Journal of Petrology Advance Access originally published online on August 31, 2005
Journal of Petrology 2006 47(1):191-230; doi:10.1093/petrology/egi072
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Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills
1 DEPARTMENT OF ENGINEERING MATERIALS, UNIVERSITY OF SHEFFIELD, SHEFFIELD S1 3JD, UK
2 SCHOOL OF EARTH, ATMOSPHERIC AND ENVIRONMENTAL SCIENCES, THE UNIVERSITY OF MANCHESTER, MANCHESTER M13 9PL, UK
RECEIVED NOVEMBER 12, 2004; ACCEPTED JULY 11, 2005
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONMajor and trace element data for the Tertiary, Shiant Isles Main Sill, NW Scotland, are used to discuss its complex internal differentiation. Vertical sections through the sill exhibit sharp breaks in chemistry that coincide with changes in texture, grain size and mineralogy. These breaks are paired, top and bottom, and correspond to the boundaries of intrusive units, confirming a four-phase multiple-intrusion model based on field relations, petrography, mineralogy and isotopes. Whole-rock chemistry is consistent with this model and necessitates only minor revisions to the intrusive and differentiation mechanisms previously proposed. The rocks contain strongly zoned minerals (e.g. olivine Fo70–5, clinopyroxene Mg# = 75–5, plagioclase An75–5) indicating almost perfect fractional crystallization, but whole-rock compositions do not show such extreme variations. Thus, while residual liquids became highly evolved in situ, they mainly became trapped within the crystal network and did not undergo wholesale inward migration. Some inward (mainly upward) concentration of residual liquids did occur to form a ‘sandwich horizon’, but the more volatile-rich, late-stage liquids that did not crystallize in situ appear to have migrated to higher levels in the sill to form pegmatitic horizons. Parental liquid compositions are modelled for the intrusive units and it is concluded that the original parent magma formed by partial melting of upper mantle that was more depleted in LREE than the sources of most Scottish Tertiary basaltic rocks. Incompatible trace elements in the picrodolerite–crinanite intrusive unit support isotope evidence that its parent magma was contaminated by crustal material. Attempts to reconcile the chemical characteristics of the sill with a recently proposed petrogenetic model based on a single intrusion of magma differentiated by novel, but controversial, processes fail comprehensively. It is predicted that the complex petrogenetic history of the Shiant Isles sill is not unusual and could become the model for other large (>50 m thick) sills.
KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill
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INTRODUCTION |
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Outcropping on the Shiant Isles, 24 km north of the Isle of Skye (Fig. 1), and intruded into Lower Jurassic sediments are some of the most northerly members of the Paleocene ‘Little Minch Sill Complex’ (Gibb & Gibson, 1989
). It has long been recognized that the largest of these, termed the Shiant Isles Main Sill by Gibb & Henderson (1984), and hereafter referred to as the sill, is internally differentiated. It has often been cited as an example of the differentiation of mildly alkaline basaltic magma (e.g. Wager & Brown, 1967; Deer et al., 1978; Gibson & Jones, 1991). Initially, the differentiation was attributed (Walker, 1930) to simple gravitative settling of olivine within a single intrusion of mafic magma. However, as new facts and evidence emerged over a series of papers, notably Drever (1953), Johnston (1953), Murray (1954a), Drever & Johnston (1959, 1965), Gibb (1973), Gibb & Henderson (1989, 1996), Foland et al. (2000) and Henderson et al. (2000), there has been an evolution in petrogenetic thinking and the sill is now regarded as a multiple intrusion. Ever more detailed models of emplacement and differentiation have arisen based on field, petrographic, mineralogical and isotopic data but, although all post-1980 models have been constrained by knowledge of the whole-rock chemistry, no comprehensive account of the internal chemistry of this classic sill has been published. [For an account of the natural and human history of the Shiant Isles, see Nicolson (2001).]
