The structure, fabrics and AMS of the Slieve Gullion ring-complex, Northern Ireland: testing the ring-dyke emplacement model

Abstract

A structural investigation of the Slieve Gullion ring-complex, part of the approximately 56 Ma Slieve Gullion Igneous Centre, County Armagh, Northern Ireland was carried out with a view to testing the ring-dyke emplacement mechanism. This investigation involved the detailed examination and mapping of critical field relationships and the measurement of visible and magnetic fabrics, within the porphyritic rhyolite (felsite) and the porphyritic granite (granophyre) parts of the ring-complex. Set against existing theories for the emplacement of this complex, our investigation failed to find steep outward-dipping fabrics and lineations that would support the emplacement of this ring-complex as a ring-dyke. Instead, we propose that the ring-complex was emplaced as a series of extrusive and intrusive subhorizontal sheets, controlled by a circular zone of deformation, and subsequently domed by the emplacement of the younger central complex. From its gently dipping bulk geometries and a disharmonically folded eutaxitic internal fabric (supported by AMS -anisotropy of magnetic susceptibilty), the earlier rhyolite is reinterpreted as a pyroclastic deposit. The rhyolite was probably deposited against the wall of a subsiding caldera and is now preserved in the SW quadrant of the complex. From primary intrusive contact geometries with pre-Palaeogene country rocks, magnetic fabrics and subtle visible foliations -all of which are gently dipping -the younger and more extensive granitic ring is suggested to have initially been a subhorizontal sheet that is now domed. Only its gently outward-dipping floor is exposed around the ring-complex, and this is for much of its circumference bounded by a circular zone of deformation -a ring-fault. This study highlights the importance of detailed structural investigation in assessing the emplacement of igneous ring-complexes, emphasizing the need to look further than a simple ring-dyke emplacement model.

Sedimentary and volcano-tectonic processes in the British Paleocene Igneous Province: a review

“…Future field investigations might be complemented by application of techniques such as Anisotropy of Magnetic Susceptibility, to study, for example, ignimbrite flow directions and 'ring-dyke' emplacement (e.g. Ort et al 2003;O'Driscoll et al 2006;Stevenson et al 2007Stevenson et al , 2008. Isotopic fingerprinting of 'exotic' pyroclastic deposits, or igneous clasts within sedimentary deposits, may help to trace their centre of origin, reconstruct drainage pathways, identify tectonic controls (e.g.…”

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…The Cloghoge pluton also hosts a number of Paleogene subvol canic rocks, termed the Slieve Gullion complex ( Fig. 1; Richey, 1927;Neeson, 1984;Cooper and Johnston, 2004b;Stevenson et al, 2008).…”

“…Several studies on granitic emplacement have utilized anisotropy of magnetic susceptibility (AMS) data (e.g., Tarling and Hrouda, 1993;Petford et al, 2000;Mamtani and Greiling, 2005;Stevenson et al, 2007aStevenson et al, , 2007bStevenson et al, , 2008Wei et al, 2014;McCarthy et al, 2015;Morgan et al, 2017). This is because the technique can provide unique forms of structural data, revealing subtle or linear fabrics that are not apparent in the field, as well as providing quantitative fabric informa tion (Owens, 1974;Jelinek, 1981;Stevenson et al, 2007aStevenson et al, , 2007b.…”

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

“…15D). Portanto, não representam diques ou intrusões anelares, dado que a definição original de tais termos inclui critérios descritivos e genéticos, específicos do momento da subsidência (Roberts 1970;Stevenson et al 2008). Sua geometria é consistente com episódios finais na câmara magmática, quando intrusões anulares ressurgentes reativam falhas periféricas ou antigos diques anelares (Johnson et al 2002;Saunders 2005;Kennedy & Stix 2007).…”