Igneous Centre Research Articles

High-precision U–Pb geochronology of the Lundy igneous complex: implications for North Atlantic volcanism and the far-field Paleocene–Eocene ash record

The Lundy Island (Bristol Channel, UK) granite is a felsic expression of the southernmost igneous centre of the North Atlantic Igneous Province that emplaced millions of cubic kilometres of magma during the Paleogene. The granite's distinctive S-type, peraluminous, two-mica ± garnet ± tourmaline composition has led to the hypothesis that eruptions from the Lundy volcanic centre may be the source of thick felsic ash layers within the early Eocene Fur Formation (Denmark) that act as key marker horizons for the onset and duration of the Paleocene–Eocene Thermal Maximum. This paper presents high-precision zircon U–Pb emplacement ages of 57.24 ± 0.11/0.12/0.13 Ma for the granite and 55.970 ± 0.021/0.030/0.070 Ma for a felsic ‘lundyite’ dyke. Trace and rare earth element patterns indicate close similarities between late-stage Lundy activity and ash layer −33 in Denmark. This ash layer was deposited during the Paleocene–Eocene Thermal Maximum carbon isotope excursion, suggesting that the Lundy volcanic centre is likely to be the source of this key ash horizon and that magmatism at Lundy likely continued into the early Eocene.

The spatial distribution of igneous centres along the Norwegian Atlantic Margin (Møre and Vøring) and their relationship to magmatic plumbing systems

A series of offshore intra-basinal igneous centres have been documented across the North Atlantic Igneous Province including the UK, Ireland and Greenland. However, inconsistent cross-border terminology implies that similar features are not present in the Norwegian offshore, which, in turns, leads to misperceptions of cross-border geological differences. This paper presents evidence for a series of Norwegian igneous centres and suggests a consistent non-genetic cross-border terminology. In the Møre Basin, several igneous centres sit close to the continent–ocean boundary (COB), which have previously been identified as seamounts and/or ‘outer highs’. To provide cross-border consistency these features are consolidated under umbrella terms: igneous centres or volcanic fissures. Further centres are probably present within the Møre Basin (east of the COB) where 3D seismic data were not available. In the Vøring Basin two new igneous centres, one intrusive and one extrusive, are identified within the continental domain. Additionally, a possible deep magmatic upwelling associated with the regionally significant T-Reflector is identified. These igneous centre end-members represent the complexity of the magmatic plumbing across the Norwegian margin. With further data it is likely that further igneous centres will be identified offshore mid-Norway.

Fault-Controlled Magma Ascent Recorded in the Central Series of the Rum Layered Intrusion, NW Scotland

Abstract The Palaeogene layered ultrabasic intrusion of the Isle of Rum forms the hearth of the Rum Igneous Centre in NW-Scotland. The regional Long Loch Fault, which is widely held to represent the feeder system to the layered magma reservoir, dissects the intrusion and is marked by extensive ultrabasic breccias of various types. Here we explore the connection between the layered ultrabasic cumulate rocks and breccias of central Rum that characterize the fault zone (the ‘Central Series’) and evaluate their relationship with the Long Loch Fault system. We show that fault splays in the Central Series define a transtensional graben above the Long Loch Fault into which portions of the layered units subsided and collapsed to form the extensive breccias of central Rum. The destabilization of the cumulate pile was aided by intrusion of Ca-rich ultrabasic magmas along the faults, fractures and existing bedding planes, creating a widespread network of veins and dykelets that provided a further means of disintegration and block detachment. Enrichment in LREE and compositional zoning in intra cumulate interstices suggest that the collapsed cumulates were infiltrated by relatively evolved plagioclase-rich melt, which led to extensive re-crystallization of interstices. Clinopyroxene compositions in Ca-rich gabbro and feldspathic peridotite veins suggest that the intruding magma was also relatively water-rich, and that pyroxene crystallized dominantly below the current level of exposure. We propose that the Long Loch Fault opened and closed repeatedly to furnish the Rum volcano with a pulsing magma conduit. When the conduit was shut, pressure built up in the underlying plumbing system, but was released during renewed fault movements to permit dense and often crystal-rich ultrabasic magmas to ascend rapidly from depth. These spread laterally on arrival in the shallow Rum magma reservoir, supplying repetitive recharges of crystal-rich magma to assemble the rhythmic layering of the Rum layered intrusion.

Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style

The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene-plagioclase-plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull SolitDykes, which propagated 100-1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased.

Recurrent Early Cretaceous, Indo-Madagascar (89–86 Ma) and Deccan (66 Ma) alkaline magmatism in the Sarnu-Dandali complex, Rajasthan: 40Ar/39Ar age evidence and geodynamic significance

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-flood basalt magmatism in the Deccan Traps province, based on a single 40Ar/39Ar age of 68.57Ma. Rhyolites found in the complex are considered to be 750Ma Malani basement. Our new 40Ar/39Ar ages of 88.9–86.8Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3±0.4Ma (2σ, melanephelinite) provide clear evidence that whereas the complex has Deccan-age (66Ma) components, it is dominantly an older (by ~20millionyears) alkaline complex, with rhyolites included. Basalt is also known to underlie the Early Cretaceous Sarnu Sandstone. Sarnu-Dandali is thus a periodically rejuvenated alkaline igneous centre, active twice in the Late Cretaceous and also earlier. Many such centres with recurrent continental alkaline magmatism (sometimes over hundreds of millions of years) are known worldwide. The 88.9–86.8Ma 40Ar/39Ar ages for Sarnu-Dandali rocks fully overlap with those for the Indo-Madagascar flood basalt province formed during continental breakup between India (plus Seychelles) and Madagascar. Recent 40Ar/39Ar work on the Mundwara alkaline complex in Rajasthan, 120km southeast of Sarnu-Dandali, has also shown polychronous emplacement (over ≥45millionyears), and 84–80Ma ages obtained from Mundwara also arguably represent post-breakup stages of the Indo-Madagascar flood basalt volcanism. Remnants of the Indo-Madagascar province are known from several localities in southern India but hitherto unknown from northwestern India 2000km away. Additional equivalents buried under the vast Deccan Traps are highly likely.

The Rum Igneous Centre, Scotland

AbstractThe publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intracaldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at ∼60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (∼59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (∼45 Ma).

Bimodal magmatism produced by progressively inhibited crustal assimilation.

The origin of bimodal (mafic-felsic) rock suites is a fundamental question in volcanology. Here we use major and trace elements, high-resolution Sr, Nd and Pb isotope analyses, experimental petrology and thermodynamic modelling to investigate bimodal magmatism at the iconic Carlingford Igneous Centre, Ireland. We show that early microgranites are the result of extensive assimilation of trace element-enriched partial melts of local metasiltstones into mafic parent magmas. Melting experiments reveal the crust is very fusible, but thermodynamic modelling indicates repeated heating events rapidly lower its melt-production capacity. Granite generation ceased once enriched partial melts could no longer form and subsequent magmatism incorporated less fertile restite compositions only, producing mafic intrusions and a pronounced compositional gap. Considering the frequency of bimodal magma suites in the North Atlantic Igneous Province, and the ubiquity of suitable crustal compositions, we propose 'progressively inhibited crustal assimilation' (PICA) as a major cause of bimodality in continental volcanism.

