Journal of Petrology | Volume 40 | Number 6 | Pages 873-908 | 1999
© Oxford University Press 1999
Geochemical Evolution of the Tertiary Mull Volcano, Western Scotland
1 Department of Geology, University of Leicester, University Road Leicester LE1 7RH, UK
2 Department of Geology, Kings College, University of London The Strand, London WC2R 2LS, UK
3 Department of Geology, University of St Andrews North Haugh, St Andrews KY16 9ST, UK
Received April 8, 1998; Revised typescript accepted September 14, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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The early Tertiary Mull volcano, western Scotland, is one of the most dissected and best exposed igneous complexes of the North Atlantic Province. The new and published geochemical data enable us to chart the magmatic evolution of the Mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres, to the youngest dykes. In this study, we identify four successive magma types within the remnant volcano. The earliest type—the Mull Plateau Group—comprises mildly alkaline basaltic rocks with steep chondrite-normalized rare earth element (REE) patterns. This type is succeeded, within the lava succession and dyke swarm, by the Coire Gorm magma type with essentially flat chondrite-normalized REE patterns. A third magma type represented within the lava and dykes—the Central Mull Tholeiites—is more depleted in incompatible trace elements than the preceding types and has flat to LREE-depleted chondrite-normalized patterns. The major intrusions and cone sheets of Mull Centre 1 and early Centre 2 belong to this magma type. Midway through the igneous activity associated with Centre 2, the magma type changed to become more alkalic and more enriched in incompatible trace elements. This magma type (the Late Mull type) is found to persist through the cone sheets and major intrusions of Centre 3, to the youngest dykes. These changes in magma composition were related to variations in the mantle source and depth of partial melting beneath Mull, and/or differences in the efficiency of melt pooling before ascent through the lithosphere. With the exception of the early Staffa magma sub-type (part of the Mull Plateau Group), the location of magma chambers, in which the bulk of contamination occurred, changed with time from deep (lower-crustal Lewisian gneiss) to shallow (upper-crustal Moine schist). Intermediate members of the Plateau Group and the Late Mull magma type are enriched in Fe, Ti and P relative to the Central Mull Tholeiites. We attribute this difference to the more alkalic nature of these suites, lower fO2, and the formation of Fe3+–P complexes in the magma. The intermediate rocks were important in magma mixing processes, with two types of mixing identified on Mull: (1) cryptic mixing between basalts and low-Fe intermediate magmas, typified by lavas and early basic cone sheets of the Central Mull Tholeiite magma type; (2) observable mingling between rhyolitic magmas and high-Fe intermediate magmas of the Late Mull type, shown by the mixed-magma bodies of the Glen More and Loch Bà ring dykes. The main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas.
‘It may safely be maintained that Mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world’ (Bailey et al., 1924
, Memoir of the Geological Survey of Great Britain, Scotland, HMSO, Edinburgh, 1924).
KEY WORDS: Mull; crustal contamination; Fe–Ti enrichment; magma mixing; mantle melting; plume
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Introduction |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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Variations in the composition of igneous rocks emplaced at rifted continental margins can, potentially, provide important clues to the nature of high-level volcanic plumbing systems, magma chamber processes and mantle sources during continental break-up. As a consequence of subsidence of these rocks below sea level or uplift and erosion following volcanism, only rarely is there an opportunity to examine the chemical evolution of a magmatic centre from the inception to the close of igneous activity. Such an opportunity is presented by the Mull volcano, part of a chain of early Tertiary igneous centres on the western seaboard of Britain. These igneous centres fed small lava fields (e.g. Antrim, Mull, Skye–Canna), broadly equivalent in age and composition to the Tertiary flood basalt fields of the Faeroes and Greenland, and seaward-dipping seismic reflector sequences along the northwest European and east Greenland continental margins [for a review, see Saunders et al. (1997)
]. Igneous activity in northwest Britain accompanied rifting of the European and North American plates above a mantle hotspot or plume to form the North Atlantic Ocean, with the production of overthickened oceanic crust close to the main rift zone (Nunns, 1983; Bott, 1988; White, 1992a). This crust is preserved as the Greenland–Iceland and Iceland–Faeroes ridges, which are inferred to define the track of the plume. From 
18 Ma to the present day, this mantle plume has produced prodigious quantities of basaltic magma to form the Icelandic lava plateau.
The Mull igneous complex (Figs 1 and 2) lies on the eastern margin of the Sea of the Hebrides Basin. This basin was the site of significant volcanism during the period 63–55 Ma, apparently caused by decompressional melting of the Iceland plume below the North Atlantic region (e.g. White, 1988; Saunders et al., 1997). The Mull igneous centre lies adjacent to the Great Glen fault (Fig. 1), which is believed to have been influential in controlling its location (Bailey et al., 1924). This fault represents a major terrane boundary between an Archaean Lewisian basement gneiss complex and Late Proterozoic Moine schists to the north of the fault, and Early Proterozoic basement overlain by Dalradian metasediments to the south [see Smith & Watson (1983), Harris (1991) and Snyder et al. (1997) for more detail]. The occurrence of small exposures of Dalradian metasediments in the centre of the Loch Don anticline, southeastern Mull (Fig. 1) supports the contention of Bailey et al. (1924) that the Great Glen fault runs through southern Mull. This contention is also supported by investigations of the crustal structure beneath Mull (e.g. Bamford et al., 1977; Hall, 1978; Bott & Tantrigoda, 1987). Seismic and gravity data suggest that the lower crust below Mull is composed of granulite-facies Lewisian gneiss, which in mid-crustal regions grades into amphibolite-facies Lewisian gneiss. At upper-crustal levels, the Moine schists have been thrust over Lewisian gneisses. Significant exposures of Moine schists are preserved within the central igneous complex, and suggest that Moine metasediments underlay the Mull Tertiary volcano, an idea consistent with the surface exposure of Moine schists on the Ross of Mull (Bailey et al., 1924). Lewisian gneisses are exposed on the islands of Iona, Tiree and Coll, which lie to the west of the Moine thrust (Fig. 1). On Mull and Morvern, a highly condensed sequence (up to 100 m) of Triassic to Cretaceous sediments overlies the basement rocks; locally, at Loch Spelve, the Triassic sequence has a thickness well in excess of 100 m (Rast et al., 1968).
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The Mull volcano combines accessibility with good exposure and a wide range of rock types (e.g. Bailey et al., 1924
). This has allowed comprehensive sampling of the lavas that surround the remnants of the volcano, together with basaltic dykes, plugs, cone sheets and ring dykes exposed on Mull and Morvern. These rocks range in composition from high-magnesium basalt to rhyolite, and include some of the earliest Tertiary lavas and youngest dykes to be emplaced in the Inner Hebrides. In this paper, we use chemical and isotopic data to assess the petrogenesis of the Mull igneous rocks, the nature of the early Tertiary volcanic plumbing system beneath Mull and variations in the composition of the mantle source with time. |
The Mull Tertiary Igneous Centre—An Overview |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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The Mull Memoir of Bailey et al. (1924)
is an important landmark in our understanding of the petrogenesis of igneous rocks, not least because it introduced to igneous petrology the concept of magma types and magma series. Perhaps as a result of the thoroughness of Bailey et al.'s pioneering study, the Mull igneous centre received relatively little attention (in comparison with Skye and Rum) from petrologists until the late 1970s. Since that time, a large amount of geochemical data has become available, mainly through postgraduate research projects (e.g. Morrison, 1979; Thomson, 1986; Kerr, 1993a; Seedhouse, 1994; Preston, 1996). These data have provided the basis for this study.
Table 1 [modified from Skelhorn et al. (1969)] provides an outline of the main Early Tertiary igneous events associated with the activity of the Mull volcano. The initial expression of igneous activity was the pre-dominantly basaltic lava succession, the present-day thickness of which is 1800 m (Bailey et al., 1924). Weathering and erosion over the last 60 my may have reduced the original thickness of the lava pile substantially. On the basis of secondary amygdale mineral assemblages in the lavas, Walker (1970) proposed that the original thickness of the lava pile was in excess of 2200 m. This estimate is supported by a recent study (Kerr, 1997) of the plugs of Mull and Morvern, which preserve a range of magma types that is more diverse than the range seen in the lava succession. These plugs have substantial contact metamorphic aureoles, and may have supplied magma to the upper parts of the lava pile. The implication is that lavas with compositions similar to those of the plugs existed near the top of the succession, but have subsequently been eroded away.
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Two distinct igneous centres were identified in the Mull Memoir (Bailey et al., 1924
), an earlier Beinn Chaisgidle centre and a later Loch Bà centre (Fig. 2). Skelhorn et al. (1969), however, showed that the Beinn Chaisgidle centre (sensu Bailey et al., 1924) has an earlier, more southerly component, which they named the ‘Glen More centre’. The loci of igneous intrusion thus appears to have moved in a northwesterly direction as the Mull magmatic centre developed, from Glen More (Centre 1) through Beinn Chaisgidle (Centre 2) to the final centre, Loch Bà (Centre 3). The major intrusive bodies of the Mull central igneous complex range in composition from gabbros to granites (Table 1). Mixing between basic and more acidic magmas is evident both petrographically and chemically, and is an important phenomenon in the development of the Mull magmatic centre (see below).
The formation of the central complex led to the establishment of a significant hydrothermal circulation system within the surrounding lavas (Bailey et al., 1924; Walker, 1970; Morrison, 1979). This hydrothermal activity has resulted in the development of zones of alteration within the lava succession. These comprise an inner greenschist-facies zone of alteration (Fig. 1) and outer zeolite-facies zone (Walker, 1970), which decreases in intensity with distance from the central complex.
Emplacement of the Mull central igneous complex was accompanied by the intrusion of two suites of basic, through intermediate-to-acidic cone sheets, which Bailey et al. (1924) termed the Early and Late cone sheet series, respectively (Table 1). (Cone sheets are relatively thin intrusions which occupy conical fissures that have a common apex centred about an intrusive complex.) In addition, a series of broadly northwest–southeast-trending (Fig. 1) basic dykes are exposed on the northern and southern coasts of Mull (Sloan, 1971; Jolly & Sanderson, 1995). Palaeomagnetic data (Ade-Hall et al., 1972) and K–Ar ages (Mussett et al., 1972) suggest that intrusion of these dykes occurred throughout the history of the Mull igneous centre. Indeed, the final pulse of igneous activity on Mull appears to have been the emplacement of basic dykes cutting the last major intrusion of Centre 3, the Loch Bà ring dyke (see Skelhorn et al., 1969). A suite of early Tertiary dykes extending some 400 km from Ayrshire to northern England (the Cowal–Cleveland swarm, Tyrrell, 1917; Holmes & Harwood, 1929) also are clearly linked to the emplacement of the Mull central igneous complex (see Tyrrell, 1917; Macdonald et al., 1988).