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Here, we describe and discuss the major and trace element whole-rock chemistry of the sill and demonstrate how it controls and underpins our preferred genetic model of multiple intrusion, magma mixing, crustal contamination and post-intrusive processes (Foland et al., 2000
; Henderson et al., 2000). We also provide a database of bulk-rock chemical analyses (Table 1) against which other petrogenetic processes and models for sills and other intrusions (e.g. Huppert & Sparks, 1980; Marsh, 1996; Latypov, 2003b) might be tested.
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INTERNAL STRUCTURE |
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Rock names
The names crinanite, picrite, picrodolerite, teschenite and olivine teschenite have been used for the analcime-bearing mafic rocks of the Shiant Isles by all previous workers, albeit with some inconsistency. According to the IUGS classification (Le Maitre et al., 1989
), almost all the Shiant Isles rocks would be olivine gabbros, if regarded as plutonic, or picrites and picrite basalts, if volcanic. To avoid confusion, allow continuity and allow distinction between petrographic units we have elected to retain the original terms and define them as used in all our previous work on the sill (Gibb & Henderson 1984). This usage is summarized below and illustrated in terms of the three main mineral constituents in Fig. 2.
- Teschenite = medium- to fine-grained rock consisting of plagioclase and clinopyroxene with minor analcime. Olivine teschenites can contain up to 20% olivine.
- Crinanite = medium- to fine-grained rock consisting of olivine (<
15%), plagioclase and clinopyroxene with minor analcime and with a distinctive ophitic or sub-ophitic olivine texture. This texture usually occurs in the sill at olivine contents below about 15% and its disappearance marks the transition from crinanite to picrodolerite.
- Picrodolerite = medium- to fine-grained rock consisting of olivine (>
15%), plagioclase and clinopyroxene with minor analcime.
- Picrite = coarse- to fine-grained rock consisting of olivine (>40%), plagioclase and clinopyroxene, with or without minor analcime. Three petrographic variants have been identified in the Shiant Isles sill and distinguished as large-feldspar picrite, medium-feldspar picrite and small-feldspar picrite (Gibb & Henderson, 1989
, fig. 5).
- Crinanite = medium- to fine-grained rock consisting of olivine (<
).
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Distribution
The thickness of the sill varies locally, with a maximum of
165 m. The main rock types are distributed as shown in the composite section (Fig. 3), which is based on sampled vertical sections through the sill, including cored drill-holes (Gibb & Henderson, 1996; Henderson et al., 2000). The sill can be subdivided into a number of petrographic units, which are capitalized in the text and figures (e.g. Lower Picrite, Picrodolerite–crinanite) to distinguish them from rock types. These are shown in Fig. 3. The modal olivine profile is generally S-shaped (Gibb & Henderson, 1992) but with a second, upper maximum. It has been described by Latypov (2003b) as "double humped".
Emplacement
The field and petrographic relationships between the various internal subdivisions and their mineralogies led to the development of a four-phase multiple intrusion model (Gibb & Henderson, 1989, 1996; Foland et al., 2000; Henderson et al., 2000). This is summarized below and illustrated in Fig. 4.
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(1) Emplacement into the sediments of magma that gave rise to the Upper and Lower Olivine Teschenites (Fig. 3). These have a combined thickness of just over 2 m, but the original intrusion was probably slightly thicker, its central part having been displaced by a subsequent intrusion (Fig. 4, Stage 2a). On emplacement, the marginal parts of the magma, which chilled against the sediments, contained
14% of olivine phenocrysts. After emplacement, sinking of these phenocrysts occurred, especially from the upper parts of the unit, just below the chilled margin. (2) Intrusion into (1), before it was completely solidified, of an olivine phenocryst-rich magma that subsequently formed the Upper and Lower Picrites with a total thickness around 24 m. During emplacement, there were substantial variations in the phenocryst content and minor variations in the composition of the suspending liquid. Both of these arose during tapping of the source magma chamber and uprise in the feeder conduit, and resulted in the internal differentiation of the Picrite unit. The Sr/Nd isotope ratios of the magmas that produced this unit and the Olivine Teschenite units (Stage 1) are indistinguishable, suggesting that both originated in the same deep magma chamber and probably gained access to the sill via the same feeder system.