Late stage oxide growth associated with hydrothermal alteration of the Western Granite, Isle of Rum, NW Scotland

[1] The Paleogene Western Granite, Isle of Rum, NW Scotland, represents a westward tilted, shallow level, felsic intrusion associated with the early growth stages of the classic early Paleogene Rum Igneous Centre. Rock magnetic data reveal an east to west variability in magnetic properties across the intrusion that is also reflected in petrographic and geochemical observations. It is argued that the subtle intraplutonic variations in oxide grain size and composition observed in the Western Granite might be a common feature of shallow epizonal igneous intrusions. As these mineralogical differences are not easily detectable without detailed rock magnetic study, this research underscores the need for comprehensive rock magnetic studies when interpreting the postemplacement history of shallow level plutons. Paleomagnetic data indicate that the eastern part of the intrusion represents the shallower part of the exposed intrusion as further evidenced by roof pendants of Lewisian Gneiss and abundant drusy (miarolitic) cavities indicative of shallow emplacement. In contrast, the western part represents a deeper (∼900 m) part of the intrusion as evidenced by the lack of roof pendants and existence of only minor drusy cavities. Rock magnetic data indicate that the dominant magnetic phase is low-Ti titanomagnetite with some titanomaghemite. Magnetic grain sizes range from pseudosingle domain in the west and multidomain in the east with a slight increase in Ti content of the titanomagnetite phases in the east as revealed by Curie point estimates. We interpret these data to indicate that late stage (subsolidus) hydrothermal deuteric fluids associated with emplacement of the Stage 2 Rum Layered Suite produced secondary growth of Fe-Ti oxide mineral phases along silicate grain boundaries in the eastern part of the intrusion. The secondary oxide growth resulted in a slightly coarser magnetic grain size fraction and an increase in Ti content of the titanomagnetite grains. The deeper structural levels of the western part of the intrusion and its distance from the contact of the Rum Layered Suite inhibited secondary oxide growth, and thus, this part of the intrusion preserves a primary magmatic oxide phase of a smaller magnetic grain size and near pure magnetite chemistry.

The Mull Palaeogene dyke swarm: insights into the evolution of the Mull igneous centre and dyke-emplacement mechanisms

AbstractGeochemical data are presented for five large Palaeogene dykes, members of the Mull swarm in southern Scotland and northern England (the Moneyacres, Hawick-Acklington, Barrmill, Muirkirk- Hartfell and Dalraith-Linburn dykes). The rock types range from basalt through andesite to dacite, although the range in individual intrusions is more restricted. The dykes are divisible into two groups; those where the compositional variation was generated by fractional crystallization of basaltic magmas, and those where it resulted from variable degrees of mixing of basaltic and silicic magmas. Several dykes are composite; the marginal facies can be more or less evolved than the central facies. The dyke magmas are thought to have originated from stratified magma chambers beneath the Mull centre and models are presented to show how the different components were derived from the chambers. Some dykes appear to have been terminated at or near the Southern Upland Fault, perhaps as a result of the chilling of early magma pulses by water in the fault. The Palaeogene dyke swarm is considerably more complex than previously recognized and has a significant input to models of the evolution of the Mull magmatic system.

Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia

AbstractThe Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.

The form, distribution and anisotropy of magnetic susceptibility of Jurassic dykes in H.U. Sverdrupfjella, Dronning Maud Land, Antarctica. Implications for dyke swarm emplacement

The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive component of the Karoo large igneous province. Five-hundred and sixty one dykes were recorded intruding Neoproterozoic gneisses and Middle Jurassic syenite plutons. 40Ar/ 39Ar geochronology data reveal two temporally distinct components: the 178–175 Ma, alkaline, Straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton; and the 206–204 Ma, tholeiitic, Jutulrøra dyke swarm found throughout the study area. The Straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction, interpreted to be the result of high magma pressure equal to the maximum principal stress. The Jutulrøra swarm displays a fan of dyke trends, with dyke thickness and spacing increasing away from the inferred point of fan convergence. Anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the Straumsvola area, with the southern/outer exposures of the Jutulrøra swarm exhibiting lateral magma transport. Although associated with a long-lived, local igneous centre comparison of palaeostress estimates for the Straumsvola dyke swarm and contemporaneous dykes in Ahlmannryggen and Vestfjella, indicates the presence of a regional scale radial stress system in western Dronning Maud Land between 178–175 Ma, supporting a mantle plume origin for the Karoo large igneous province.

Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland

Major volcanic activity on the Isle of Rum commenced with the eruption of thick (>100 m) intra-caldera rhyodacite ash-flow sheets fed from steep-sided feeder conduits in the proximity of the Main Ring Fault. Twenty plagioclase phenocrysts of the rhyodacite were analysed using single crystal 40 Ar/ 39 Ar laser dating, yielding a mean apparent age of 60.83 ± 0.27 Ma (MSWD=3.65). On an age v. probability plot the feldspars do not, however, show a simple Gaussian distribution, but a major peak at 60.33 ± 0.21 Ma and two smaller shoulders at c . 61.4 Ma and 63 Ma. These older ages are interpreted to represent recycled and largely re-equilibrated feldspars. The age peak at 60.33 ± 0.21 Ma is interpreted to represent the intrusion and eruption age of the rhyodacites. This new age constraint overlaps with that for the ultrabasic intrusion, implying that the latter was already forming at depth and supplying necessary heat during the early felsic activity phase, and quickly thereafter migrated upwards to shallow structural levels and intruded the volcano's earlier deposits. Combined with previously published ages, these new age data highlight an extremely rapid succession of events at the Rum centre, the whole sequence occurring in potentially <500 ka.

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.

A NEW EXPOSURE OF A CALDERA FAULT SEGMENT AT THE SLIEVE GULLION IGNEOUS CENTRE: IMPLICATIONS FOR THE EMPLACEMENT OF THE EARLY RING-COMPLEX

A contact zone between porphyritic microgranite of the early ring-complex of the Paleocene Slieve Gullion igneous centre and Lower Palaeozoic metasedimentary rocks of the Longford—Down inlier was exposed along a 300m section excavated during construction of the new M 1 motorway. The outcrop displays a thin sliver of Longford—Down metasediment in contact with porphyritic microgranite in a steep, and locally intensely crushed, contact zone that dips away from the ring-complex. This outcrop pattern is reminiscent of a 'caldera-superfault'. Given the recent discussion on sheet versus ring-dyke emplacement of the early ring-complex at Slieve Gullion, this new evidence argues in favour of the traditional ring-dyke model with magma ascending along an active ring fracture associated with caldera subsidence.

A NEW EXPOSURE OF A CALDERA FAULT SEGMENT AT THE SLIEVE GULLION IGNEOUS CENTRE: IMPLICATIONS FOR THE EMPLACEMENT OF THE EARLY RING-COMPLEX

A new exposure of a caldera fault segment at Slieve Gullion Igneous Centre: implication for the emplacement of the early ring-complex

Magmatic Layering and Magnetic Fabrics in the Palaeogene Carlingford Later Gabbros, Co. Louth, Ireland

