Magmas, Hebridean Igneous Province
| Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey. |
Introduction
Much of our present-day understanding of magmas and magmatic processes is the consequence of research undertaken on the Palaeogene rocks of the Hebridean Igneous Province. Following on from pioneering studies on Skye and the Small Isles (Harker, 1904, 1908), the most significant breakthroughs were made by the Geological Survey with their investigations of the volcanic and intrusive rocks of Mull (Bailey et al., 1924), a landmark study that established a framework for most subsequent research. The geochemistry and genesis of the magmatism of the Hebridean Igneous Province have been discussed and summarised in several seminal publications. In particular, Thompson (1982a, b) provided incisive reviews and new ideas on a wide range of topics, and Saunders et al. (1997) discussed the geochemistry of the volcanic rocks of north-west Scotland in the context of the whole North Atlantic Igneous Superprovince. Magmatism in the Hebridean Province has recently been reviewed by Bell and Williamson (2002).
Early concepts
Magma-types and magma-series
The important concepts of magma-types and magma-series were established by Bailey et al. (1924) through their study of the lavas and intrusive rocks of Mull. A dominant Normal Mull Magma-series was identified, consisting of: the Plateau Magma-type (olivine-rich basalt lavas), the Non-porphyritic Magma-type (olivine-poor basalt lavas), the Intermediate to Sub-acid Magma-type (various minor intrusions), and the Acid Magma-type (granites of the central complex). The Plateau Magma-type was considered to be the parental magma of the Normal Mull Magma-series. The other magma-types were derived from it by magmatic differentiation, and each was regarded as the product of crystallisation of successively more silicic residual liquids. Bowen (1928) concluded that fractionation of olivine was the most likely process involved in the production of the evolved liquids. An alternative model was proposed by Kennedy (1930), who considered that the Non-porphyritic Magma-type was parental to the Plateau Magma-type, but later (Kennedy, 1933) he suggested that both magma-types were parental, renaming them tholeiitic basalt (hypersthene-normative) and olivine basalt (nepheline-normative, and later renamed alkali olivine basalt by Tilley, 1950). Subsequently, the experimental studies of Yoder and Tilley (1962) confirmed that one type could not yield the other through crystal-liquid fractionation under low pressure conditions.
Major-element compositions
The landmark paper by Thompson et al. (1972) on the major-element geochemistry of Skye lavas provided, for the first time, a clear insight into the relationships of parental and derivative magmas. They discovered that, although the Skye lavas are petrographically of alkali olivine basalt type, they vary between nepheline-normative and hypersthene-normative (P914153). They assigned the lavas to a Skye Main Lava Series, within which the nepheline-normative flows have a major-element composition typical of alkali olivine basalt, but several of the minor- and trace-element concentrations are typical of tholeiitic basalts (Thompson et al., 1980). On Mull, a similar range in composition occurs within the Plateau Magma-type (Morrison et al., 1980).
Experimental studies led Thompson (1974) to conclude that the most primitive (i.e. magnesium-rich) basalts of the Skye Main Lava Series achieved their final compositions at pressures of up to 17 kbar, well within the upper mantle. These magmas were then erupted relatively fast, with little opportunity for re-equilibration prior to eruption. However, the major-element data for other flows from the Skye Main Lava Series, the Mull Plateau Magma-type and elsewhere in the Hebridean Igneous Province, indicate that subsequent fractionation processes occurred at pressures of around 9 kbar, near to the base of the crust (Thompson and Gibson, 1991). In effect, the continental crust acted as a mechnical filter, causing magmas to pond and fractionate at depth (Saunders et al., 1997). Two evolutionary trends were recognised by Thompson et al. (1972):
- alkali olivine basalt—hawaiite—mugearite—benmoreite
- alkali olivine basalt—Si-rich and Fe-poor intermediate magmas—trachyte (P914154)
The Benmoreite Trend evolved under high pressure (deep within the crust) with Fe-enrichment, in contrast to the low pressure (shallow) conditions, with Fe-depletion, of the Trachyte Trend.