Geophysical studies by Bott & Tantrigoda (1987) indicated a +50 mGal gravity anomaly over the Mull central complex, which was interpreted as a 6.5–13 km thick, mafic to ultramafic intrusive body with a volume of between 2000 and 3600 km3. By analogy with Skye (Bott & Tuson, 1973), the granitic rocks on Mull appear to form a thin skin on top of this mafic cylinder, with the granites of Centres 1 and 2 (Glen More and Beinn Chaisgidle) being 1 km thick and those of Centre 3 (Loch Bà) being 2 km thick. These granites represent only 5–9% of the total intrusive mass of the central complex (Bott & Tantrigoda, 1987).
In southeastern Mull, the trace of the Great Glen fault appears to have been deflected 5 km by intrusion of the central complex (Bailey et al., 1924; Walker, 1975). A further structural consequence of the emplacement of this large body of magma into the upper crust was the development of a series of arcuate (annular) folds in southwest Mull (Fig. 1). This folding is earlier than any of the exposed intrusions of the central complex (Bailey et al., 1924; Skelhorn et al., 1969), but is younger than the remnants of the early Tertiary lava succession exposed in southeast Mull (see Bailey, 1962).
Age and magnetic polarity of samples from the Mull igneous centre
The first radiometric age data for Mull lavas were obtained by Evans (1969), R. D. Beckinsale [unpublished data, reported by Mussett et al. (1972)] and Mussett et al. (1972). Samples from Ardtun and Bunessan, southwest Mull, gave K–Ar ages of 61.2 ± 2.0 Ma, 60.9 ± 2.0 Ma and 59.7 ± 2.0 Ma (1
errors), respectively, whereas a suite of lavas from Gribun in western Mull gave K–Ar ages ranging from 63 Ma to 46 Ma. The Gribun lavas appear to have suffered variable degrees of alteration and argon loss, which probably accounts for the wide range of ages. Basic dykes from the northwest coast of Mull also were dated by K–Ar methods (Evans, 1969; Mussett et al., 1972). These rocks gave ages ranging from 62 Ma to 45 Ma, again reflecting argon redistribution associated with partial alteration of the samples.
More recently, Mussett (1986) dated 20 whole-rock samples from Mull using 40Ar/39Ar incremental heating methods. Data obtained for 10 of these samples did not meet the acceptance criteria of Lanphere & Dalrymple (1978) but data for 10 other samples were interpreted as reliable estimates of the true crystallization ages. The Mull lavas gave 40Ar/39Ar ages ranging from 61.1 ± 1.4 Ma to 59.4 ± 0.9 Ma (1 errors), consistent with the K–Ar ages obtained for Ardtun and Bunessan lavas. In addition, several acidic rocks were analysed. The Loch Uisg granophyre (Centre 1) gave an age of 58.1 ± 1.6 Ma, and the Beinn a’ Ghraig granophyre and Loch Bà ring dyke gave ages of 57.0 ± 0.8 and 56.5 ± 1.0 Ma (1 errors), respectively.
A recent palynological study of interlava sediments from Tertiary Hebridean lavas (Bell & Jolley, 1997) placed the initiation of volcanic activity on Mull at 55 Ma, i.e. 4 my younger than the 40Ar/39Ar ages of the lavas. However, as pointed out by Kerr & Kent (1998) the reliable 40Ar/39Ar ages (Mussett, 1986) for the Mull lava succession were not discussed by Bell & Jolley (1997). Moreover, new 40Ar/39Ar dating studies (J. G. Fitton & L. Chambers, personal communication, 1997) of the Mull lava succession have confirmed (within error) the ages previously determined by Mussett (1986). It would thus appear that the palynological age for the inception of the Mull lavas is in error, and more work is required before this technique can be used as a reliable dating technique for the Tertiary Hebridean lava successions.
The 40Ar/39Ar ages outlined above suggest eruption of the lava sequence and emplacement of the intrusive centre over a fairly short period of time, perhaps 4–5 my, beginning at 61 Ma. This is broadly consistent with the results of palaeomagnetic studies of Mull lavas and dykes (e.g. Ade-Hall et al., 1972; Hall et al., 1977; Mussett et al., 1980; Dagley et al., 1987). The palaeomagnetic data suggest a polarity sequence of R–N–R– or R–N–R–N–R, the latter possibly being the result of thermal overprinting (Dagley et al., 1987). The R–N–R–N–R sequence could correspond to chrons 26R–26–25R–25–24R on the timescale of Berggren et al. (1995), i.e. the period from 61 Ma to 55 Ma.
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Geochemistry of the Mull Igneous Complex |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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Lava succession
Bailey et al. (1924)
divided the Mull lavas into two main types, the ‘Plateau magma type’ and the ‘Non-porphyritic Central type’, on the basis of a few chemical analyses and a multitude of petrographic observations. These two magma types were subsequently recognized world-wide and became the alkaline and tholeiitic types, respectively (Tilley, 1950; Wager, 1956). On Mull, this nomenclature has stood the test of time, with the Plateau magma type becoming the Mull Plateau Group (MPG, Morrison et al., 1980) and the Non-porphyritic Central type becoming the Central Mull Tholeiites (CMT, Kerr, 1995b). In recent years, a flow-by-flow chemical study (Kerr, 1993a, 1995b) of 600 lavas from the Mull succession has identified a third main type of lava on Mull, the Coire Gorm (CG) magma type (Figs 3 and 4).
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It should be emphasized that the three groups of Mull lavas identified by Beckinsale et al. (1978)
on the basis of 18 lava samples do not correspond to the three main magma types noted above. These workers proposed that each of their groups was derived by different degrees of partial melting of a lithospheric mantle source, with no crustal input and only minimal fractional crystallization. This interpretation now appears outmoded; extensive geochemical studies of the Mull lavas have suggested that the source of the lavas was asthenospheric mantle, and that the bulk of Mull magmas were variably contaminated by Lewisian (and/or Moine) crustal rocks before eruption (e.g. Morrison et al., 1980; Thompson et al., 1982, , 1986; Kerr, 1994, 1995a, 1995b; Kerr et al., 1995). Thus, Group 1 of Beckinsale et al. (1978) has been identified by subsequent workers as uncontaminated Plateau Group lavas, the Beckinsale et al. Group 2 as Plateau Group lavas contaminated with Moine schist and their Group 3 as Plateau Group lavas contaminated with Lewisian gneiss.
The sequence of Tertiary lavas exposed on Mull can be summarized as follows. The Mull Plateau Group forms the lower 730 m of the lava pile, and crops out throughout northern and western Mull [see Fig. 1 and Kerr (1995a) for further details]. The Coire Gorm lavas (at least 250 m thick) stratigraphically overlie the Plateau Group lavas and are exposed only at the top of Ben More. Several of the earliest Coire Gorm lavas are intercalated with the final more evolved lava flows (trachytes) of the Plateau Group. The Central Mull Tholeiite lavas are at least 900 m thick (Skelhorn et al., 1969) and occur in southeastern Mull, and within the Beinn Chaisgidle caldera (Centre 2) (Fig. 1). As lavas of the Central Mull Tholeiite type are not found within the main lava succession on Ben More, it is reasonable to assume that they were erupted after the Coire Gorm lavas.
The geochemical differences between the three main Mull lava types are illustrated in Figs 3 and 4 (see also Table 2). The element ratios, (Sm/Yb)cn, Ce/Y, La/Nd and Ti/Zr, were selected because they are not affected significantly by crustal contamination (although higher La/Nd ratios may be caused by contamination). In general, variations on these diagrams mainly reflect differences in the degree of partial melting or in the nature of the mantle source region [as well as the effects of low-pressure fractional crystallization in the case of falling F/(F + M) and very low Ti/Zr values]. The Plateau Group lavas represent a transitional tholeiitic-alkalic series, ranging in composition from high-magnesium (14 wt % MgO) basalt to trachyte [Figs 3 and 4a–d and Kerr (1993a)]. The basalt and hawaiites of the Plateau Group mostly have Ti/Zr ratios of <105 and (Sm/Yb)cn ranging from 1.6 to 3.5 (Fig. 4e and f). The Coire Gorm basalts have major element compositions that are similar to those of the Plateau Group basalts, but possess flatter rare earth element (REE) patterns and have Ti/Zr > 100 (Fig. 4). The Central Mull Tholeiite basalts have generally higher CaO, and lower abundances of incompatible trace elements, when compared with the Plateau Group and Coire Gorm lavas (Fig. 4a–d), (La/Sm)cn ranges from 0.6 to 1.5, (Sm/Yb)cn- < 1.9 and Ti/Zr ratios of >100 in the Central Mull Tholeiites (Fig. 4e and f).
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The different trace element ratios noted above suggest a change in the depth of partial melting with time, from deep, garnet-present melting (Plateau Group lavas) to shallow melting with little residual garnet in the source (Coire Gorm and Central Mull Tholeiite lavas). A similar observation has been noted for the Skye Tertiary dyke swarm (Mattey et al., 1977
) and lava succession (Wood, 1979; Thompson et al., 1980; Ellam, 1992), the early Tertiary Rockall Trough and Hatton Bank lavas (Morton et al., 1988) and in basalts from East Greenland (Fram & Lesher, 1993). On Mull, this change was interpreted by Kerr (1994, 1995b) to indicate partial melting below progressively thinning lithosphere as the magmatic system developed.
Subdivision of the Mull Plateau Group lava sequence
Kerr (1995a) subdivided the Plateau Group lavas into nine chemically distinctive sub-types (Tables 2 and 3). These sub-types are believed to reflect modification of the primary mantle-derived magmas by processes such as fractional crystallization, crustal assimilation and contamination by lithospheric mantle rocks. The earliest Plateau Group sub-type to be erupted, known as the Staffa magma sub-type (I; Table 3), appears to be extensively contaminated by Moine and Lewisian metasediments (Thompson et al., 1986; Kerr, 1999). Some of the later, more-alkalic high-magnesium basalts (IV) and basaltic hawaiites (VII) have assimilated small amounts of silicic crust (<5% contaminant; Kerr et al., 1995) from the Lewisian basement. However, not all of the high-magnesium basalts are crustally contaminated; a sub-type (V) with 13–14 wt % MgO and low Ba/Nb ratios (<15) has also been found (Table 3). These lavas are inferred to provide the closest approximation to the composition of the primitive asthenospheric melts (Kerr, 1995b). The more evolved basalts and hawaiites (VI and VII) are generally less contaminated with Lewisian crust (Ba/Nb <25; 
Nd > + 5) when compared with the high-magnesium basalts (Kerr et al., 1995). These, and the mugearites (VIII), benmoreites and trachytes (IX) appear to be related to the least contaminated Plateau Group basalts (V) by simple fractional crystallization (e.g. the modelled trends in Fig. 4a–d). Similarly, the basalts, intermediate rocks and trachytes belonging to the Skye Main Lava Series are related by simple fractional crystallization (Thompson et al., 1980).