A detailed magnetic fabric study of the annular partially layered Palaeogene Carlingford 'Later Gabbros', Co. Louth, is combined with field observations to constrain existing emplacement models for the intrusion. The Anisotropy of Magnetic Susceptibility (AMS) fabrics are generally quite inconsistently oriented. Only in three of seven traverses of the intrusion is consistency in orientation observed, suggesting that little can be drawn from the data by way of interpretation. However, the traverse in the north of the intrusion, at Ballinteskin, corresponds to an area of considerable syn-magmatic deformation of layering. Magnetic fabrics in this area exhibit consistently oriented inward dipping and inward plunging magnetic foliations and lineations respectively. Comparison of this data with a recent AMS study of the Great Eucrite of Ardnamurchan, NW Scotland, suggests that though central subsidence may have occurred in the Later Gabbros too, it was not as important as in Ardnamurchan. This is supported to some degree by the presence of a shallowly dipping (-20?) intrusive contact between the Later Gabbros and an extensive granophyre intrusion, which also suggests that the granophyre is the older intrusion. This conclusion is inconsistent with previous interpretations of the granophyre as the youngest intrusion in the Carlingford igneous centre. Introduction and geological setting The Carlingford Igneous Complex is the oldest of three closely associated Palaeogene intrusive centres in NE Ireland dated (U-Pb zircon age) at 61.4 ? 0.8 Ma; the others are the Slieve Gul lion Complex and the Mourne Mountains Com plex (Gamble et al. 1999; Cooper and Johnston 2004 and references therein; Fig. 1). An early series of basalt lavas exposed on the southern margin of the complex is believed to mark the earliest Palaeogene activity in the area. Two separate phases of gabbro are believed to have intruded into these basalts, the earlier repre sented by several isolated areas which crop out in the SW of the complex around Trumpet Hill (Fig. 1). The later suite of gabbros (the Car lingford 'Later Gabbros') form the prominent Irish Journal of Earth Sciences 24 (2006), 37-50. ? Royal Irish Academy 37 This content downloaded from 207.46.13.129 on Sun, 26 Jun 2016 06:11:25 UTC All use subject to http://about.jstor.org/terms 38 Irish Journal of Earth Sciences (2006)

Magmatic Layering and Magnetic Fabrics in the Palaeogene Carlingford Later Gabbros, Co. Louth, Ireland

A detailed magnetic fabric study of the annular partially layered Palaeogene Carlingford 'Later Gabbros', Co. Louth, is combined with field observations to constrain existing emplacement models for the intrusion. The Anisotropy of Magnetic Susceptibility (AMS) fabrics are generally quite inconsistently oriented. Only in three of seven traverses of the intrusion is consistency in orientation observed, suggesting that little can be drawn from the data by way of interpretation. However, the traverse in the north of the intrusion, at Ballinteskin, corresponds to an area of considerable syn-magmatic deformation of layering. Magnetic fabrics in this area exhibit consistently oriented inward dipping and inward plunging magnetic foliations and lineations respectively. Comparison of this data with a recent AMS study of the Great Eucrite of Ardnamurchan, NW Scotland, suggests that though central subsidence may have occurred in the Later Gabbros too, it was not as important as in Ardnamurchan. This is supported to some degree by the presence of a shallowly dipping (∼20°) intrusive contact between the Later Gabbros and an extensive granophyre intrusion, which also suggests that the granophyre is the older intrusion. This conclusion is inconsistent with previous interpretations of the granophyre as the youngest intrusion in the Carlingford igneous centre.

Geometry and mode of emplacement of dike swarms around the Birnudalstindur igneous centre, SE Iceland

Dike geometries around the well-exposed periphery of the Birnudalstindur igneous centre (SE Iceland) are constrained by moving averages of strike, dip, thickness and dilation by 775 mafic dikes, mapped along three strategically placed transects. On the basis of spatial analysis of dike strikes, a rift-parallel swarm is distinguished from a cross-cutting tri-axial swarm pattern of ‘brown dolerites’ that clearly post-dates the volcano's cone sheet swarm. Dikes are on average orientated at right angles to the lava pile and consequently used to constrain the ‘flexured’ geometry of the host lava pile, subsequently back-rotated to horizontal. This produces two end-member scenarios, which can be tentatively used to evaluate the dynamic formation of Icelandic crust. Dike dilations above a prominent stratigraphical transition into hyaloclastite breccias are markedly lower than in the underlying plateau lava pile, suggesting that vertical dike propagations were inhibited along this density/stress boundary. Lined up with the Birnudalstindur igneous centre, average dike thicknesses decrease towards the axes of both the rift-parallel dike swarm and the rift-perpendicular branch of the tri-axial swarm. This arguably links all dike swarms to the Birnudalstindur igneous centre, even if it remains inconclusive whether rift-parallel dikes fed and/or were injected laterally away from its sub-volcanic magma chamber. It seems more likely that the slightly offset tri-axial swarm of brown dolerites was preferentially emplaced along a peripheral bulge that was created around the ‘down-sagging’ volcano.