(3) Intrusion of a substantial pulse of magma between the Lower Picrite and the Upper Picrite (Fig. 3) to form the 135 m thick Picrodolerite–crinanite unit. This gave rise to the Lower Discontinuity, which is a petrographic break visible in the drill-cores and conspicuous in the field (Gibb & Henderson, 1989, figs 3 and 4). A (presumed) similar upper contact (transient discontinuity; Fig. 4) between the new magma and overlying picrite has been obliterated by subsequent intrusion (Stage 4). At the time of emplacement, the adjacent picrite must have been a rigid crystal mush, cooler than the new magma to cause chilling, but still with sufficient interstitial liquid to allow extensive subsequent mixing across the Lower Discontinuity (Gibb & Henderson, 1996; Foland et al., 2000). However, the Sr/Nd isotope ratios were initially homogeneous throughout the Picrodolerite–crinanite unit and distinct from those of the Olivine Teschenite and Picrite units, suggesting a separate (albeit probably related), more evolved source (Foland et al., 2000). This, together with the absence of xenoliths of olivine teschenite or picrite in the unit, could be an indication that the magma accessed the sill via a separate feeder system. The magma contained phenocrysts of olivine (10%) and calcic plagioclase (1%) that settled out after emplacement to differentiate the unit into picrodolerite passing upwards into crinanite (Fig. 3).
(4) Finally, a fourth intrusion of magma ascended through the country rock along a separate feeder dyke, becoming contaminated by the sediments. As it passed through the upper parts of the sill, it picked up xenoliths of more and less solidified crinanite before spreading out laterally along the junction between the top of the Picrodolerite–crinanite and the Upper Picrite to form the 4 m thick Granular Olivine Picrodolerite (abbreviated as GOPd). This intrusion eroded away the pre-existing crinanite–picrite contact before chilling against both adjacent units. The upper surface of the GOPd chilled against completely crystallized picrite to form the Upper Discontinuity (Fig. 3) but the underlying crinanite was still sufficiently ‘mushy’ to allow post-intrusive mixing of the liquid phases of the two magmas across the interface (Foland et al., 2000). As intrusion of the GOPd progressed, the feeder dyke widened, reducing the amount of sediment contamination and increasing the flow rate and olivine phenocryst content of the magma. The last two enabled the onset of flow differentiation in the feeder, with the result that the magma was emplaced into the sill with an olivine-rich central ‘plug’. This unit, which appears to be exposed only at the NW corner of Eilean an Tighe (Fig. 1), may not persist laterally throughout the whole sill.
The emplacement of all four magma pulses must have occurred over a relatively short time span, as each intrusion was emplaced before its immediate predecessor had completely solidified. One result of this was that after the intrusion of the GOPd, the sill underwent much of its remaining cooling as a single thermal unit, as evidenced by the continuity of the massive columnar joints across unit boundaries. During the later stages of this cooling, hydrothermal alteration and Sr exchange with the enclosing sediments occurred, the evidence of which is most marked in the marginal parts of the sill. These effects are most noticeable in the alteration of the primary feldspathic minerals to analcime and zeolites, alteration of olivine to serpentine and the local precipitation of sulphide minerals (Gibb & Henderson, 1996).
Nd and Sr isotope data (Foland et al., 2000) show that two distinct, albeit genetically related, magmas were involved in the formation of the sill. A more primitive, "picritic" magma, characterized by nine analysed picrites and olivine teschenites, has 143Nd/144Nd = 0·51281–0·51287 (2
= 0·00001) and 87Sr/86Sr = 0·7035–0·7037 (2 = 0·0001) and a more evolved "crinanitic" magma, characterized by 10 analysed picrodolerites and crinanites, has 143Nd/144Nd = 0·51252–0·51256 (2 = 0·00002) and 87Sr/86Sr = 0·7047–0·7049 (2 = 0·0001). The former was emplaced to form the Olivine Teschenite (Stage 1) and Picrite (Stage 2) units, while the latter gave rise to the Picrodolerite–crinanite (Stage 3). The emplacement of the GOPd (Stage 4) represented a return to the more primitive picritic magma, although with some contamination by the country rock and mixing with the crinanitic magma.