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Cone sheets
Each of the three igneous centres on Mull is associated with a separate suite of cone sheets (Fig. 2), which range in composition from basic to acidic (Table 4; Bailey et al., 1924
; Thomson, 1986). Bailey et al. subdivided the cone sheets into three broad groups: early basic cone sheets (EBCS), early acidic-to-intermediate cone sheets (EACS) and late basic cone sheets (LBCS), which are essentially ferrodioritic in composition. Subsequent work by Thomson (1986) revealed that the picture is more complicated than this: Centre 1 cone sheets comprise early basic and early intermediate-to-acidic varieties; Centre 2 cone sheets comprise both early and late, basic and intermediate-to-acidic varieties; Centre 3 cone sheets comprise late basic and late intermediate-to-acidic varieties. Cone sheets from all three intrusive centres preserve evidence of magma mixing [see below and Thomson (1986)]. For the purposes of this study, we have used 4 wt % MgO to divide the more basic cone sheets from the more acidic varieties.
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Basic cone sheets
There are significant field and petrographic differences between the EBCS and LBCS. The EBCS generally are coarse grained and contain olivine (often pseudomorphed). Individual cone sheets are generally thicker than those of the LBCS, which contain no olivine and are generally fine grained. The two suites also differ with respect to their chemistry (Table 4; Thomson, 1986)
. Figure 5a and c reveals that at equivalent MgO contents, the LBCS possess levels of incompatible trace elements that are slightly higher than those measured in the EBCS. This is particularly evident in the concentrations of the REE (Table 4 and Fig. 5c). Plots of incompatible elements and isotopic ratios (Fig. 6) and major elements against MgO (Fig. 7) also serve to illustrate these differences. Thomson (1986) concluded that the differences in trace element contents between the EBCS and LBCS are not related to varying degrees of crustal contamination. She suggested that the two cone sheet suites represent different degrees of partial melting, or are related by fractional crystallization.
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One of the most notable features of the LBCS, in comparison with the EBCS, is a marked enrichment in Fe, Ti and P (Fig. 7), as a result of the late onset of Fe–Ti oxide and apatite fractionation. Thomson (1986)
attributed this to crystallization of the LBCS under conditions of low fO2, when compared with the EBCS. In this regard, it is noteworthy that some of the Tertiary plugs in north Mull and Morvern (Kerr, 1997), together with the youngest of the basic dykes (see below), display compositions that are similar to those of the LBCS.
On a primitive mantle-normalized multi-element plot (Fig. 5a), basic cone sheets from Mull show strong negative Nb anomalies. Elsewhere in the Hebrides, such anomalies in basalts are inferred to reflect crustal contamination, particularly when associated with low 206Pb/204Pb and high 87Sr/86Sr ratios (e.g. Thompson et al., 1982, , 1986; Kerr et al., 1995). In the Mull cone sheets, Nb contents have been reduced by assimilation of crustal rocks; however, the large-ion lithophile elements (Rb, Ba, K, Sr) show substantial increases over their original concentrations in these rocks, resulting in greatly exaggerated Nb anomalies. Crustal influences are seen also in the Nd–Sr isotopic compositions of the Mull cone sheets (Thomson, 1986). For example, Fig. 6b shows that these rocks possess high 87Sr/86Sr and low 143Nd/144Nd ratios relative to depleted mantle compositions. Given these features, Moine metasediments appear to represent the most likely contaminants of these cone sheets (Fig. 6b). This idea is supported by studies of cone sheets at Loch Scridain (Fig. 2; Kille et al., 1986; Thompson et al., 1986; Preston & Bell, 1997; Preston et al., 1998), which belong to the EBCS and are probably related to Centre 1 magmatism. The Loch Scridain cone sheets generally have 143Nd/144Nd 0.5119 (Preston et al., 1998), comparable with Nd isotope ratios measured for Moine rocks [see Fig. 6b, and Thompson et al. (1986) and Thomson (1986)]. These cone sheets also contain numerous partially melted xenoliths of Moine metasediments (Kille et al., 1986; Preston et al., 1998).
Acidic–intermediate cone sheets
Thomson (1986) used elemental and isotopic data to demonstrate that the more acidic cone sheets and the Mull granites (see next section) do not represent crustal melts, as the bulk compositions of felsic Moine and Lewisian rocks are very different from the compositions of the acidic cone sheets. Representative primitive mantle-normalized multi-element plots of the early acidic cone sheets (Centres 1 and 2) and the late acidic cone sheets (Centres 2 and 3) (Fig. 5b) are very similar to those of the basic cone sheets for the most incompatible elements. This implies that the acidic cone sheets are also crustally contaminated. On a plot of 87Sr/86Sr vs MgO (Fig. 7d), the basic and acidic cone sheets of Centres 2 and 3 display excellent negative correlations, i.e. within each centre, the most evolved rocks are the most contaminated. This suggests that crustal assimilation largely occurred in tandem with fractional crystallization. A similar mechanism is proposed for Centre 1 intrusions (Fig. 7d), although it should be noted that Sr isotopic data for these rocks display slightly more scatter than those for cone sheets from Centres 2 and 3. In addition, the Centre 1 cone sheets appear to contain a greater proportion of the contaminant when compared with those of Centres 2 and 3 [see Thomson (1986)]. Interestingly, some of the Centre 3 basic and acidic cone sheets have Ba/Nb ratios significantly lower than those of other Centre 3 cone sheets, implying that the low Ba/Nb cone sheets are not as highly contaminated with crust, or are contaminated with material which is less Ba-enriched.
At equivalent MgO contents, the early and late acidic–intermediate Mull cone sheets possess subtly different chemical signatures (Figs 5 and 7), with Centre 2 again having representatives of both suites. The late acidic–intermediate cone sheets are similar to their more basic counterparts in that they possess generally higher levels of incompatible trace elements (Fig. 5b and d). In addition to a small negative Ti anomaly (Fig. 5b), the late acidic–intermediate cone sheets have high P relative to the early acidic cone sheets. This suggests that the onset of apatite crystallization was delayed in the late cone sheets. A trend towards Fe–Ti and P enrichment, which was evident in the LBCS, is seen to continue in the late more acidic cone sheets up to about 3 wt % MgO for Fe and Ti and 2.5 wt % MgO for P (Fig. 7). Geochemical modelling using the TRACE 3 program (Nielsen, 1988) suggests that the late basic and acidic cone sheet magmas could be related to each other by fractional crystallization at low fO2 (Fig. 7b). Differentiation at slightly higher fO2 (more oxidizing conditions) could explain the relationship between the early basic and acidic cone sheets (Thomson, 1986).
As will be shown, the rocks from all three centres of the Mull central complex display field, petrographic and chemical evidence for magma mixing. Mixing between the acid and basic end members of the Centre 1 cone sheets is particularly well demonstrated in plots such as Fig. 7a–c, which possess a marked inflection because of the fractionation of Fe–Ti oxide or apatite. Magma mixing will result in trend lines that do not follow the predicted liquid line of descent, but rather cut across below the apex of the inflection as demonstrated in Fig. 7a–c.
Figure 6d reveals that there is a correlation between (La/Nd)cn and initial 87Sr/86Sr for the early Mull cone sheets (both basic and acidic rocks). In general, EBCS samples with the lowest 87Sr/86Sr (<0.7060) also possess the lowest values of (La/Nd)cn (<1). In contrast, LBCS samples with 87Sr/86Sr ratios of <0.7065 have (La/Nd)cn > 1 (Fig. 6d). This suggests that contamination by Moine metasediments will increase the (La/Nd)cn ratio of a basaltic magma, an idea consistent with high light REE (LREE) concentrations in the Moine metasediments [i.e. (La/Nd)cn > 2.5; Thompson et al., 1986; Kerr, 1999]. Moreover, the data suggest that the parent magmas of the EBCS may have been significantly depleted in the LREE relative to the parent magmas of the LBCS (see also Fig. 5c). The least contaminated and more mafic EBCS have chondrite-normalized REE patterns similar to those of the Central Mull Tholeiites (Fig. 5c) and so appear to belong to this magma type. In contrast, the least contaminated and more mafic LBCS magmas are moderately enriched in the LREE, and are compositionally similar to some late plugs and dykes on Mull. We will call this more enriched (than the Central Mull Tholeiite type) magma type the Late Mull (LM) magma type.
Major gabbroic and granitic intrusions
The major intrusions of the Mull central igneous complex mostly take the form of ring dykes and stocks (Bailey et al., 1924). With four exceptions [the Ben Buie and Corra-bheinn layered gabbros, and the An Cruachan and Gaodhail augite diorites; Fig. 2 and Skelhorn et al. (1969, , 1979)], the major intrusions are granitic or are mixed-magma bodies containing a significant granitic component. Only a very small amount of whole-rock geochemical data are currently available for the major basic intrusions, and these come from the chilled margin of the Ben Buie gabbro, the last major intrusion of Centre 1 [Skelhorn et al. (1979) and Table 5]. Samples of this gabbro have high-Ca tholeiitic compositions and flat to LREE-depleted chondrite-normalized REE patterns (Fig. 5c). The Ben Buie gabbro is thus compositionally similar to some of the EBCS and the youngest preserved lavas. On the basis of these characteristics, these rocks are considered to belong to the Central Mull Tholeiite magma type (see above).
