Igneous geology: regional setting, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Palaeogene rocks that comprise the lava fields, dyke swarms, sill-complexes and central complexes of the Hebridean Igneous Province are part of the much larger, North Atlantic Igneous Superprovince (P914120). The magmatism was the consequence of a large mantle plume, which first impacted on the base of the continental lithosphere towards the end of the Cretaceous Period. Magmatism continues today on Iceland and along the Mid-Atlantic Ridge but represents a much smaller, less productive stage in the evolution of the plume.
Seismic reflection and refraction studies have revealed that the north Atlantic Ocean between north-west Europe and Greenland comprises thick sequences (up to 6000 m) of seaward-dipping reflectors. These are attributed to extrusive igneous rocks, erupted at the time of rifting and the initiation of ocean floor spreading (White, 1988). Much of the volcanic activity occurred above sea level, because of the dynamic support provided by the plume (White and Mackenzie, 1989). This is demonstrated by the presence of inter-lava palaeosols recovered from deep-sea boreholes (e.g. Viereck et al., 1988). In addition, large amounts of igneous rock, up to 1500 m thick, were underplated during the initial stages of rifting. The landward equivalents of the offshore lavas include the thick and laterally continuous flood basalt lavas of west and east Greenland, and the less extensive lava fields of the Faroe Islands, north-west Scotland and north-east Ireland.
The vast majority of the lava sequences, both offshore and onshore, were erupted subaerially, predominantly from fissure-type feeders, now represented by laterally continuous linear dyke swarms. The north-west-trending dyke swarms that are associated with the Skye and Mull lava fields of western Scotland developed almost at right angles to the rift margin and indicate contemporaneous north-east–south-west extension of the north-west European margin.
The central complexes represent sites where magma emplacement into the upper continental crust along the rift margin reached anomalously large proportions. Along the north-west European continental margin, the central complexes occur within a relatively narrow, north-trending zone about 40 km in width (P914119). The zone can be traced from Lundy, in the Bristol Channel, north through the central complexes in north-east Ireland to those that occur along the western coast of Scotland. The St Kilda archipelago is formed by a central complex that lies to the west of this zone. It is most appropriately grouped with the many submerged central complexes now recognised to the west of Shetland, which occur on the south-east side of the Rockall Trough and within the Faroe–Shetland Basin (Ritchie and Hitchen, 1996; P914120). In broad terms, the continental crust was thinned (stretched) during the initial stages of rifting and associated volcanic activity (compare with Meissner et al., 1986) in a north–south zone running through western Britain and north-east Ireland, and also farther to the west. Crustal extension permitted access to the surface for mantle-derived magmas; some were erupted directly to the surface while others ascended in stages, ponding at the mantle-crust boundary or in reservoirs at various levels within the crust (Morrison et al., 1985).
The central complexes of western Scotland appear to be sited where older Palaeozoic (Caledonian) or Mesozoic structures cross the north–south zone. For example, the North Arran Granite Pluton occurs on the trace of the Highland Boundary Fault, the Mull Central Complex occurs on the trace of the Great Glen Fault, and the Skye and Rum central complexes are situated on, or close to, the Camasunary–Skerryvore Fault. Typically, the central complexes occur within areas of basement rocks exposed in the elevated footwalls of Mesozoic faults (Roberts and Holdsworth, 1999; P914121).
The lava fields of western Scotland are thickest within fault-bound basins containing syn-rift terrigenous Triassic sedimentary rocks (up to 500 m in thickness) overlain by marine to brackish Jurassic sandstones, limestones and mudstones (Binns et al., 1974; Walker, 1979). The Skye Lava Field, comprising the thick sequences of flows preserved on Skye and Canna, is contained within the Sea of the Hebrides Basin, bounded to the north-west by the Minch Fault. The sedimentary rocks of the basin have a feather-edge against Lewisian and Torridonian rocks of the adjoining Skye–Rum Horst. To the south, the Mull Lava Field is contained within the Inner Hebrides Basin, bounded to the north-west by the Camasunary–Skerryvore Fault (P914121), with Precambrian rocks of the Skye–Rum Horst forming the footwall. The fault displaces lavas of the Eigg Lava Formation and Mull Lava Group but not those of the younger Skye Lava Group. The south-east limit of the Inner Hebrides Basin was not fault-controlled, and there lavas of the Mull Lava Field overstep Jurassic strata and thin remnants of overlying Cretaceous rocks to lie directly upon basement rocks of the Moine Supergroup north and south of Loch Sunart.
The Mull Lava Field is considered to extend north to include the thin sequence of lavas preserved in the Ben Hiant area on the south side of the Ardnamurchan peninsula. The lava sequences of Muck and Eigg also appear to form a northern extension to the Mull lavas (Fyfe et al., 1993, fig. 40) and may be contemporaneous with the earlier part of that succession. The basaltic lavas on Eigg are capped by a thick rhyodacitic pitchstone lava, which was erupted into a palaeovalley eroded into the earlier flows. The rhyodacite lava is considered to be the youngest flow preserved onshore in the Hebridean Province (P914126).