It has recently been suggested (Latypov, 2003b) that internally differentiated mafic–ultramafic sills with "double humped", S-shaped modal profiles like (and including) the Shiant Isles Main Sill are formed by intrusion of a single pulse of magma in which vigorous, wholesale internal turbulent convection becomes established. Notwithstanding the arguments against such convection occurring in sills (Marsh, 1989; Gibb & Henderson, 1992), Latypov proposed that it leads to very high-temperature gradients across liquid marginal boundary layers, enabling Soret diffusion to generate compositional variations that are the reverse of normal fractional crystallization. We shall show that the whole-rock chemistry, like the field and petrographic evidence, and the Sr–Nd isotope chemistry, is inconsistent with such an origin for the Shiant Isles sill.
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ANALYTICAL METHODS |
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Sill rocks (151 samples) and their host sediments (four samples) were analysed for major elements and 31 trace elements (including REE), although not every sample was analysed for all elements. For 146 of the samples, major elements were determined at the University of Sheffield by X-ray fluorescence (XRF) analysis of lithium tetraborate fusion discs. Additionally, Fe2+ was determined by the ammonium metavanadate method of back-titrating excess metavanadate with standardized ferrous ammonium sulphate solution; CO2 released by boiling concentrated phosphoric acid was adsorbed on ‘Sofnolite’ and weighed by difference and H2O+ released by heating was condensed and weighed. For eight samples, the major elements were determined by emission spectroscopy at the University of Manchester. For the majority of the samples, the trace elements V, Cr, Mn, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb and Ba were determined by XRF analysis of pressed powder pellets with ‘SX’ binder (van Zyl, 1982
) at the University of Sheffield. For 50 samples, V, Cr, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb, Lu, Hf, Ta, Pb, Th, and U were determined by ICP-MS at the University of Manchester and for 32 samples, Rb, Sr, Nd and Sm were analysed by isotope dilution at Ohio State University, Columbus, USA. The ICP-MS analyses were carried out by a method based on that of Pearce et al. (1997) using a VG Elemental Plasmaquad 2 STE. Samples were digested in HF/HNO3 in a C.E.M. MDS-2000 microwave system, transferred to PTFE basins, taken to dryness twice and finally taken up in 2% HNO3. The ICP-MS data were normalized using the Sr value obtained by isotope dilution or XRF analysis. Where the same trace elements were analysed by more than one of XRF, ICP-MS or isotope dilution, agreement for most elements is better than 10% (relative). The exceptions are some transition elements (mainly V, Cr and Ni) analysed by ICP-MS, which showed the effects of molecular interferences from Cl– (from residual perchloric acid) or Ca-bearing complexes. For rocks with <5 ppm Rb and Nb, XRF data were unreliable. Consequently, we prefer XRF data for the transition elements, isotope dilution or ICP-MS data for Rb, and ICP-MS data for Nb.
The accuracy of the emission spectroscopy data for major elements and of the ICP-MS data for trace elements was assessed by analysing the USGS standard rock BHVO-1 with each batch of samples. The results for the standard are given in Table 1, together with the recommended values (Govindaraju, 1989) from which it can be seen that agreement is satisfactory except for our Ta values, which, consequently, have not been used.
Representative whole-rock analyses of the sill rocks are given in Table 1. Wherever we have values determined by more than one method, those for the preferred method are presented, as indicated in Table 1. Only REE values determined by ICP-MS have been used in the figures and for interpretations.