A distinctly high concentration of calcium and low potassium, titanium and phosphorous, akin to tholeiitic Mid Ocean Ridge-type basalts has been determined for some of the igneous rocks from Skye (Esson et al., 1975), for example the Preshal More lava (Williamson and Bell, 1994), certain dykes within the axial portion of the Skye Dyke Swarm (Mattey et al., 1977), and the cone-sheets of the Cuillin Centre (Bell et al., 1994). These are referred to as the Preshal More Magma-type and are considered to have evolved under low pressure in the upper crust (Thompson, 1982b; P914153). The Non-porphyritic Magma-type of Mull, subsequently renamed the Central Mull Tholeiite Magma-type, probably had a similar low-pressure origin (Kerr, 1995a; Kerr et al., 1999). However, certain tholeiitic basalt lavas belonging to the Staffa Lava Formation on Mull appear to have a different origin, involving fractional crystallisation and contamination of Mull Plateau magmas (Kerr, 1998).
Contamination processes
Magmas rising through the continental crust beneath the Hebridean Igneous Province were likely to have interacted with and been contaminated by country rock including, granulite-facies (lower crustal) Lewisian gneisses, amphibolite-facies (upper crustal) Lewisian gneisses, Moine pelites and psammites, and Torridonian sandstones and siltstones (Chapter 2). Each of these crustal materials has a distinctive isotopic signature (P914155; P914156), which would be imparted as a recognisable geochemical imprint if such materials were assimilated by the magmas during ascent. During such contamination processes, the magmas may also have undergone fractional crystallisation, and it is the heat liberated for the crystallisation that provided the thermal energy required by the contamination process. Therefore, it is important to assess whether contamination occurred before, during or after fractional crystallisation. In particular, the combined process of assimilation during fractional crystallization may be recognised from the geochemical signatures of various suites of extrusive and intrusive rocks.
Using the isotopic ratios, and in some instances isotopic abundances, of strontium (Sr), neodymium (Nd) and lead (Pb) in the Skye Main Lava Series of the Skye Lava Field, variable amounts of both lower and upper crustal (Lewisian gneiss) contamination have been detected, with the major contribution coming from granulite-facies lower crust (Carter et al., 1978; Moorbath and Thompson, 1980; Dickin, 1981;Thompson, 1982 a, b; Thompson et al., 1982; Thirlwall and Jones, 1983; Dickin et al., 1987). Similar contamination models have been deduced for the Mull Plateau Magma-type of the Mull Lava Field (Kerr et al., 1999).
The contamination history of the Preshal More Magma-type is completely different from that of the Skye Main Lava Series and the Mull Plateau Magma-type. In the Preshal More magmas, contamination was concurrent with fractionation and the dominant contaminant was upper crustal, amphibolite-facies Lewisian gneisses (P914156). The genetically related rocks of the Cuillin Centre of the Skye Central Complex have also had a similar contamination history (Dickin et al., 1984a; Bell et al., 1994). The tholeiitic basalt lavas of the Staffa Lava Formation, close to the base of the Mull Lava Group, are also contaminated, although contrasting models are offered to explain the type of magmas involved, the nature of the contaminants and the timing of contamination (Thompson et al., 1986; Kerr et al., 1999).
Depth of magma generation
On the basis of experimental studies (Thompson, 1974) and trace-element modelling (Thompson et al., 1980), it may be concluded that the primary melting events that yielded the Skye Main Lava Series magmas occurred in the mantle at a depth of about 60 km and involved partial melting of spinel lherzolite. The Preshal More Magma-type was generated by subsequent larger degrees of melting of the residual mantle material. The remaining unmelted mantle was of harzburgitic composition.
In a re-examination of the Skye lavas, Scarrow and Cox (1995) concluded that the parental magmas were of picritic composition. They developed a model for the production of parental melts involving decompressive melting of abnormally hot mantle, with the final segregation of the melts occurring over the depth range of 60 to 112 km and at a temperature of 1390° to 1510°C. This depth estimate is not at significant variance with that proposed by Thompson et al. (1972) and Thompson (1974). The melts that segregated at the greatest depths were nepheline-normative, whereas hypersthene-normative magmas were produced towards the top of the melting column.
