The structure, fabrics and AMS of the Slieve Gullion ring-complex, Northern Ireland: testing the ring-dyke emplacement model
Carl T. E. Stevenson, Brian O'Driscoll, Eoghan P. Holohan, Rebecca Couchman, R. John Reavy, Graham D. M. Andrews, 2008. "The structure, fabrics and AMS of the Slieve Gullion ring-complex, Northern Ireland: testing the ring-dyke emplacement model", Structure and Emplacement of High-Level Magmatic Systems, K. Thomson, N. Petford
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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.
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Structure and Emplacement of High-Level Magmatic Systems
There are continual rounds of annual conferences, special sessions and other symposia that provide ample opportunity for researchers to convene and discuss igneous processes. However, the origins of laccoliths and sills continue to inspire and confound geologists.
In one sense, this is surprising. After all, don’t we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this volume, the answer is clearly ‘no’.
This book contains contributions on physical geology, igneous petrology, volcanology, structural geology, crustal mechanics and geophysics that cover the entire gambit of geological processes associated with the shallow emplacement of magma. High-level intrusions in sedimentary basins can also act as hydrocarbon reservoirs and as sources for thermal maturation.
In drawing together a diversity of perspectives on the emplacement of sills, laccoliths and dykes we hope to advance further our understanding of their behaviour.