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Detailed petrological studies by Pankhurst et al. (1978)
and Walsh et al. (1979) showed that the Mull granitoids can be divided into two compositionally distinct groups: those of Centre 1 (‘early granites’) and those belonging to Centres 2 and 3 (‘late granites’). The compositions of the early and late Mull granites are shown in Figs 6c–d, 7d, 8 and 9, and Table 5. The late granites have high K2O, Na2O, Al2O3, MnO, Zr, Y and Nb, and low CaO, P2O5, TiO2, Sr, V, (Ce/Yb)cn and initial 87Sr/86Sr relative to values for the early granites (Walsh et al., 1979). A significant overlap and obvious trends between the compositional fields of the granites and the acidic cone sheets of Centres 1 and 2 (e.g. Figs 7–9) suggest that the early and late granites are related to their respective acidic (and basic) cone sheet magmas by fractional crystallization with variable amounts of contamination by Moinian and/or Lewisian crust. Centre 1 acid magmas appear to be the most contaminated by crust (highest initial 87Sr/86Sr; Fig. 7d), whereas Centre 2 magmas may contain a smaller proportion of crust (lowest 87Sr/86Sr; Fig. 7d) (Walsh et al., 1979) than both Centres 1 and 3.
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Petrographic observations and geochemical modelling suggest that the crystallization sequence for the early magma suite (basic and acidic cone sheets and granites, predominantly from Centre 1) initially involved plagioclase and pyroxene, with small amounts of olivine. As crystallization continued, Fe–Ti oxide(s) joined the fractionating assemblage at
4 wt % MgO, followed by apatite at 3 wt % MgO. The point at which Fe–Ti oxide(s) and apatite started to fractionate is somewhat difficult to estimate from Fig. 7a and b because of extensive mixing between basic and acidic members of the early magma suite (Thomson, 1986). In the late magma suite, comprising most of the Centre 2 and Centre 3 rocks, the onset of Fe–Ti oxide and apatite fractionation was suppressed until 3 wt % MgO and 2 wt % MgO, respectively. Therefore, because plagioclase and clinopyroxene were the main fractionating phases over a wide compositional interval in the late acidic suite, these cone sheets and granites have abundances of Sr, CaO (Fig. 8) and V (not shown) that are lower than those of equivalent rocks from Centre 1. At the point of apatite and Fe–Ti oxide fractionation in the late acidic magmas, the residual melt was highly enriched in P, Ti, V and Fe, such that these phases were able to form a significant proportion of the fractionating assemblage. Consequently, relative to the early magma suite, removal of these elements from the melt was more efficient. The net result is that the late granites of Centres 2 and 3 have generally low concentrations of P2O5, TiO2, Fe2O3* and V when compared with those of Centre 1 (Walsh et al., 1979).
The late granites of Centre 3 are, by and large, the most evolved of the three centres (e.g. Figs 8 and 9). Low Ba and K2O in some of the more evolved rocks suggest that the parent magmas may have begun to fractionate alkali feldspar. The most fractionated granite of all (74.6–76.7 wt % SiO2; Walsh et al., 1979) is a large dyke (Toll Doire; Fig. 2) associated with the main portion of the Beinn a’ Ghraig granophyre. One of these Toll Doire granite samples has a Zr content significantly lower than that of other Toll Doire samples (Fig. 8c), and at such high degrees of differentiation this may be indicative of zircon fractionation.
Figure 9a shows the slightly more alkaline nature of the late cone sheets and late granites, with many of the samples plotting above the alkali–tholeiite divide; this is in contrast to the early cone sheets and associated granites, which almost all plot below the divide. This is consonant with higher incompatible trace element contents (at equivalent MgO contents) in the late cone sheet and granite series as opposed to the early series (Figs 5 and 8). These features suggest that the parental magmas of the late series were derived either from a more enriched mantle source region or by smaller degrees of mantle melting.
Major mixed-magma intrusions
Glen More ring dyke
The Glen More ring dyke (Fig. 2) is the youngest intrusion in Mull Centre 2. At its western termination the ring dyke consists of coarse- to medium-grained olivine gabbro passing upwards into quartz dolerite, diorite, granodiorite, and granophyre (Bailey et al., 1924; Bor, 1951). Although poorly exposed, boundaries between these units appear to be gradational. Evidence of magma mixing occurs in the form of acidic patches, veins and enclaves in the gabbro and diorite, and sub-rounded clots and xenoliths of intermediate (ferrodioritic) composition in the granophyre [for details of field and petrographic relationships, see Bailey et al. (1924)].
H. H. Thomas and E. B. Bailey [in Bailey et al. (1924)] proposed that the Glen More ring dyke formed by in situ differentiation, resulting in the segregation and upward filtration of residual acidic material through the solidified gabbro–dolerite [see also Koomans & Kuenen (1938)]. This suggestion was refuted on petrographic grounds by Holmes (1931) and Fenner (1937), who believed that the ring dyke originated by mixing between intermediate and acidic magmas. Bor (1951) offered a compromise solution, in which fractionation of a gabbroic mineral assemblage produced a residual liquid of intermediate composition; subsequently, this magma became hybridized by mixing with acidic liquid. The acidic melt was considered to have formed by differentiation of a batch of basaltic magma intruded after the gabbro–dolerite body had crystallized. Field observations by Marshall (1984) and least-squares modelling of whole-rock and clinopyroxene compositions (Seedhouse, 1994) broadly support Bor's hypothesis.
The first major study on the Glen More ring dyke since Bor (1951) was carried out by Seedhouse (1994). Thirty samples for major and trace element analysis (see Table 6) were collected from throughout the height of the ring dyke. Chemically, the gabbros and diorites from the ring dyke possess compositions which are similar to those of the LBCS, in that they display a tendency towards high Fe2O3*, TiO2 and P2O5 and are slightly more enriched in incompatible trace elements when compared with the EBCS (Fig. 7a–c; Table 6). On first consideration, it appears that the parent magmas of the Glen More ring dyke were more magnesian than the most mafic representatives of the LBCS. Seedhouse (1994) has shown that olivines from the most mafic gabbro (sample GM13; Table 6) possess maximum forsterite contents of 72, which would have been in equilibrium with a magma containing 5–6 wt % MgO. Seedhouse proposed that the more mafic gabbros of the Glen More ring dyke contain cumulus olivine and augite, and that the parent magma of the gabbros and diorites was akin to the composition of sample MT3 (Table 6), which contains 5.4 wt % MgO. Fractionation of olivine, augite and plagioclase from this parental magma produced the diorites and the cumulate gabbros (Fig. 7). On primitive mantle-normalized multi-element plots (Fig. 10), the ring dyke samples have high Rb/Nb ratios (comparable with those of the LBCS) and are most probably contaminated with Moine metasediments. As for the LBCS, fractionation of Fe–Ti oxides and apatite was suppressed until <4 wt % MgO and 2 wt % MgO, respectively. Figure 10b–c shows the increasing influence of plagioclase, apatite and Fe–Ti oxide fractionation, in the form of negative Sr, P and Ti anomalies, with increasing degree of magmatic evolution. The granophyres of the Glen More ring dyke are, not surprisingly, very similar in composition to the other granites of Centres 2 and 3 (see Fig. 10b). It is probable that these granophyric magmas were derived from a parental magma similar to the Glen More diorite, in a deeper, probably older magma chamber (Seedhouse, 1994).
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It can be seen from Fig. 7 that the granophyric and more intermediate rocks plot along rather tightly constrained trends. Three lines of evidence suggest that magma mixing is the most likely explanation for these trends (Seedhouse, 1994
): (1) the upper part of the granophyre has a chilled margin of the same composition, indicating that the granophyric magma was intruded into the ring dyke and did not form by in situ differentiation; (2) the intermediate rocks have a blotchy heterogeneous nature; (3) the margins of clinopyroxene and plagioclase crystals in diorite enclaves display resorption textures where they are in contact with the granophyres. Mixing has occurred between a residual dioritic magma and a separate granophyric magma intruded from below. The blotchy appearance of the intermediate rocks means that mixing has not been complete or vigorous. Possible reasons for this, and the mechanics of magma mixing in relation to the Glen More and Loch Bà Ring dykes will be discussed below.
Loch Bà ring dyke
The Loch Bà ring dyke is a fine-grained, sometimes flow-banded, rhyolite body up to 400 m thick and some 8 km in diameter (Fig. 2). The ring dyke forms part of a silicic–mafic intrusive complex centred on Glen Cannel [Mull Centre 3 of Skelhorn et al. (1969)] and, as such, represents one of the last Tertiary igneous events of the Mull volcano. The field and textural relationships of the intrusion have been discussed in detail by Bailey et al. (1924), Lewis (1968), Marshall (1984) and Sparks (1988), all of whom noted the presence within the rhyolite of abundant mafic inclusions consisting of predominantly glassy or cryptocrystalline material. Sparks (1988) interpreted the ring dyke as a rheomorphic welded tuff emplaced during the collapse of the Centre 3 caldera. Following Marshall (1984), he considered the ring dyke to have been emplaced from a compositionally zoned magma chamber, in which extreme differentiation of tholeiitic magma had occurred. In this situation, assimilation of silica-poor cumulate materials derived from andesitic liquids was suggested to modify the composition of less evolved magmas, resulting in significant departures from the ‘normal’ liquid line of descent (Sparks, 1988, pp. 458–459). Sparks believed that the Loch Bà intrusion evolved in a manner similar to that suggested by Hunter & Sparks (1987) for the Skaergaard complex of East Greenland, i.e. along a path of silica enrichment and iron depletion following the precipitation of magnetite.
Sparks (1988) reported only major element concentrations from the Loch Bà ring dyke, and so detailed comparison with other rocks from Centres 2 and 3 is not possible [but see Marshall (1984)]. The dark glassy rocks range in composition from ferrodiorite (53 wt % SiO2, 19 wt % Fe2O3*) to rhyolite (71 wt % SiO2). The occurrence of ferrodiorites commonly with elevated TiO2 (Fig. 7b) places the Loch Bà ring dyke within the same petrogenetic framework as the rest of the late Centre 2 and Centre 3 igneous activity. The Loch Bà rhyolites, when essentially free from dark inclusions, range in composition from 70 to 76 wt % SiO2, and are comparable in composition with acidic cone sheets and granites from Mull Centres 2 and 3 (Figs 7–9).
Basaltic dykes
The Mull regional dyke swarm is one of the largest in the Hebrides, extending southeastward across southern Scotland and northern England (e.g. Tyrrell, 1917; Holmes & Harwood, 1929), and northwestward to the Outer Isles (Jehu & Craig, 1925). On Mull, the structure, distribution and emplacement style of the dykes was studied by Sloan (1971) and Jolly & Sanderson (1995). These workers showed that the mean thickness and spacing of dykes increase with distance from the central intrusive complex, consistent with a decrease in the amount of crustal extension (21% to 2%). Mafic dykes occur in all parts of the swarm, but felsic dykes (felsites, quartz porphyries, trachytes) are largely confined to the area close to the intrusive complex. On the basis of field relationships, it is thought that intrusion of the dykes occurred throughout the period of Tertiary igneous activity.