The timing of emplacement of the linear regional dyke swarms relative to the lava fields is difficult to deduce directly from field relationships. There are few recorded examples of dykes feeding flows, and on Skye, Mull, Muck, Eigg and in south-east Rum intense dyke swarms cut the lavas. Furthermore, on Skye, Rum and Mull, earlier members of the central complexes are commonly intruded by numerous north-west-aligned dykes. However, from detailed mapping within the Hebridean Igneous Province (e.g. Williamson and Bell, 1994) and throughout the North Atlantic Igneous Superprovince (Upton, 1988), it is likely that the vast majority of the lavas were erupted from fissure systems, now represented by dykes, and from plugs developed on the dykes (Kerr, 1997). The magmas were also intruded to form sill-complexes, which were preferentially developed in the sedimentary rocks of the Mesozoic basins. Only towards the ends of the individual pulses of igneous activity were lavas possibly erupted from more-localised volcanic edifices overlying the central complexes.
Timing and igneous stratigraphy
The age of the lavas in relation to the central complexes can be deduced reasonably easily in some places. Clear intrusive contacts between members of the central complexes and earlier lavas are quite common, and are accompanied by pronounced high-grade thermal and hydrothermal aureoles. Conversely, where a lava field developed subsequent to the emplacement of a central complex, the interflow sedimentary deposits may contain material derived from the complex, if it was unroofed and eroded during the construction of the lava pile.
The age relationships of the rocks within this province have not always been readily resolved by radiometric and other age data (compare with Mussett et al., 1988). However, the relative ages are now becoming clearer through a combination of more precise radiometric dating techniques, palynological dating of interflow sedimentary rocks, and palaeomagnetism (e.g. Pearson et al., 1996; Bell and Jolley, 1997; Hamilton et al., 1998; Chambers and Pringle, 2001, Chambers et al., 2005). Published precise radiometric age determinations and biostratigraphical ages based upon palynofloral assemblages are summarised, together with other data, in Table 8 and P914126. The lavas of the province span an interval of at least three million years, from about 61 Ma to at least 58 Ma. The interval corresponds to Chron 26R of the magnetic time scale (Berggren et al., 1995). All of the Hebridean onshore central complexes appear to have been initiated during this interval, and it is apparently only in Skye, and possibly Mull, that significant activity occurred at later dates.
There is only one precise radiometric age determination for the Skye Lava Field, although there is a considerable body of palynological data. The Portree Hyaloclastite Formation (Palagonite Tuffs of Anderson and Dunham, 1966) at the base of the Skye Lava Group and the Preshal Beg Conglomerate Formation near the top of the group contain pollen assemblages that indicate an age range of 58.23 to 58.0 Ma. (Jolley, 1997), and an Ar-Ar age of 58.91 ± 0. 18 Ma has been obtained from the trachytic Sleadale Tuff beneath the Preshal Beg Conglomerate (Bell and Williamson, 2002). Petrologically distinct lavas of the Talisker Formation, which directly overlie the Preshal Beg conglomerates, are the youngest lavas within the Skye Lava Field (p. 65). Although the Talisker lavas and the Preshal Beag conglomerates are separated from the underlying lavas by a clear erosional break, which coincides with a significant change in magma-type, the time interval represented was probably short (Hamilton et al., 1998).
The lavas of the Canna Lava Formation preserved in north-west Rum, which belong to the Skye Lava Group (Williamson and Bell, 1994; Emeleus, 1997), are demonstrably younger than the Western Granite, one of the earliest members of the Rum Central Complex. The lavas rest upon a weathered surface of the granite, and interstratified conglomerates on Rum, Canna, Sanday and south-west Skye contain clasts derived from the granite and from other, younger, members of the Rum Central Complex. Thus, the lavas were erupted in the interval of less than 1.6 million years, between the emplacement and unroofing of the Rum Central Complex (60.5–60.1 Ma) and the emplacement of the gabbros of the Cuillin Centre on Skye (58.9 Ma), which intrude the Skye Lava Group.
In the Small Isles, the Sgurr of Eigg Pitchstone Formation is one of the latest eruptive events in the Hebridean Province (58.7 Ma), whereas the tuffs at the base of the Eigg Lava Formation on Muck have given the oldest ages (60.6 Ma). Neither formation has yet yielded useful material for palynological dating, although plant remains are well known from conglomerates beneath the Sgurr of Eigg pitchstone (p. 77).