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VERTICAL VARIATIONS IN WHOLE-ROCK CHEMISTRY |
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It has been known since the pioneering work of Walker (1930)
that the sill is vertically differentiated. This is illustrated by the variations in major and trace element chemistry (Fig. 5) for the three vertical sections described by Gibb & Henderson (1996, figs 2 and 3). However, in subsequent discussions, we focus on vertical chemical variations in the context of the composite section constructed from these by Gibb & Henderson (1996) and Henderson et al. (2000) in order to provide an overall framework for petrogenetic discussion. By doing so, the important geochemical changes that occur inward from the upper and lower cooling surfaces of the sill can be emphasized.
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Composite section
The composite section (Figs. 3 and 6a) was originally constructed from the Hole 2 and Hole 3 sections (Figs. 1 and 5) with the +97 m height in the former being equated with the –40 m depth in the latter, as deduced from petrographic and modal data and supported by mineral chemistry (Gibb & Henderson, 1996
). Heights, which for the sampled sections are vertical above sea level, were recalculated for the composite section to vertical above the base of the sill and are given in italics, following previous practice (Gibb & Henderson, 1996). We have now used the whole-rock chemistry to refine this model by adjusting the overlapping trends vertically for each element in Hole 3 and Hole 2 until the best visual fits were obtained. For all elements except Sr, this fit was within a few metres of that based on modal and petrographic criteria and the average fit is virtually identical, thus further increasing confidence in the composite section (Fig. 6a). It is evident that no amount of vertical adjustment would produce a reasonable fit for Sr. However, Foland et al. (2000) have shown that much of the Sr was introduced during post-solidification alteration of the sill rocks by hydrothermal fluids derived from the country rocks. We therefore discount the mismatch for Sr and indeed will not use Sr for any of the petrogenetic deductions.
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Geochemical variations with height
Element distributions with height in the lower three-quarters of the sill are of two main types. In the first, the element concentrations are controlled by the modal amounts of olivine and chrome spinel, the latter being common as inclusions in olivine throughout the picrites and lowermost picrodolerites (Gibb & Henderson, 1996
, fig. 3). Chemical parameters exhibiting this type of distribution include MgO, Ni and Cr (not shown). Concentrations increase slightly upward through the olivine teschenites, then across the junction with the picrites (Fig. 6). They continue to increase upwards through the picrites to a maximum before decreasing towards the Lower Discontinuity. Above the discontinuity, the concentrations decrease rapidly upward through the picrodolerites, reaching a relatively constant value in the crinanites. FeTotal, FeO and Mn also exhibit this type of distribution, but with less marked variations in the picrites and smaller decreases through the picrodolerites (Table 1). Mg# [= Mg/(Mg + Fe2+), atomic proportions] also has a similar distribution profile (Fig. 6). [Because variable oxidation effects are likely to have been associated with late- and post-magmatic hydrothermal alteration, we have calculated Mg# taking magmatic Fe2+ to be 85% of the total Fe content (Brooks, 1977).]
The second type of element distribution is essentially a mirror image of the first (Figs. 5 and 6a). Elements with these ‘mirror image’ profiles are mainly those concentrated in plagioclase (Al, Ca, Na, K, Sr), augite (Ca, Ti), iron oxides (Ti, V) and apatite (P, REE), or are incompatible (e.g. Zr, REE, Nb, Th). As the plagioclase/pyroxene ratio changes only slightly throughout the sill (Gibb & Henderson, 1996, fig. 3), these element variations appear to be largely inverse to the olivine content.
The vertical distribution of olivine and compositional variations in olivine (Johnston, 1953; Gibb & Henderson, 1996) and clinopyroxene (Gibb, 1973) are ample proof of the sill's vertical differentiation in terms of Mg#. In addition, throughout the Lower Picrite unit, especially in Hole 2 (Fig. 5a), the Ca# [= Ca/(Ca + Na + K), atomic proportions] has a D-shaped profile, indicating that the unit is slightly more primitive in its centre than at the edges. Above the discontinuity, there is a slight but steady decrease in Ca# up through the picrodolerites and crinanites.