The incompatible trace-element contents of lavas thoughout the Hebridean Igneous Province are low when compared with continental flood lava sequences worldwide. The depletion of these elements from the mantle source probably took place during Permian times when highly alkaline (lamprophyric) magmas were extracted from the mantle below the entire Hebridean area by partial melting (Morrison et al., 1980; Thompson, 1982b). Differences between the Skye Main Lava Series and the Mull Plateau Magma-type could be due to lateral mantle heterogeneity.
Ultrabasic and basic magmas of the central complexes
Interpretation of the geochemical affinities of the coarse-grained ultrabasic and basic units of the central complexes is difficult, as they have all been modified by a variety of fractionation processes. However, it should be possible to deduce the magma-types involved from material forming their chilled margins. In practice, data on these lithologies are extremely sparse and are not always easily interpreted (pp. 115—116). A more promising insight may be provided by fine-grained rocks of the abundant minor intrusions, which were intruded contemporaneously with the emplacement of the ultrabasic and basic rocks.
Opinions on the nature of the parental magmas vary. The abundant cone-sheets which focus on the central complexes of Skye (Bell et al., 1994), Mull (Kerr et al., 1999) and Ardnamurchan (Holland and Brown, 1972; Gribble, 1974; Thompson, 1982a, Geldmacher et al., 1998) are of either the Preshal More Magma-type, or the Central Mull Tholeiite Magma-type and its differentiated equivalents. These tholeiitic magma-types, which equilibrated at relatively low pressures (about 3 kbar), are regarded as likely parent magmas, and this conclusion is supported by the similar composition of the chilled margin of the Ben Buie Gabbro on Mull (Skelhorn et al., 1979; Kerr et al., 1999; Chapter 9). Somewhat different parental magma compositions have been proposed for the ultrabasic and basic rocks of Rum. There, direct evidence for the involvement of more-primitive picritic magmas comes from a chilled margin to the Eastern Layered Intrusion, with 15 to 20 per cent MgO (Volker, 1983; Greenwood et al., 1990) and from an aphyric dyke with 13.5 per cent MgO (McClurg, 1982; Upton et al., 2002).
Minor intrusions
The dyke swarms, sill-complexes and volcanic plugs or bosses provide a wealth of information about the temporal and areal occurrence of most of the magma-types in the Hebridean Igneous Province. They may also provide in situ evidence of some of the high crustal level (low pressure) processes considered to have affected the lavas. Thus, crustal xenoliths are found in various stages of incorporation in certain sills, varied lithologies corresponding to near-complete magma-series occur within individual intrusions, dyke swarms show changes from alkali olivine basalt members to tholeiitic basalt members with time, and in composite intrusions completely contrasting rock compositions (commonly silicic and basic) occur together, and are observed in various stages of mixing and mingling.
Dyke swarms
The geochemistry of the Skye Dyke Swarm has been studied in detail by Mattey et al. (1977). Approximately 70 per cent of the dykes are of Preshal More Magma-type and are particularly abundant within the axial region of the swarm, close to the Cuillin Centre, although the type persists throughout the length of the swarm forming, for example, almost all of the Skye dykes on Harris and at Arisaig.
The dominant compositions of the Mull Dyke Swarm are of the Mull Plateau and Central Mull Tholeiite magma-types (Kerr et al., 1999). The tholeiitic dykes of north-east England (Macdonald et al., 1988) are the most south-easterly representatives of the Mull Dyke Swarm. They belong to the Central Mull Tholeiite Magma-type and have elevated initial 87Sr/86Sr values (Moorbath and Thompson, 1980), implying substantial contamination by crustal materials, most likely Moine pelite.