The petrography and compositions of Mull-related dykes have been studied by Lamacraft (1978), Thompson (1982) and Kent (1995; see also Table 7). The majority of the dykes analysed belong to the Plateau Group and Central Mull Tholeiite magma types, with a smaller number of Coire Gorm-type dykes (Table 7). The Plateau Group dykes generally have low MgO, Ni and Cr contents (typically 4–6 wt %, <100 ppm and <60 ppm, respectively), and high abundances of Fe2O3* and TiO2 (up to 17 wt % and up to 3.4 wt %, respectively) (Fig. 11). With the exception of hawaiite samples, most of these dykes are depleted in Nb relative to primitive mantle. Coire Gorm-type dykes have MgO 6–7 wt %, moderate Fe2O3* (12–14 wt %) (Fig. 11), and show relative depletion in Nb. The Central Mull Tholeiite-type dykes include high-magnesium samples (up to 11 wt % MgO; Kent, 1995), but usually are moderately evolved (7–8 wt % MgO, up to 250 ppm Ni and up to 550 ppm Cr) (Fig. 11). All analysed Central Mull Tholeiite-type dykes have <4 ppm Nb (Table 7).
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) exposed along the northern coast of Mull (Chemical analyses of Mull-related Tertiary dykes in southern Scotland and northern England, including the Cleveland en echelon dyke suite, reveal that they too are of Central Mull Tholeiite type, i.e. tholeiitic basalts exhibiting enrichment in the LREE and with essentially flat chondrite-normalized heavy REE (HREE) patterns (Macdonald et al., 1988
). These dykes possess high initial 87Sr/86Sr ratios (up to 0.7123; Moorbath & Thompson, 1980) and appear to be extensively contaminated by Moine metasediments. Thus, they are compositionally very similar to the Mull EBCS, and may even have been derived from the same magma reservoir. The lateral injection of 85 km3 (Macdonald et al., 1988) of basaltic magma for up to 400 km from the Mull centre may be linked to the collapse of the Centre 1 caldera. This suggestion is supported by calculations showing that the volume of magma that would need to be removed from an underlying chamber to accommodate the collapse of the Centre 1 caldera is approximately the same as that inferred for the Cleveland dyke suite [see Macdonald et al. (1988)].
Plugs of north Mull and Morvern
In a recent study, Kerr (1997) assessed the geochemistry and petrogenesis of the previously little-studied plugs of north Mull and Morvern. Field evidence suggests that the plugs may have acted as feeder conduits for the lava succession; moreover, the compositions of several of the plugs closely resemble the Central Mull Tholeiite and Plateau Group lavas. Despite substantial contact metamorphic aureoles, several Mull plugs have compositions that are not preserved in the lava succession. Certain of these plugs are low-Fe benmoreites and trachytes (Kerr, 1997) resembling evolved lavas from Skye (Thompson et al., 1972). The low-Fe plugs also are similar in composition to the Mull early acidic cone sheets (see above). Like these, the plugs are probably fractionates of a Central Mull Tholeiite-type magma.
Most of the more basic plugs (Table 8) resemble rocks of the Beinn Dearg More magma type (Thompson & Morrison, 1988), represented by a group of incompatible trace element enriched, late-stage dykes on Skye (Fig. 12). Relative to the Beinn Dearg More magma type, the more basic Mull plugs are less enriched in incompatible trace elements and less alkaline. In addition, these Mull plugs show a tendency towards Fe enrichment, not observed in the Beinn Dearg More dykes (Fig 12b). The trace element enriched character (relative to the Central Mull Tholeiite magma type; Fig. 12) of these more mafic Mull plugs is, however, reminiscent of that of the Late Mull magma type—as represented by the LBCS and the late Mull dykes (see previous section on cone sheets). Kerr (1997) speculated that lavas with compositions similar to those of the Late Mull magma type plugs were erupted, but were near the top of the lava succession, and so have been eroded away.
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Some of the Late Mull magma type plugs have MgO contents well in excess of 10 wt % (Table 8). These rocksare enriched in Ba (e.g. Ba/Nb >25; Fig. 12a) and appear to have assimilated some Lewisian crust. The magmas supplying the high-magnesium plugs probably came through the crust well away from the large upper-crustal magma chambers of the Mull central intrusive complex, thus avoiding excessive fractionation and contamination by upper-crustal (Moine) rocks. This means that they are closer in composition to their parental magmas when compared with the LBCS, which as we have shown earlier are the more-evolved and contaminated representatives of the Late Mull magma type.
Dykes cutting the Loch Bà ring dyke
During mapping of the Loch Bà intrusion (Figs 1 and 2), E. B. Wright and J. E. Richey (unpublished field slips, dated 1910–1913) noted the occurrence of basaltic dykes cutting the ring dyke along its northwestern edge close to Loch Bà. These dykes represent the final pulse of basaltic magmatism on Mull. Discussed briefly by Bailey et al. (1924), the geochemistry of these late dykes has not been studied previously.
We have collected and analysed a representative set of samples, major and trace element data for which are given in Table 7 and Fig. 12. Two samples are low-alkali, high-Ca tholeiites belonging to the Central Mull Tholeiite magma type. The remaining samples have low to moderate MgO (3.6–6.7 wt %) and are enriched in incompatible trace elements relative to Central Mull Tholeiite-type basalts. These basaltic dykes can be classified as belonging to the Late Mull magma type (Fig. 12). In common with the LBCS, the Late Mull magma type dykes show an Fe–Ti enrichment trend, with two samples (LB7 and LB8) possessing >20 wt % Fe2O3* (Fig. 12) and >3.3 wt % TiO2. These Late Mull magma type dykes have slightly high Ba/Nb (19–52) relative to sample LB6 (Central Mull Tholeiite type), suggesting that they have assimilated small amounts of Lewisian crust (Fig. 12c).
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Discussion |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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Magma mixing processes
The deeply dissected Tertiary volcanoes of the Hebrides form a classic area in which to study composite intrusions. Such bodies are believed to represent quenched magmatic systems in which intimate, but incomplete, mixing between physically and chemically diverse magma types has occurred (Harker, 1904
; Skelhorn, 1959). Field and petrographic evidence suggests that the most primitive mixing end-member in many Hebridean composite igneous intrusions is a ferrodioritic liquid, derived from a parental tholeiitic magma by fractional crystallization processes (e.g. Harker, 1904; Bell, 1983; Marshall & Sparks, 1984; Sparks, 1988).
Two distinct types of magma mixing are observed within the igneous rocks of the Mull Tertiary volcano. First, there is what we term ‘observable magma mingling’, i.e. clear field evidence of mixing, such as convolute or crenulated contacts between silicic and mafic rocks, or ragged inclusions and pillows of mafic and/or intermediate material within a silicic host. This type of mixing is exemplified by the Loch Bà and Glen More ring dykes. Similar bodies are found in several other British Tertiary igneous centres, notably St Kilda, Skye and Ardnamurchan. They can take the form of either net-veined intrusions, such as the Mullach Sgar complex on St Kilda (Marshall & Sparks, 1984), or mixed-magma bodies, such as the Glamaig granite and Marsco summit gabbro on Skye (Thompson, 1968, , 1980; Bell, 1983). Second, there is what can be described as near-complete or cryptic magma mixing (hybridization), where the only evidence of magma mixing comes from whole-rock composition or from petrographic evidence in the form of disequilibrium phenocrysts or partially resorbed and regrown phenocrysts. Petrographic evidence from the Plateau Group lavas, in the form of resorbed and regrown plagioclase phenocrysts, suggests that cryptic magma mixing was also an important process early in the evolution of the Mull igneous centre (Kerr, 1998). In the discussion below, we focus on the EBCS, which display chemical and petrographic evidence of magma mixing. In contrast to observable magma mingling, the ‘mafic’ end-member in the EBCS is not of ferrodioritic composition, but rather is a moderately evolved basalt with 7–8 wt % MgO (Fig. 7). The silicic end-member is a diorite (Fig. 7–9), i.e. a composition less evolved than that suggested to give rise to observable magma mingling (e.g. Bell, 1983).
In the following section, we explore possible explanations for the two styles of magma mixing identified within the Mull Tertiary igneous complex. We address two related questions:
- Why do the early cone sheets (Centre 1 and early Centre 2) display much greater evidence for cryptic magma mixing than the late cone sheets (late Centre 2 and Centre 3)?
- What were the principal factors that determined the various styles of magma mixing on Mull?
Aside from evidence of cryptic magma mixing, the Mull early cone sheets are more contaminated with upper-crustal rocks and extend to compositions that are more mafic or more evolved than those of the late cone sheets (Figs 7–9). These two features are probably related, because the initially more mafic nature of the early cone sheet magmas may have permitted them to assimilate a larger volume of the country rock than was possible for the late cone sheet magmas [see Campbell (1985) and Huppert & Sparks (1985)]. Additionally, a high heat flux into the chamber from which the early cone sheet magmas were derived could have contributed to the efficiency of the magma mixing process. In the Mull late cone sheets, there appears to have been some mixing between magmas lying at either side of the apex on a plot of MgO vs TiO2 (Fig. 7). In these rocks, mixing occurred between magmas with concentrations of TiO21 wt % higher (at equivalent MgO contents) than those of the early cone sheets. The major element compositions of these mixing end-members were not appreciably different from one another.
In their modelling of the Mullach Sgar complex on St Kilda, Sparks & Marshall (1986) identified a zone in TiO2–MgO space (at high proportions of silicic magma relative to mafic) where mixing will not occur (Fig. 13). The Mull late cone sheets and the Loch Bà and Glen More ring dykes largely fit this model, but the Mull early cone sheets display a complete mixing spectrum right across the zone of ‘prohibited’ mixing proposed by Sparks & Marshall (1986) (Fig. 13). Figure 14, a plot of density vs MgO, illustrates why this might be the case. Some of the most mafic early cone sheets lie in a density minimum [when olivine has been joined on the liquid line of descent by plagioclase or clinopyroxene; see Stolper & Walker (1980)]. With increasing fractionation of a plagioclase- and pyroxene-dominated assemblage, the density of a residual magma will increase to a maximum just before the onset of Fe–Ti oxide fractionation, after which the density drops sharply. Figure 13 shows two mixing zones: one for the magma mingling in the Glen More and Loch Bà ring dykes (Zone 1) and the other for the Mull early cone sheets (Zone 2). It is noticeable that the overall gradient for Zone 2 is much gentler than that for Zone 1. This suggests that the higher density of Hebridean Fe–Ti-rich magmas acted as a substantial thermodynamic barrier to mixing, in contrast to mixing between more silicic end-members and basalts lying at the density minimum. Although the viscosity contrast between the more evolved Fe–Ti-rich magma and the silicic magma will be less than that between the more mafic, density minimum, basalt and the silicic magma, relative density appears to have been the most important factor in determining the intensity and efficiency of mixing [see Oldenburg et al. (1989)]. In addition, we observe that the silicic end-member for the early cone sheet mixed magmas is less evolved (1–2 wt % MgO; Fig. 13b), and so less viscous and more dense, than the silicic end-member proposed by Sparks & Marshall (1986) for magma mixing at Mullach Sgar, St Kilda.
