laurentian margin morphology is compatible with that demonstrated for parts of the Newfoundland-Quebec sectors of the Appalachian sector of the orogen.

Silurian turbidites used to reconstruct a volcanic terrain and its Mesoproterozoic basement in the Irish Caledonides

The Lettergesh Formation, Co. Galway, Ireland, comprises a 1.5 km thick sequence of turbidites deposited c. 430 Ma ago during closure of the Iapetus Ocean. Petrographic and geochemical data show that the turbidites consist overwhelmingly of detritus derived from a young volcanic terrain. Palaeocurrent data indicate that the detritus was derived from the north. Modelling of the chemical compositions of the turbidites indicates provenance from a high-K calc-alkaline, subduction-related volcanic province built on continental crust. A subordinate ultrabasic component may represent detritus from either an ophiolite or tectonically emplaced serpentinite. The turbidites are characterized by low variability of initial ɛ Nd (−6.5 to −3.2) and highly variable initial 87Sr/86Sr ratios. Modelling of the isotopic data suggests a mean crustal residence age of 1440–1600 Ma for the continental foundation of the volcanic terrain. Such a model age is consistent with a mixture of crust having TDM ages of c. 1.9 Ga and c. 1.3 Ga, both of which are known from the Annagh Gneiss Complex of NW Ireland.

Turbidites from the Clew Bay Complex, Ireland: provenance based on petrography, geochemistry and crustal residence values

The Clew Bay Complex in the Irish Caledonides represents a Caledonian element separating the Irish Scottish Dalradian to the north from the South Mayo Ordovician arc basins to the south. The petrographic. geochemical and crustal residence characteristics of the Silurian sandstoncs within the complex are presented. Petrological analysis indicates that the sandstones were derived from a transitional continental provenance, possibly a passive continental margin or the cratonic flank of ;1 foreland basin. Whole rock geochemistry confirms this provenance type and demonstrates the absence of any significant ophiolite detritus. Sni-Nd model ages indicate a possible derivation from Upper Dalradian or Torridonian rocks, although a mixed provenance may also be considered. These data indicate distinct differences between these turhidites and those of the nearby North Galway succession and suggest that the Clew Bay Complex could be regarded as a suspect terrane with respect to at least parts of the South Mayo arc zone. (’; 1996 by John Wiley & Sons, Ltd.