The xenolithic monchiquite dyke at Loch Roag, on Harris, is compositionally unique among the Palaeogene dykes of the Hebridean Igneous Province. It is of basic composition (MgO = 9 per cent), rich in K2O (3.5 per cent) and has elevated concentrations of barium (Ba) and the Light Rare-Earth Elements (Menzies et al., 1987). The composition is similar to minor intrusions of Late Palaeozoic age that are quite common in western Scotland (e.g. Upton et al., 1998).
Volcanic plugs
The Cnoc Rhaonastil Boss on Islay comprises dominant alkali olivine-dolerite together with volumetrically minor pods of coarse-grained nepheline-syenite (Chapter 7; Hole and Morrison, 1992; Preston et al., 1998b, 2000a, b). The major- and trace-element compositions are highly enriched in incompatible elements, which can be adequately explained by the fractionation of olivine and plagioclase from an alkali basalt magma; such enrichment is not reported from elsewhere in the Hebridean Igneous Province. The intrusion provides a unique insight into the composition of liquids produced by extreme fractional crystallisation, most likely in a closed system in a high-level magma chamber. It indicates the possible composition of the end-stage liquids produced by extreme fractionation of the dominant Skye Main Lava Series and Mull Plateau Magma-type that is not recorded in the lava piles, but is approached in the Shiant Isles sills (see below).
Sill-complexes
Three geochemically distinct groups are recognised in the Loch Scridain Sill-complex of Mull (Preston and Bell, 1997; Preston et al., 1998a, 1999).
- Group 1 consists of aphyric tholeiitic basalts and basaltic andesites with Preshal More Magma-type affinities
- Group 2 comprises plagioclase- and pyroxene-phyric andesites and dacites
- Group 3 are rhyolites
The sills show progressive and extreme enrichment in incompatible elements and have elevated initial Sr- and Pb-isotope ratios. These properties are attributed to fractional crystallisation concurrent with assimilation of a partial melt derived from Moine metasedimentary rocks. The Group 3 rhyolites originated as partial melts derived from pelites and the Group 2 sills appear to represent magmas produced by simple bulk mixing of Group 1 and Group 3 magmas. Cognate xenoliths provide samples of early-precipitated cumulate assemblages and quartzite xenoliths from the Moine basement are common. Distinctive mullite-bearing aluminous buchites are considered to represent the end-product of two-stage melting of Moine pelite: initial partial melting resulted in the removal of rhyolitic melts (compare with the Group 3 sills), which was followed by bulk melting of the residual aluminous pelites within the sill conduits. Complex reactions between the aluminous melts and the tholeiitic sill magma produced a hybrid magma that subsequently crystallised to form the plagioclase-spinel-corundum rims that encase the aluminous xenoliths (Dempster et al., 1999).
The sills of the Little Minch Sill-complex on Skye range in composition from picrite through picrodolerite to analcime-bearing olivine-dolerite ('crinanite') and, on the Shiant Isles, to silica-undersaturated alkali syenites (Gibb and Gibson, 1989; Gibson, 1990; Gibb and Henderson, 1996). The sills are of alkaline affinity, with geochemical characteristics comparable with those of the earlier Skye Main Lava Series (see above). Isotopic data indicate contamination with up to 20 per cent of amphibolite-facies Lewisian gneisses, concurrent with fractionation, thus implying that the sill magmas underwent distinctly different evolutionary paths to those of the Skye Main Lava Series lavas as they ascended through, and reacted with, the crust.
Silicic rocks
The silicic rocks include the main granitic lithologies of the central complexes, which range in composition between monzogranite and peralkaline granite, together with rare minor intrusions (mainly dykes), rhyolitic lavas and pyroclastic rocks. The origins of the silicic rocks have been the subject of controversy and the topic is covered in several reviews (J D Bell, 1976; Gass and Thorpe, 1976;Thompson, 1982a, b).
One of the major points of controversy has been the relative importance of fractional crystallisation of basaltic melts compared with the partial fusion of country-rock lithologies in yielding granitic liquids. Fractional crystallisation was preferred in the early models (Harker, 1904, 1908), although this was contested by some (e.g. Reynolds, 1954), and subsequently partial melting models became favoured (e.g Brown, 1963; Wager et al., 1965; Dunham, 1968, Thompson, 1969). Within the environs of the central complexes, where significant masses of ultrabasic and basic magmas were fractionating within crustal reservoirs, there was the potential for both processes to have operated.