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It is significant that magma mingling involving an Fe–Ti-rich magma is very commonly preserved in the ring dykes of the British Tertiary province (Sparks & Marshall, 1986
). These ring dykes have formed as a result of the collapse of a central caldera into an underlying zoned magma chamber, followed by the intrusion of magma along the ring dyke fracture. Thus, mixing has been physically forced upon the system by caldera collapse and, in the case of Loch Bà, exsolution of gas, which was possibly caused either by pressure release or influx of a denser magma (see Thompson, 1980; Sparks, 1988).
In our discussion of the Mull–Morvern plugs, we noted that they preserve compositions more primitive (up to 11 wt % MgO) than those of the LBCS (up to 5.5 wt % MgO). We reasoned that this was because they were emplaced well away from the density filtration trap of the Mull central igneous complex. A similar explanation can be advanced as a reason why the EBCS extend to 10 wt % MgO. In the early stages of activity in the central complex, the magmatic plumbing system may have been relatively simple, with few magma chambers. Thus, it would have been relatively easy for density-minimum basalts to ascend from Moho or lower-crustal magma chambers (where they had fractionated from primitive olivine tholeiites) into high-level magma reservoirs. As the plumbing system developed, more magma chambers (i.e. traps) would have developed in the crust, making it increasingly difficult for density-minimum basalts to pass into the upper crust without undergoing significant further fractionation (to form Fe–Ti-rich magmas).
In summary, the emphasis placed by previous workers (e.g. Bell, 1983; Marshall & Sparks, 1984; Sparks & Marshall, 1986) on the importance of magma mixing in the petrogenesis of Hebridean Fe–Ti-rich intermediate magmas appears unjustified. Rather than being fundamental to the mixing process, the high density of Fe–Ti-rich end-member magmas may actually have inhibited their complete hybridization with more silicic liquids. Therefore, intrusive bodies displaying observable magma mingling (see above) are ‘frozen’ examples of a very ineffective magma mixing process, which has been severely hindered by the large density contrast between an Fe–Ti-rich magma and a more silicic magma. In contrast, mixing between density-minimum basalts and diorites was more effective and complete because of the smaller density contrast between the two magmas (Figs 13 and 14).
Origin of Fe–Ti–P enriched intermediate magmas
The crystallization products of high-Fe–Ti–P intermediate magmas are found within the Plateau Group lava sequence and the late cone sheets of Mull Centres 2 and 3 (e.g. Bailey et al., 1924; Thomson, 1986). In this section, we focus on the processes that may have given rise to these rocks.
One of the main factors in controlling the onset of Fe–Ti oxide crystallization is oxygen fugacity (fO2.) At relatively high fO2 values (more oxidizing) Fe–Ti oxide will precipitate at higher MgO contents than when the magma has a lower fO2 (more reducing). However, an explanation based solely on fO2 arguments cannot explain the concomitant enrichment in P2O5 (as well as Fe and Ti), and so other factors must also play a role in determining the saturation point of Fe–Ti oxide components in the magma.
The existence of Fe (and Ti) enrichment trends in tholeiitic magma series has been recognized for some time (e.g. Fenner, 1929). More recently, it has been noted that P is enriched in many high-Fe–Ti magmas, suggesting a link between Fe–Ti enrichment and P enrichment (see, e.g. Juster et al., 1989; Toplis et al., 1994). Phosphorus is highly soluble in basaltic melts (Watson, 1979; Harrison & Watson, 1984), and due to the high electronegativity of the P5+ ion, forms very stable complexes with 3+ ions, particularly Fe3+ (Toplis & Carroll, 1994). Complexing between P and Fe3+ ties up these elements, resulting in a delay in the onset of apatite and Fe–Ti oxide fractionation. Critically, however, this complexing does not change the Fe3+/Fe2+ ratio of the melt.
We noted above that in the Mull LBCS, apatite fractionation commences at values of MgO lower than those at which Fe–Ti oxide is precipitated (i.e. 2.5 wt % as opposed to 3.0 wt % MgO; Fig. 7). If the onset of Fe–Ti oxide fractionation is a result of the breakdown of P–Fe3+ complexes, why does apatite fractionation not commence at the same time as that of Fe–Ti oxide? The reason may be that P forms bonds with other trivalent ions in the melt, for example, the REE and Al3+. Alternatively, the onset of Fe–Ti oxide fractionation may simply represent the build-up of free Fe3+ in the melt as a result of fractionation of olivine and pyroxene (which contain more Fe2+ than Fe3+). In this situation, the P–Fe3+ complexes may not break down until the commencement of apatite fractionation.
Following Juster et al. (1989) and others, Toplis & Carroll (1994) have noted several effects of increasing P concentrations in a basaltic magma. In the context of Mull, the most important of these is the dissolution of olivine, which will reduce the Fe2O3/FeO ratio of the melt. Olivine dissolution can occur either by reaction with O2 to form magnetite, or by reaction with SiO2 (in the liquid phase) to form pigeonite (e.g. Juster et al., 1989; Toplis & Carroll, 1995, , 1996). The absence of pigeonite in the Plateau Group and Late Mull magma types is not only a reflection of their alkaline nature, but suggests also that the first reaction may be more significant in basaltic rocks of the Mull Tertiary igneous centre. It is important to note that olivine resorption increases with rising fO2. Toplis & Carroll (1995) suggested that olivine may react to form a magnetite component in the liquid, rather than the solid; however, if this reaction occurs in a highly oxidized magma chamber (such as that in which the Mull EBCS are inferred to have formed; Thomson, 1986), magnetite may have crystallized. Therefore, partial dissolution of olivine in the EBCS magmas may have been responsible for the early onset of Fe–Ti oxide fractionation. This in turn resulted in the EBCS magmas being less enriched in Fe–Ti–P when compared with the LBCS. Alternatively, the less enriched character of the EBCS may be due to the fact that they are inferred to have been more extensively contaminated with oxidized and hydrated upper-crustal rocks, relative to the LBCS (Thomson, 1986). Assimilation of such crust would have resulted in a more oxidized magma, thereby promoting the earlier (higher-MgO) onset of Fe–Ti oxide fractionation.
It is interesting to note that on Mull, it is the more alkalic igneous rocks (Plateau Group and Late Mull magma types) that display Fe–Ti–P enrichment, whereas the more tholeiitic Coire Gorm and Central Mull Tholeiite magma types do not show such pronounced enrichment (Figs 7, 12 and 13). A likely reason for this is that low degrees of partial melting of anhydrous garnet lherzolite will result in (alkalic) magmas that are enriched in Ti and P relative to concentrations of these elements in (tholeiitic) magmas formed by higher degrees of partial melting within the spinel stability field (e.g. Thompson, 1974). The Plateau Group and Late Mull magma types are believed to represent small-degree partial melts of garnet lherzolite, in contrast to the Coire Gorm and Central Mull Tholeiite magma types, which are inferred to result from larger degrees of melting (see below). Experiments by Hirose & Kushiro (1993) show that partial melting in the spinel stability field will generate basaltic magmas which have, on average, FeO* concentrations 2 wt % less than those of melts formed within the garnet stability field (see Klein & Langmuir, 1987). Thus, having started out with lower contents of FeO* (and presumably low Ti and P), the Coire Gorm and Central Mull Tholeiite parent magmas were less likely to have evolved to high-Fe–Ti–P liquids.
To summarize, a combination of factors, in addition to variable fO2, may have been responsible for the Fe–Ti–P enrichment observed in the Plateau Group and Late Mull magma types. These include the depth of final melt segregation, the behaviour of Fe and P ions in the melt, partial dissolution of olivine in the melt and the depth of contamination of the parent magmas.
Temporal evolution of lithospheric contamination
Several contamination trends can be identified within the Mull lavas, dykes and cone sheets, as shown in Fig. 6. The Mull igneous centre does not appear to record a simple progression with time from lower- to upper-crustal contamination of basaltic magmas, as proposed for Skye by Dickin (1981) and Thompson et al. (1982). The main temporal trends pertaining to crustal contamination are summarized in Fig. 15.
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Early Mull lavas belonging to the Staffa magma sub-type (Kerr, 1999
) are Plateau Group lavas that have assimilated substantial amounts (20–30%) of Moine metasediments (Thompson et al., 1986). Why these early lavas should have progressed to high crustal levels and evolved by assimilation–fractional crystallization (AFC) processes is unclear, especially when one considers that many of the overlying Plateau Group lavas are contaminated with lower-crustal granulite-facies rocks (e.g. Morrison et al., 1980; Kerr et al., 1995). Thompson et al. (1986) suggested that one explanation for this was that the earliest magmas ascended along a zone of weakness associated with the Great Glen fault, which runs through southern Mull. This ‘express route’ to the upper crust was not exploited by later magmas, possibly because the Great Glen fault may have become inactive at this time.
Following eruption of lavas belonging to the Staffa magma sub-type, the locus of contamination shifted to the lower crust (0.9 GPa; Thompson, 1982; Kerr et al., 1995). Some Plateau Group magmas reached the surface having undergone virtually no contamination, and even the most contaminated Plateau Group lavas are believed to contain only 3–5% highly fusible silicic granulite-facies Lewisian gneiss (Kerr et al., 1995). This contamination is evidenced by the lavas having moderately radiogenic 87Sr/86Sr values (<0.7045; Figs 6 and 15) and very unradiogenic 206Pb/204Pb (Dickin, 1981; Kerr et al., 1995). As the most mafic (hottest) Plateau Group lavas are the most contaminated, Kerr et al. suggested the contamination mechanism was one of ‘assimilation during turbulent magma ascent’ (see Huppert & Sparks, 1985). An alternative to this, which does not require turbulent flow in dykes supplying the Mull lavas, is an AFC-type process in which the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (r = 2.0–2.7; Reiners et al., 1995). Although such a process allows relatively large amounts of crustal assimilation (up to 18% of the initial magma mass) with very little accompanying differentiation of the basaltic melt, it is doubtful if it is thermodynamically feasible (Huppert & Sparks, 1985).
