Mineralogical Magazine, October 2004, Vol. 68(5), pp. 725–738 IntrusivehistoryoftheSlieveGullionringdyke,Ireland: implications for the internal structure of silicic sub-caldera magma chambers 1 1, 2 3 1 1 S. MCDONNELL ,V.R.TROLL *, C. H. EMELEUS ,I.G.MEIGHAN ,D.BROCK AND R. J. GOULD 1 Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland 2 Department of Earth Sciences, University of Durham, DH1 3LE, UK 3 Institute of Lifelong Learning, Queen’s University Belfast, Belfast BT7 1NN, UK ABSTRACT The Palaeogene Slieve Gullion Igneous Complex comprises a layered central intrusion surrounded by a slightly older ring dyke. The ring dyke contains two major intrusive rock types. About 70% of the ring dyke is occupied by porphyritic granophyre and 30% by porphyritic felsite. Locally complex relationships between the two lithologies are observed. Major and trace element compositions suggest that there are two distinct chemical groups within each lithology: a Si-rich felsite, concentrated in a ~1 m wide zone at the outer margins of the dyke which grades into a less Si-rich felsite towards the interior. Similarly, a Si-rich granophyre, concentrated in the centre of the intrusion grades outwards into a Si-poor granophyre facies. These rock relationships and geochemical variations suggest that a complex magma chamber hosted a stratified granitic magma body and various wall/floor magma facies. Low density, high-Si felsite magma from the top of the chamber was tapped first, followed by less Si-rich felsite magma as evacuation proceeded. The granophyres probably originate from the chamber walls/floor, representing more mushy equivalents of the felsite magma. Little granophyre magma was tapped during the early stages of the evacuation sequence. As evacuation continued, probably aided by trap-door caldera collapse, the ‘granophyre magmas’ intruded the already emplaced and slightly cooled felsite, forming the complexly zoned structure of the Slieve Gullion ring intrusion. KEYWORDS: Slieve Gullion, ring dykes, magma chamber dynamics, caldera collapse. Introduction with the majority of these becoming more silicic corewards (‘normally zoned intrusion’; cf. RING DYKES are cylindrical sheet intrusions that Fridrich and Mahood, 1984) but reverse zonations form whenmagma rises alonga steep, usually have also been documented, e.g. in the Sande outward-dipping, ring fracture. They commonly cauldron, Norway and the Little Chief granite, delimit central collapsed blocks and display a sub- California (cf. Stephens and Halliday, 1980; circular outcrop pattern. These intrusions were Fridrich and Mahood, 1984). first described in the Tertiary igneous centres of The Slieve GullionTertiary Complex is located the British Isles by Bailey et al. (1924) and later west of the Mourne Mountains in south County by Richey (1932, 1961), who recognized them in Armagh, NorthernIreland,the southernportion both Britain and Ireland. Many ring dykes and crossing into County Louth (Ireland). Along with sheet intrusions display compositional zoning the Carlingford Centre, the Mourne Mountains (Hildreth, 1979; Smith, 1979; Mahood, 1981), and the Antrim basalts, Slieve Gullion forms the NE-Ireland part of the British-Irish Tertiary * E-mail: [email protected] Igneous Province (Upton, 1988; Gamble et al., DOI: 10.1180/0026461046850215 1999), believed to be related to plume activity and # 2004 The Mineralogical Society S. MCDONNELL ET AL. Lower Palaeozoic 99 01 03 05 07 09 11 13 25 Pre-Tertiary meta-sediments rocks Newry granodiorite 23 Porphyritic felsite Lislea Porphyritic granophyre Tertiary 21 rocks Vent agglomerate Slieve Gullion (577m) Mullaghbawn Central complex intrusives 19 N Fault Forkhill 17 Town/village 2 km 21 GridlineGT FIG. 1. Geological sketch map of the Slieve Gullionringcomplex. the opening of the North Atlantic (White and north around Lislea, an average width between McKenzie, 1989; Saunders et al., 1997). The 0.75 and 1 km is recorded. The ring dyke has Slieve Gullioncomplex consistsof a roughly been intruded into both the Lower Palaeozoic circular arrangement of hills, the ring dyke, metasediments of the Longford-Down belt and ~12 km in diameter, encompassing a central the south-west portionof the CaledonianNewry intrusion that forms Slieve Gullion (~577 m) granodiorite pluton (Meighan and Neeson, 1979) (Richey and Thomas, 1932; Emeleus, 1962; (Fig. 1). The ring dyke consists of two principal Elwell et al., 1974; Gamble, 1979). The ring felsic rock types, porphyritic felsite and dyke varies inwidth across the complex. At porphyritic granophyre (Fig. 2), which are Forkhill it measures ~1 km inwidth, decreasing surrounded by a series of brecciated country to 0.25 km west of Mullaghbawnvillage. Inthe rocks. The latter are historically termed the Vent FIG. 2. Hand specimens of porphyritic felsite (left) and porphyritic granophyre (right). The felsite consists of a dark grey matrix containing phenocrysts of quartz and Or-rich K-feldspar, ~2 mm in diameter. Flow-banding defined by fiamme is a common feature. The granophyre is light grey in colour and contains larger Or-rich K-feldspar phenocrysts (~7.5 mm) along with phenocrysts of quartz similar in size to those in the felsite. (1-Euro coin, 23 mm wide, for scale). 