Isotopic studies have shown that certain of the Skye granites have modified (elevated) radiogenic strontium and lead signatures, because of the presence of crustal material. Consequently, the granites were most likely formed, in part, by partial melting of Lewisian gneisses (Moorbath and Bell, 1965), and Pb isotope data (Moorbath and Welke, 1969) clearly point towards a mixed origin, involving both mantle and crustal lead (P914156). Some of the granites have isotopic signatures that are very similar to those of the basic rocks of the lava fields and central complexes (Walsh et al., 1979; Moorbath and Thompson, 1980; Dickin, 1981; Dickin et al., 1984a). As with the basic rocks, contributions from various combinations of lower (granulite-facies) and upper (amphibolite-facies) crustal Lewisian gneisses, Moine pelite and psammite and Dalradian pelite are recognised.
Thompson (1982b) identified three compositional groups of granites throughout the British and Irish sectors of the North Atlantic Igneous Superprovince, based on mineralogical (and hence major-element) composition, trace-element composition and radiogenic isotope signatures. He argued that the primitive and peralkaline granites, which are the only types represented in the Hebridean Igneous Province, can be related to basaltic parental magmas through simple crystal fractionation involving plagioclase, alkali feldspar, quartz, iron-titanium oxides and apatite.
The two main basic magma-types that could fractionate to yield granitic liquids are those of the Skye Main Lava Series/Mull Plateau Magma-type and those of the Preshal More Magma-type. Investigations of the rare-earth element geochemistry of the Skye and Mull granites (Thorpe et al., 1977; Thorpe, 1978; Meighan, 1979; Walsh et al., 1979) showed an apparent match between the light-rare-earth-element profiles of the granites and those of the Skye Main Lava Series and Mull Plateau Magma-type, suggesting that they are related through fractional crystallisation. Thompson (1982b), however, advocated an origin through the mixing of 80 per cent of uncontaminated Preshal More basalt with 10 per cent each of granulite-facies and amphibolite-facies Lewisian gneisses. The abundant availability of Preshal More magmas during the growth of the Hebridean central complexes (with the possible exception of Rum), and the intimate association of tholeiitic rocks of this type with silicic rocks in many composite and other intrusions (see below), strongly support this contention, although derivation of the silicic rocks by fractionation of crustally contaminated Skye Main Lava Series/Mull Plateau magmas cannot be ruled out.
Peraluminous silicic liquids have been generated experimentally by melting Lewisian gneisses (granodiorite—tonalite) and Torridonian sedimentary rocks (feldspathic sandstone and siltstone) at 1 kbar water pressure (Thompson, 1981). These rocks are all completely melted by 930°C, a similar temperature to that required for the complete melting of granitic rocks from the Hebridean Igneous Province (Thompson, 1983). Therefore, if the granites are the product of the wholesale melting of the Lewisian and Torridonian lithologies, they should have similar trace-element signatures. However, the trace-element profiles for the granitic components of the Lewisian gneisses are very different from those of the Paleocene granites (Thorpe et al., 1977; Thompson, 1982b). Consequently, the granites cannot represent high-percentage melts of these relatively fusible lithologies.
Therefore, the granites most likely have a dual origin. From a detailed isotope study of the Skye granites, Dickin (1981) concluded that the dominant contaminant is Lewisian amphibolite-facies gneiss, similar to the contaminant introduced into the Preshal More Magma-type (P914156). On rising to shallower crustal levels, contaminated Preshal More magmas underwent fractionation in the amphibolite-facies Lewisian gneisses of the upper crust. The heat generated by this process resulted in partial melting of the gneisses, generating silicic melts which then mixed with silicic melts produced by fractional crystallisation of the contaminated Preshal More magmas. It is estimated that the proportion of crustal input was between 5 and 33 per cent of the mass of each intrusion, with a larger crustal fraction occurring in the younger granites, which had undergone a longer period of differentiation in the upper crust.