Development of the Mull central intrusive complex at 59 Ma marked a return to predominantly upper-crustal (Moine) contamination, evidenced by high 87Sr/86Sr and high Rb/Nb ratios in the Mull cone sheets (Figs 5a, 6 and 15; Thomson, 1986). Lead isotopic data are not available for most of the cone sheets and granites. However, Pb isotope analyses of the (Moine-xenolithic) EBCS around Loch Scridain (Thompson et al., 1986; Preston et al., 1998) suggest that although contamination is overwhelmingly by rocks of Moine composition, the EBCS have also assimilated small amounts of granulite-facies Lewisian gneiss. In detail, it appears that Mull Centre 1 cone sheets and granites have been crustally contaminated to a greater extent than those of Centres 2 and 3. One reason for this is that the Centre 1 magmas were among the first to invade the crust, and so could have melted a substantial proportion of the most fusible Moine rocks. This would have left less-fusible crust available as a contaminant for magmas associated with Centres 2 and 3 (see Thompson, 1982). In addition, Centre 1 magmas may have been more mafic (hotter) when they reached the upper crust, when compared with magmas of Centres 2 and 3. Figure 7d shows that a good negative correlation exists between 87Sr/86Sr and MgO in Centre 1 rocks, consistent with AFC-type processes (see Thomson, 1986). The lower initial 87Sr/86Sr ratios and Rb/Ba of Centre 2 and 3 rocks could indicate smaller amounts of contamination, or may be a reflection of deeper magma chambers (Fig. 15), partly in Moine rocks and partly in upper-crustal amphibolite-facies Lewisian rocks. Resolution of this issue awaits a Pb isotopic investigation of rocks from the Mull central igneous complex.
The Centre 2 cone sheets and granites have low 87Sr/86Sr (mean = 0.7070) relative to those of Centre 3 (mean = 0.7090; Fig. 6). Thomson (1986) ascribed this to differing ratios of the mass of crust assimilated to the mass of material fractionated (see DePaolo, 1982). Although this is certainly plausible, several other explanations can be advanced. One possibility is that the basaltic magmas of Centre 2 were contaminated with a greater proportion of Lewisian crust than of Moine. The generally lower initial 87Sr/86Sr of Lewisian crust (Fig. 6b) could mean that Centre 3 magmas actually assimilated about the same volume of crust as the magmas of Centre 2, but that most of this crust consisted of less radiogenic Lewisian material. Another possibility is that Centre 2 magmas intruded a region of crust from which much of the readily fusible material had been stripped out by earlier magmas (those of Centre 1, the locus of which lies close to that of Centre 2; Fig. 2). In contrast, Centre 3 magmas intruded crustal rocks 5 km to the northwest of the earlier intrusive centres. This crust may have seen little previous throughput of magma.
In summary, the development of the Mull magmatic complex appears to have been associated with a cycle of magma chamber (crustal contamination) locations ranging from initially shallow to lower crustal, followed by a progressive return to upper-crustal magma chambers. Basaltic melts belonging to each of the three main intrusive centres may have occupied magma reservoirs at different depths within the upper crust.
Mantle melting processes and sources
From the chemical evidence presented above, it is possible to piece together a magmatic sequence for Mull. Primitive magmas parental to the earliest basaltic lavas (the Plateau Group) had relatively steep chondrite-normalized HREE patterns [e.g. (Nd/Yb)cn >2] and slightly down-turned chondrite-normalized LREE patterns [(La/Nd)cn <1] (Figs 4e and 5c). These basalts were followed by Coire Gorm-type lavas, which possess essentially flat chondrite-normalized REE patterns. The Coire Gorm lavas were in turn succeeded by basalts belonging to the Central Mull Tholeiite magma type, a group which is also represented in the EBCS suite. The Central Mull Tholeiite magma type, when free from the effects of contamination, possesses relatively flat chondrite-normalized middle REE (MREE) to HREE patterns [e.g. (Eu/Yb)cn <1.3] and is moderately depleted in the LREE [(La/Nd)cn 
1]. The final Mull magma type is the Late Mull magma type, found in the LBCS suite and the Mull–Morvern late dykes and plugs. Rocks of this magma type generally have chondrite-normalized MREE–HREE patterns [e.g. (Eu/Yb)cn >1.4] that are steeper than those of the Central Mull Tholeiite magma type. The Late Mull magma type also are enriched in the LREE [(La/Nd)cn >1] when compared with Central Mull Tholeiite-type basalts.
It is widely held (see, e.g. Wood, 1979; Ellam, 1992) that basaltic igneous rocks showing significant depletion in the MREE–HREE relative to chondritic abundances were derived from a garnet-bearing source region. During the evolution of the Mull Tertiary volcano, the mantle source of the basalts changed with time from garnet-bearing (Plateau Group lavas) to a garnet-poor, spinel-bearing source (Coire Gorm and Central Mull Tholeiite lavas, EBCS suite), and back to a garnet-bearing source (Late Mull magma type). Similar, but not identical, temporal geochemical trends have been established in basalts erupted elsewhere along the northwest European and East Greenland margins (see above). The origin of these trends is unclear, but could be related to variations in lithospheric thickness with time (Ellam, 1992; Fram & Lesher, 1993; Kerr, 1994), temporal differences in the efficiency of polybaric melt pooling (Brodie, 1995) or generation of basaltic melt oceanward of the rifted continental margins, followed by transport of melt in dykes to the site of final eruption (White, 1992b). We now examine each of these hypotheses in turn.
Variations in lithospheric thickness
Modelling based on the relative abundance of REE in the Plateau Group lavas reveals that mantle melting cannot have occurred entirely within the stability field of garnet lherzolite (e.g. Brodie, 1995; Kerr, 1995b). Therefore, it is likely that the Plateau Group lavas were derived from a melt column that straddled, and tapped melt from, the garnet–spinel transition zone. The transition zone spans a depth of 80–95 km for mantle potential temperatures of 1450–1500°C (see Watson, 1993), appropriate for Mull Tertiary basalts (Kent, 1995; Kerr, 1995b). Point-and-depth averages calculated for the mantle melting column of the Plateau Group lavas are located within the garnet–spinel transition zone (Brodie, 1995; Kerr, 1995b) and it is likely that there was melting both below and above these depths, in the garnet and spinel stability fields, respectively (Brodie, 1995). Together with evidence from the Mesozoic sedimentary record of the Sea of the Hebrides Basin (e.g. Morton, 1987), these calculations suggest that during eruption of the Plateau Group lavas, the thickness of the lithosphere mechanical boundary layer (the ‘permanent’ part of the plate; McKenzie & Bickle, 1988) was of the order of 70–75 km. If correct, this implies that no significant thinning of the Hebridean lithosphere had occurred since the Jurassic (contra Thompson & Gibson, 1991).
It is possible that the flatter chondrite-normalized HREE patterns of the Coire Gorm and Central Mull Tholeiite magma types are a reflection of melting beneath thinner lithosphere (see Ellam, 1992); i.e. the point-and-depth average of melting, and indeed most of the melting column, was within the spinel stability field. Assuming no temporal change in the mantle potential temperature (and by implication, the height of the melting column), the lithosphere mechanical boundary layer is required to have been 60 km thick to place the melting column of the Coire Gorm–Central Mull Tholeiite magma type largely within the spinel stability field. This change in boundary layer thickness would be consistent with a stretching factor (b) of 1.2. Structural studies of the Mull dyke swarm (Sloan, 1971; Jolly & Sanderson, 1995) indicate that crustal extension of up to 20% occurred locally adjacent to the Mull central igneous complex. However, on a regional scale, early Tertiary stretching associated with dyke emplacement was less than b = 1.07 (Speight et al., 1982; England, 1988). Similarly, Kerr (1994) has noted that the regional extension observed in the Mull dyke swarm can only account for, at most, one-third of the extension inferred from the chemical differences between the Plateau Group and the Central Mull Tholeiite–Coire Gorm types.
Several explanations have been proposed to account for the discrepancy between the amount of early Tertiary upper-crustal extension recorded in the basaltic dyke swarms and the thickness of the Hebridean lithosphere mechanical boundary layer, as inferred from geochemical studies of the Coire Gorm–Central Mull Tholeiite magma types and equivalent lavas on Skye. For example, Kerr (1994) suggested rapid erosion of the lithosphere mechanical boundary layer beneath the Sea of the Hebrides Basin by the hot Iceland plume. However, convective removal by a plume of 15 km of lithosphere in 1–2 my (the interval during which the Plateau Group and Coire Gorm–Central Mull Tholeiite magma types were emplaced) may not be thermodynamically feasible (e.g. Fowler, 1990). A more reasonable estimate is erosion of 1–2 km of lithosphere over this time period.
England (1992) has proposed that discontinuous, depth-dependent stretching of the Hebridean lithosphere resulted in extension of the ductile lower lithosphere over an area wider than that affecting the upper crust. This depth-dependent stretching may have been maintained by magma migration through the lithosphere. Unfortunately, the presence of underplated basaltic material beneath the Sea of the Hebrides Basin (Brodie & White, 1994) makes it difficult to test this hypothesis using thermal subsidence data.
The restriction of Coire Gorm and Central Mull Tholeiite lavas to within, and close to, the central complex suggests that these magmas were only generated below the region of the central complex and that it was only this localized region that experienced substantial thinning. Such localized extension can be seen at the surface, as the most intense stretching (as recorded by the dyke swarms) is observed close to both the Mull and Skye central complexes (Speight et al., 1982). Additionally, dykes have a tendency to become wider and more abundant with increasing depth. For example, within the lava successions of the Hebrides, dykes increase in abundance with decreasing height (Speight et al., 1982). Therefore, the amount of extension in the lower lithosphere beneath the Mull central complex may have been significantly greater than that observed in the upper crust. Average upper-crustal stretching associated with the Mull centre corresponds to a beta factor of 1.06 (Sloan, 1971). At the surface, this is distributed over 45–50 km, which corresponds to 3–4 km extension. However, if during the later development of the Mull volcano, the length scale over which this 3–4 km of extension occurred was, at the base of the lithosphere, confined to the width of the central complex (<20 km), then this focused lower-lithosphere extension would have a localized beta factor of 1.2. Thus we tentatively suggest that melting to form the Plateau Group lavas could have been fairly widespread beneath Mull, whereas melting to produce the Coire Gorm and Central Mull Tholeiite magmas was more localized below a stretched region not much wider than the central complex.