The Mull Lava Field is invaded by the Mull Central Complex. Radiometric age determinations and geochemical signatures (Chambers and Pringle, 2001) indicate that construction of the Mull and Skye lava fields may have been more or less contemporaneous. This is, however, difficult to reconcile with the palynological evidence. The Mull flora constitutes a widely recognised, well-defined and short-lived assemblage that flourished in warm, subtropical conditions, the late Paleocene thermal maximum, and is completely different from the older flora obtained from Skye and the Small Isles (Jolley, 1997). The late Paleocene thermal maximum is dated at 54.98 Ma on the geological time scale of Berggren et al. (1995), but on Mull rocks containing the diagnostic pollen flora are overlain by lavas that have yielded isotopic U-Pb and Ar-Ar dates between 57.5 and 60.54 Ma (e.g. Chambers and Pringle, 2001). Furthermore, within the post-lava Mull Central Complex, the Corra-bheinn Gabbro and Loch Bà Felsite intrusions are dated by the U-Pb method at 58.3 and 58.5 Ma, respectively (Hamilton et al., 1998).
The Skye Central Complex contains a large number of intrusive units, the oldest of which are various layered gabbroic and ultrabasic rocks, together with a number of confluent cone-sheets that comprise the Cuillin Centre. A U-Pb age of 58.9 Ma was obtained from a gabbroic pegmatite formed early in the development of this centre (Table 8). Amongst the cone-sheets is a relatively late-stage suite of dolerites and basalts, which were emplaced at very shallow depths, and are compositionally identical to flows of the Talisker Formation (Walker, 1993b; Bell et al., 1994). It may therefore be inferred that the later stages in the formation of the Cuillin Centre, which represents the eroded roots of a basaltic central volcano, were synchronous with and relate to the outpouring of the distinctive flows of the Talisker Formation. The Talisker lavas flowed onto a land surface formed from older and compositionally different formations of the Skye Lava Group. There is a close correspondence between the radiometric age for early activity in the Cuillin Centre and that for the eruption of the trachytic tuff underlying the Preshal Beg Conglomerate Formation, and the biostratigraphical age of the Preshal Beg Conglomerate Formation. Consequently, the Cuillin Centre was developing contemporaneously with at least part of the Skye Lava Field. The younger granite centres of the Skye Central Complex yield ages that range from about 58.5 to 57 Ma for intrusions of the Srath na Creitheach and Western Red Hills centres, to about 56 Ma for the Beinn an Dubhaich Granite and a pitchstone dyke that intrudes the nearby Beinn na Caillich Granite, both belonging to the Eastern Red Hills Centre (P914126). These ages suggest that the growth of the Skye Central Complex may have spanned up to three million years, compatible with the complex palaeomagnetic record (Mussett et al., 1988).
The Ardnamurchan Central Complex was emplaced into basaltic lavas. No reliable age data are available for these hydrothermally altered flows, although those around Ben Hiant, along the eastern side of the central complex, are considered to be an outlier of the Mull–Morvern Lava Field. Radiometric age determinations on the Great Eucrite and tonalite intrusions of Centre 3 give ages of about 59 Ma.
The radiometric age of 60.6 Ma obtained from the oldest lavas in the Eigg Lava Formation is similar to those from rocks near the base of the Mull Lava Field (Table 8). Investigations of the offshore geology between western Mull, Ardnamurchan and the Small Isles show that the lavas of Mull are linked to those of Muck and Eigg (Fyfe et al., 1993, fig. 40).
The episodes of lava eruption in the Hebridean Igneous Province are summarised in P914126 and appear to constitute a few discrete pulses within a period of less than three million years. However, the age and duration of activity within the central complexes are currently less well constrained. The Rum Central Complex postdates the Eigg Lava Formation but predates the Skye Lava Group. The majority of the Skye Central Complex most likely postdates the Skye Lava Group, but the youngest Talisker Lava Formation may have been erupted from a central volcano now represented by parts of the Cuillin Centre. The Mull Central Complex postdates the Mull Lava Group. The St Kilda Central Complex, the Blackstones Central Complex, the North Arran Granite Pluton, the Central Arran Ring-complex and the Ailsa Craig Granite have all been dated by radiometric techniques, but some have significant errors. They also have the disadvantage of not being associated with lavas to offer a precise comparison with pollen and spore age data.
Evidence that the plume-driven igneous activity in the Hebridean Igneous Province occurred sporadically rather than continuously may be provided by the temporal spacing of detritus fans in offshore Palaeogene sedimentary rocks (White and Lovell, 1997). The detritus fans resulted from increased uplift and erosion in the sedimentary source areas, which may, in turn, have been caused by pulses of magmatic underplating from a mantle plume. The underplating may also have triggered the igneous activity, as the correlation between major periods of fan deposition and maxima in igneous activity in the Hebridean Igneous Province is quite striking. The very clear break in magmatism between emplacement of the Rum Central Complex and accumulation of the Canna Lava Formation could be one result of pulsed activity.