Late Caledonian granitoids and timing of deformation in the Iapetus suture zone of eastern Ireland

Abstract

Late Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture Mine' in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…Terrestrial Old Red Sandstone (ORS) facies dominate with placer and fluvial sequences, whilst marine facies began to develop by the late Upper Devonian (Clayton et al, 1980;MacCarthy, 1990). Syn-to post-orogenic intrusions (for example, the Donegal, Leinster, Newry and Corvock granites) associated with the final closure of the Iapetus Ocean and the late stages of the Caledonian Orogeny (also known as the Acadian Orogeny) were emplaced during the Silurian and Early Devonian, c.430-380 Ma (Chew and Stillman, 2009;Halliday et al, 1980;Murphy, 1987;O'Connor, 1975O'Connor, , 1989O'Connor and Brück, 1978;). The late Upper Devonian marine transgression continued into the Lower Carboniferous when shale and siltstone deposits from the Lower Tournasian were overlain by thick limestone sequences that host important Pb-Zn deposits (Banks et al, 2002;LeHuray et al, 1987;Wilkinson, 2003).…”

“…the Mournes Mountains) through remobilisation of preexisting orogenic mineralisation (see Moles et al, 2013). In relation to the Mourne Mountains placer gold, the radiogenic Pb is likely to have been derived from the Lower Palaeozoic sediments of the Southern Uplands which were derived in part from the more radiogenic SET crust (Hutton and Murphy, 1987;Murphy, 1987).…”

U-Pb zircon geochronology of silicic tuffs and chronostratigraphy of the earliest Old Red Sandstone in the Munster Basin, SW Ireland

“…The hachured zone shows the suggested timing of late Caledonian (Acadian) deformation based on information given by Murphy (1987) and McKerrow (1988). The 410-390 Ma range bar depicts the time span of syn-Acadian pluton intrusion (Murphy 1987); the median age of c. 400 Ma is widely considered to date the peak Acadian (DI) event (e.g. Murphy 1987).…”

“…The 410-390 Ma range bar depicts the time span of syn-Acadian pluton intrusion (Murphy 1987); the median age of c. 400 Ma is widely considered to date the peak Acadian (DI) event (e.g. Murphy 1987). The chronostratigraphy of the alluvial fan lithosomes within the Valentia Slate and Bailinskelligs Sandstone Formations of NW lveragh (filled triangles) is not known in detail.…”

“…The numerical age of earliest ORS in the Munster Basin remains to be specified, but must be older than the Killeen and Keel-Enagh ages. The Munster Basin must have initiated subsequent to (1) the end of late Caledonian (Acadian) deformation, probably given by syntectonic granite intrusion at c. 400 Ma in the Leinster massif and eastern Ireland (Murphy 1987), and (2) the deposition of the post-Acadian Caherbla Group in the southern part of the Dingle Basin (Fig. 18).…”