The apparent return to slightly deeper melting in late Centre 2 and Centre 3 may represent a slight rethickening of the lithosphere in the waning stages of plume activity, possibly contributed to by magmatic underplating and progressive melt addition to the lithosphere in the form of sills.
Efficiency of melt pooling
Melt fractions generated at different depths within a mantle melting column may be extracted and pooled with varying degrees of efficiency before their ascent through the lithosphere in dykes (e.g. Elliott et al., 1991; Nicholson & Latin, 1992). In this scenario, the Plateau Group basalts represent well-mixed, point-and-depth average melt compositions, whereas the Coire Gorm–Central Mull Tholeiite magma types are imperfect aggregates of trace element depleted melts derived largely from the upper part of the melting column (see Brodie, 1995). This explanation is attractive because it does not necessitate changes in melting conditions within the mantle. However, as noted by Brodie (1995) for East Greenland and Preshal More (Skye) basalts, it is unclear why the efficiency of polybaric melt pooling should vary systematically with time.
Kerr (1995b) has argued on geochemical grounds (LREE and MREE contents) that the Coire Gorm lavas are not remelts of the residue from which the Plateau Group magmas had previously been extracted (and Fig. 11 clearly shows this). This implies a change in the composition of the mantle source of Mull basalts in the interval between eruption of the Plateau Group and Coire Gorm magma types. In contrast, the Fairy Bridge basalts of Skye (which are similar in composition to the Coire Gorm magma type) possibly can be explained by progressive remelting of the residue remaining from extraction of the Skye Main Lava Series, without recourse to a change in mantle source composition (see Wood, 1979; Thompson et al., 1980). Why should the melting regime beneath Mull have been different from that below Skye? The answer is not yet clear. However, Kerr's (1995b) observations on the Coire Gorm magma type suggest that (in-)efficiency of polybaric melt pooling does not offer a complete explanation for the compositional differences between successive Mull magma types; in the case of the Mull Plateau Group to Coire Gorm transition, source differences also have a role to play.
Melt generation to the west of the UK continental shelf
A third possibility is that certain Hebridean magma types were generated 200–300 km to the west of the Sea of the Hebrides Basin, beneath the Rockall Trough or Faeroes–Shetland basin (e.g. White, 1992a, 1992b; Brodie, 1995). After melt pooling at the base of the lithosphere, it is proposed that they were transported southeastward via linked dyke and sill systems. England (1992) has discussed the possible significance of such linked systems within the Sea of Hebrides Basin (see above). In the hypothesis of White (1992a, 1992b), the lithospheric thickness beneath this basin remained essentially constant during the early Tertiary. Magma types such as the Plateau Group and Skye Main Lava Series were generated beneath unstretched lithosphere, whereas the Coire Gorm–Central Mull Tholeiite and Preshal More magmas were produced below stretched lithosphere to the west of the UK continental shelf. The Late Mull magma type and similar rocks on Skye (the Beinn Dearg More magma type; Thompson & Morrison, 1988) represent melts produced once more beneath the Sea of Hebrides Basin. This explanation avoids the need for rapid changes in lithospheric thickness or systematic variations in the efficiency of melt pooling with time.
Unfortunately, there are major problems with the model of White (1992a, 1992b). First, the occurrence of multiple intrusive centres and associated dyke swarms in the Sea of Hebrides Basin and on Arran suggests that, over time, melt generation became focused below particular areas. An example of melt focusing would be the Black Cuillins igneous complex of Skye, which acted as a feeder to the Preshal More lavas and dykes (e.g. Esson et al., 1975). The Hebridean igneous complexes appear to lie close to the intersections of extensional faults that were active during magmatism (England, 1988). Exposed dykes become increasingly scarce with increasing distance from these intrusive centres, suggesting that melt was not transported via dykes towards the Sea of Hebrides Basin. Second, it is not intuitively obvious why melt generated beneath stretched and thinned lithosphere (the Rockall Trough and Faeroes–Shetland basin) should migrate eastwards towards a zone of essentially intact lithosphere (the Sea of Hebrides Basin; see above). It would have been much easier for this melt to have been erupted within the offshore basins.
In summary, all the models outlined above have problems. We favour a model that invokes some localized thinning (predominantly by extension, but with a contribution from lower-lithospheric erosion), combined with selective magma extraction from the top of the melting column. The subsequent eruption of the slightly deeper-derived Late Mull magma type (compare the Beinn Dearg More type on Skye), is possibly a reflection of melting beneath lithosphere that had become rethickened by underplating or intrusion.
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Concluding Summary |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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- Magma mixing is a much more common process in the evolution of the Mull volcano (and probably other Hebridean igneous centres) than was previously realized. Two fundamental types of magma mixing occur, and density appears to be the main controlling factor determining which type of mixing predominates. For example, the relatively small density difference between low-Mg basalts and intermediate low-Fe magmas results in near-complete (but cryptic) magma mixing, as shown by the early Mull cone sheets and Central Mull Tholeiite lavas. In contrast, the greater density contrast between high-Fe–Ti intermediate and rhyolitic magmas results in observable (i.e. incomplete) magma mingling, exemplified by mixed magmas of the Glen More and Loch Bà ring dykes.
- Although low fO2 undoubtedly contributes to the formation of the high-Fe–Ti–P intermediate magmas of the Mull Plateau Group and Centres 2 and 3, it is not the sole cause of elemental enrichment. Other important factors are (a) complexing in the magma between the strongly electronegative P5+ ion and Fe3+ and REE (3+) ions, and (b) the depth of final melt segregation, with the more alkaline magmas being more likely to become enriched in Fe–Ti–P.
- The location of crustal contamination, and so of magma chambers, varied throughout the evolution of the Tertiary Mull volcano in a semi-systematic manner. The basic magmas of the early Plateau Group have been mostly contaminated with granulite-facies Lewisian crust, suggesting the presence of lower-crustal magma chambers. The magmas of the Mull central igneous complex are contaminated with upper-crustal Moine metasediments, reflecting the former presence of high-level magma chambers beneath the central complex. The most contaminated magmas found within the eroded Mull volcano are associated with Centre 1, probably because at this stage the upper crust still contained a reasonable supply of readily fusible material. As the volcanic centre evolved, so less easily fusible crust was available; as a result, the later magmas are less contaminated.
- A progression from Mull Plateau Group magmas with relatively steep (LREE-enriched) chondrite-normalized REE patterns to Central Mull Tholeiite magmas with essentially flat chondrite-normalized HREE patterns strongly suggests a shallowing with time of the mantle source. This may be due to lithospheric thinning (perhaps locally beneath the central igneous complex), or preferential extraction of melt from the top of a melt column beneath lithosphere of essentially constant thickness. We find no evidence to support the idea that the Mull (and other Hebridean) magmas were generated to the west of the British Isles and transported in dykes to their point of eruption.
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Appendix: Analytical Methods |
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TOPABSTRACTIntroductionThe Mull Tertiary Igneous...Geochemistry of the Mull...DiscussionConcluding SummaryAppendix: Analytical MethodsReferences |
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Major and trace element analyses of the Mull cone sheets were obtained by inductively coupled plasma–atomic emission spectrometry (ICP-AES) at King's College, London, using a Philips PS1900 spectrometer. For major elements and Zr, rock powders were fused with LiBO2 to which La had been added as an internal standard. The fused bead was dissolved in dilute HNO3 and made up to volume. For the other trace elements, the rock powders were decomposed in HF and perchloric acid, evaporated to dryness and dissolved in dilute HCl. Distilled H2O was then added to make up to volume. The samples were then introduced into the plasma, one at a time, for simultaneous elemental analysis. Analytical methods and operating conditions are similar to those described by Thompson & Walsh (1989)
. Precision for most elements is between 1% and 3%; for the REE, precision is better than 5%.
Data for the Glen More ring dyke were obtained at the University of St Andrews, using a Philips PW1450 automated X-ray fluorescence (XRF) spectrometer. Operating conditions, analytical techniques and precision and accuracy are similar to those described by Fitton et al. (1998). Analyses of the Mull dykes were obtained by XRF at the University of Edinburgh; details of laboratory methods, standards, precision and accuracy have been described by Kent (1995) and Fitton et al. (1998)
Strontium isotopic data were obtained at the British Geological Survey Isotope Unit, Gray's Inn Road, London. Analyses were made on a VG Micromass 30 mass spectrometer. All data were corrected to the Eimer & Amend standard value of 87Sr/86Sr = 0.70800. Neodymium isotopic data were obtained at the University of Leeds. Samples for Nd isotope analysis were decomposed by HF–HNO3 in a PTFE bomb for 48 h at 150°C. Neodymium was separated using a three-column technique, with further purification of the Nd carried out using a small version of the first columns. Neodymium was analysed as the metal ion species on a VG Micromass 30 mass spectrometer. Non-radiogenic 148Nd/144Nd was measured immediately after 143Nd/144Nd for most samples and acts as an internal standard. Further details have been discussed by Thirlwall (1982)
A complete data set containing all the data used in this paper can be found in the electronic appendix located at http://www.oup.co.uk/petroj/hdb/Volume_40/Issue_06/
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Acknowledgements |
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A.C.K. was supported by the University of Leicester and a Special Research Fellowship awarded by the Leverhulme Trust. R.W.K. was supported by a Fermor Fellowship from The Geological Society of London. B.A.T. was funded by a University of London postgraduate studentship at King's College, and J.K.S. by an NERC postgraduate studentship awarded to C.H.D. at the University of St Andrews. Nick Walsh, Bob Beckinsale and Matthew Thirlwall are thanked for inductively coupled plasma, Sr and Nd isotope analyses, respectively. Ron Hardy, Nick Marsh, Godfrey Fitton, Dodie James and Ed Stephens provided valuable help with X-ray fluorescence work. Peter Dagley is thanked for providing samples of the North Mull Dykes. Discussions with Bob Thompson, Raymond Skelhorn, Andy Saunders and Mike Norry helped crystallize many of the ideas presented in this paper. Constructive reviews by Henry Emeleus, Godfrey Fitton and an anonyomous referee are appreciated, as is the editorial work of Marge Wilson.
This study is dedicated to the memory of Ansel Dunham.
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FOOTNOTES |
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Present address: The Open University (East Anglian Region), Cintra House, 12 Hills Road, Cambridge CB2 1PF, UK. 
Present address: Cairn Energy, 50 Lothian Road, Edinburgh EH3 9BYL, UK.
* Corresponding author. Telephone: +44 116 2523639. Fax +44 116 2523639. e-mail: ack2{at}leicester.ac.uk
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