Sr and Nd isotope evidence for successive crustal contamination of Slieve Gullion ring-dyke magmas, Co. Armagh, Ireland

The Palaeogene Slieve Gullion Igneous Centre in southern Armagh, Northern Ireland, consists of a layered central intrusive complex surrounded by a prominent and slightly older ring-dyke that intrudes both Lower Palaeozoic sedimentary rocks and the Caledonian Newry Granodiorite pluton (452 Ma). The ring-dyke comprises two major rock types: porphyritic felsite and porphyritic granophyre. We analysed both ring-dyke lithologies, both types of country rock, and a local Palaeogene basalt dyke sample for Sr and Nd isotopes. Trace element and whole rock data for this suite suggest that there are two distinct groups of both felsite and granophyre: one Si-rich and one Si-poor, most likely representing two magmas from a zoned chamber and their mushy chamber wall equivalents (McDonnell et al. 2004). Isotope data show the low-Si rocks to be higher in radiogenic Sr than the high-Si rocks, which is inconsistent with a simple AFC-scenario of increasing sediment assimilation with higher degrees of differentiation. However, using MORB-type basalt as a starting composition, the low-Si ring-dyke rocks can be modelled through AFC with Lower Palaeozoic sedimentary rock as the contaminant. The decreasing 87Sr/86Sr trend from low-Si to high-Si dyke rocks, in turn, represents a second stage of contamination. Selective assimilation of the most fusible portions of Newry Granodiorite, which is lower in radiogenic Sr than the local sedimentary rocks, appears to be the most plausible solution. The Sr and Nd data are consistent with (a) at least a two-stage contamination history during upper crustal residence and storage, whereby fractionating magmas of basaltic and intermediate composition are contaminated by local sedimentary rocks, giving rise to rhyolite magmas that experience additional shallow contamination by Newry Granodiorite, and (b) a zoned rhyolite magma chamber where high-Si magma is stored in the upper part of the chamber where crystallization and crustal contamination are most extensive.

First results from shallow stratigraphic boreholes on the eastern flank of the Rockall Basin, offshore western Ireland

The results of an integrated sedimentological and seismic stratigraphical analysis of three borehole sites on the eastern flank of the Rockall Basin, offshore western Ireland are reported. Two sites were drilled on the western slope of the Porcupine High, above the North and South Bróna basins (boreholes 83/20-sb01, 83/24-sb01 and 83/24-sb02), and one on the northern flank of the Porcupine High (16/28-sb01), above the Macdara Basin. The cores establish that the half-graben basins marginal to the eastern Rockall Basin contain Jurassic deposits and that they were inverted sometime in the Late Jurassic or Early Cretaceous. An angular unconformity above the Bróna basins is overlain by a condensed, tripartite Cretaceous succession (‘brownsand’, ‘greensand’, chalky micrite) that records stepwise deepening, with evidence for a Cenomanian-Turonian phase of normal faulting. Above the Macdara Basin, the unconformity is overlain by a basalt that was cored at the 16/28 site and is interpreted to represent a flow of Cretaceous age derived from the Drol Igneous Centre. At all three borehole sites, Cretaceous strata are onlapped (or downlapped) by Paleocene-Eocene strata that display evidence of a minor episode of fault reactivation above the Bróna basins. Cored Eocene strata vary from clastic to carbonate-prone from north to south and smectitic clays are common at the 16/28 site. Post-Mid-Eocene westward tilting of the Rockall slope rotated the Eocene stratigraphy and the underlying Cretaceous deposits (including the lava flow in the 16/28 area) at least 3° down to the west. Slope development resulted in extensional sliding and the erosion of the C30 deep-water unconformity that is onlapped by Miocene slope deposits. C30 was cored in the 83/20 area where it cuts down into Cretaceous strata and is crusted with phosphates and the Cretaceous beneath Mn-impregnated.