Magma mixing
Direct evidence for the co-existence of basic and silicic magmas is provided by a wide range of intrusions, including composite dykes and sills with contrasting basaltic and silicic components (Chapters 7; 8), intrusions such as the Loch Bà Ring Dyke of Mull where a suite of intermediate and basic inclusions is scattered through a rhyolitic matrix, and the Marsco Hybrids of Skye in which magma mixing has resulted in compositionally homogeneous hybrids (Chapter 9).
Examination of the composite intrusions commonly reveals that both members also contain evidence of pre-emplacement magma mixing; diffuse clots and small xenoliths of basic material are scattered throughout the silicic rocks, and corroded xenocrysts of quartz, alkali feldspar and sodic plagioclase are common in the basic members. Thus, the composition of the 'end-member' magmas that were originally involved in mixing may be obscure. Where recognised, the basic units are typically somewhat more evolved than basalt, and are usually basaltic andesite or ferroandesite/ferrodiorite, consistent with their generation by about 50 per cent fractional crystallisation of tholeiitic basaltic magma (Bell, 1983; Marshall and Sparks, 1984; Sparks, 1988; Bell and Pankhurst, 1993).
More-complete magma mixing has occurred in certain intrusions in the central complexes. In the Loch Bà Ring Dyke, the dominant rhyolitic material contains typically glassy inclusions that range in composition from ferrobasalt to rhyolite (including welded tuff), with tholeiitic andesites the most abundant (Blake et al., 1965; Walker and Skelhorn, 1966; Sparks, 1988). The compositional range in the Marsco Hybrids is similar to that in the Loch Bà Ring Dyke (Harker, 1904; Wager and Vincent, 1962; Wager et al., 1965; JD Bell, 1966; Thompson, 1969; BR Bell, 1983). The central ferrodiorite contains rare quartz xenocrysts and is thus, in part, a hybrid composition.
The likely spatial relationships and dynamics of the magmas in the magma chambers have been modelled by several investigators. Wager et al. (1965) envisaged a compositionally stratified magma chamber, with mechanical mixing occurring along the interface between the basic and overlying silicic magmas, each of which was convecting separately. The annular mass of hybrid magma so formed at the interface was intruded as the composite Marsco Hybrids ring-dyke.
The dispersed basic inclusions of the Glamaig Granite were explained by Thompson (1980a) as relicts of rounded blebs of basaltic magma that formed as the consequence of violent disruption in a convecting system initiated when basaltic magma invaded overlying silicic magma (compare with Sparks et al., 1977). From a detailed examination of the Loch Bà Ring-dyke, Sparks (1988) concluded that it was formed from a strongly zoned magma chamber in which there were two separately convecting parts, one vertically zoned in composition from tholeiitic basaltic andesite upwards into dacite, the other an overlying cap of rhyolitic magma. Emplacement occurred when rapid subsidence of the central block within the ring-fracture forced thorough mixing of the lower, hotter magmas with the cooler rhyolitic cap. Superheating of the rhyolitic magma led to exsolution of volatiles and rapid expansion, and as the two magmas mixed the resultant emulsion-like magma was injected up the ring-fracture as a gas-charged pyroclastic fluid.
Consideration of these examples of mixed magma intrusions, and of other bodies including intrusions in Centre 2, Ardnamurchan (Skelhorn and Elwell, 1966), the Mullach Sgar intrusions of St Kilda (e.g. Harding, 1966), the Glen More Ring-dyke, Mull (Bailey et al., 1924; Holmes, 1936; Fenner, 1937; Kerr et al., 1999) and composite intrusions on Arran (Kanaris-Sotiriou and Gibb, 1985), highlight not just the consequences of the co-existence and interaction of basic and silicic magmas, but also the dominant, driving force that the basalt magmas represent in almost all aspects of the magmatism of the Hebridean Igneous Province.