726 INTRUSIVE HISTORY OF SLIEVE GULLION RING DYKE Agglomerates of Forkhill and the Camlough the central plain moat between the ring dyke and Breccias. Earlier Tertiary intrusions of dolerite Slieve Gullionhill (Fig. 1), where generally,it is and gabbro and some Tertiary basalt have also extremely rotten. Overall it is a coarse-grained, beenmapped inthe area (Richey andThomas, grey granodiorite. Equigranular (2À3 mm) grains 1932; Emeleus, 1962). of quartz, feldspar and varying amounts of biotite The complex relationships between the various and hornblende are visible in hand sample with ring-dyke rocks were previously recognized by smaller amounts of secondary muscovite identifi- Emeleus, (1956, 1962) and thought to result from able in thin section. The modal proportions of a two-stage intrusive history. We employed field mafic minerals vary considerably from ~10 to observations, petrography, major and trace- ~60% and this variation can be seen both on an element analysis, and viscosity and density outcrop and a regional scale. calculations to further clarify the relationship between the felsite and granophyre rocks and to Tertiary rocks reconstruct in detail the emplacement history of the ring-dyke intrusions. In particular, we address Early Tertiary basalts the questionof whether the ring-dyke rocks Early Tertiary basalts are exposed at several represent unrelated magmatic episodes that locations across the centre, e.g. south of happen to intrude the same structural weakness, MullaghbawnMountain(IH 9878 1713) and or whether the rocks represent different magma Glendooey. In hand specimen, they appear dark types of the same magmatic episode, i.e. are grey with anoverall red tinge.Although they are genetically linked. Our results shed light on the very fine-grained, some isolated lath-shaped behaviour of felsic magmas incaldera-fault plagioclase feldspar crystals, Fe-Ti oxides and conduits and have implications for the structure chlorite are apparent. In thin section, ophitic of the sub-Slieve Gullionmagma chamber and augite may be seen. These basalts are quite potentially similar intrusions elsewhere. vesicular and many vesicles contain secondary mineralization (carbonate Ô zeolites). Field occurrence and petrography Vent Agglomerate Country rocks Vent Agglomerate outcrops are predominantly Lower Palaeozoic metasediments exposed adjacent to the ring dyke at Mullaghbawn These sediments form part of the Central Belt Mountain, Slievebrack, Croslieve and Cashel of the Longford-Down inlier, to the west of the Mountain (IH 9772 1977). This breccia is Slieve GullionCentre(Fig. 1). Towards the composed chiefly of Newry granodiorite clasts south-west of the centre the metasediments ranging from sub-millimetre to metre size, with underlie rolling agricultural land and are generally pieces of gabbro, basalt and Palaeozoic sediments fine-grained. Chlorite, muscovite and quartz can also present (Richey and Thomas, 1932; Emeleus, be seeninthinsection.Internal laminations 1956). This brecciationis the result of highly gas- withinbeds canresult insome colour variation charged eruptions prior to caldera collapse and from lighter grey to green/grey. The rocks are thus predates the emplacement of the ring dyke well bedded with bed thickness ranging from ~10 (Emeleus, 1956, 1962). Outcrops generally have a to ~40 cm. Laminations and coarsening upwards reddish, oxidized tinge to them and are strongly sequences may be seen in both hand specimen and weathered. The clasts are contained within a dark thin section. Moving northwards near Aughanduff grey/greenmatrix which appears to consistalmost (IH 9806 1771), the metasediment facies is dark- entirely of very finely comminuted minerals grey incolour, with grainsof quartz ~1 mm in derived from the Newry granodiorite. In thin diameter clearly visible amongst finer-grained section, quartz and feldspar make up >50% of chlorite and muscovite. Close to the ring dyke, the matrix components, with hornblende, biotite and metasediments have undergone further thermal opaque minerals also present. While this breccia metamorphism, with hornfelsed textures and is generally clast supported, some individual disturbed bedding visible. outcrops have a higher matrix to clast ratio. Few internal structures or sedimentary structures are