Element distributions through the uppermost part of the sill are essentially the inverse of those in the lower part, again with two types that are mirror images of each other. In the first type (e.g. MgO, Ni, Cr), concentrations are relatively constant through the Upper Olivine Teschenite (Fig. 6b). Across the junction with the Upper Picrite, there is a marked increase that continues down through the picrites to the Upper Discontinuity. Within the GOPd, the profiles reflect the tripartite petrographic subdivision of this unit with its central olivine-rich ‘plug’ (Foland et al., 2000; Henderson et al., 2000). In the crinanites just below the GOPd, the situation becomes complicated by the presence of pegmatitic horizons. In the highest crinanite, concentrations are similar to those in the Upper Olivine Teschenite, then decrease downwards over 15–20 m to the lowest values found in the crinanites and remain relatively constant for 30 m before increasing again.
In the second, ‘mirror image’-type of distribution (e.g. Al2O3, CaO, Na2O and the incompatible trace elements), the concentration varies slightly within the Upper Olivine Teschenite then falls sharply across the junction with the Upper Picrite, through which it continues to decrease towards the Upper Discontinuity (Figs. 5b and 6b). A marked increase across the Upper Discontinuity is followed by a decrease into the olivine-rich centre of the GOPd, before another increase into the lower part of this unit. Distribution patterns at the base of the GOPd are a little erratic, possibly reflecting the fact that this part of the unit shows the highest degrees of contamination (Foland et al., 2000). Perhaps significant in this context is Zr, which, in the lowest analysed GOPd, reaches levels exceeded only in the pegmatitic horizons (Fig. 5b). Below the GOPd, concentrations in the uppermost crinanites return to values similar to those in the olivine teschenites (Figs. 5b and 6b) before increasing downward to converge with the concentrations in the crinanites at the top of the Hole 2 section. For most of the incompatible trace elements maximum concentrations occur around the 120 m level in the composite section, where values are typically two to three times those at the top and bottom of the Picrodolerite–crinanite unit.
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BOUNDARIES OF INTRUSIVE UNITS |
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The subdivision of the sill and the consequent multiple intrusion model arose from the recognition of sharp internal boundaries based on field and petrographic evidence, especially changes in modal mineralogy and grain size (Gibb & Henderson, 1989
, 1996; Henderson et al., 2000). Such boundaries, at least those across which there are mineralogical changes, should be marked by changes in the whole-rock chemistry. However, where a pulse of magma was emplaced into a hot incompletely crystallized earlier intrusion, rapidly chilled impermeable margins might not have formed. Consequently, there would have been potential for mixing, either of the bulk magmas or of the interstitial liquid from the earlier magma with the new magma. Any such mixing would blur changes in chemistry across the junctions. Indeed, Foland et al. (2000) invoked just such a mechanism to account for the variation in isotope chemistry within, and across the lower boundary of, the GOPd unit.
Upper margin of the sill
The upper margin has been sampled and analysed only from Hole 3. The extremely fine-grained marginal rock contains 14% of small euhedral to subhedral phenocrysts of olivine (<0·9 mm long) and sparse microphenocrysts of plagioclase (<0·3 mm long) (Fig. 7). The analysed sample (SC988, Table 1) is 5 cm below the actual contact with the sediments, but is petrographically similar to the marginal rock, albeit slightly less fine-grained.
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Corrigan (1982)
ascertained the cooling rate through the plagioclase crystallization interval across basaltic dykes by reproducing experimentally the plagioclase grain size. The composition of the upper margin of the sill is similar to Corrigan's "Dyke 1". Applying Corrigan's experimental data to the sill (avoiding obvious phenocrysts) indicates that the rock 1 cm below the contact cooled through the nucleation and growth intervals of the plagioclase at a rate in excess of 10°C/h. Such fast cooling argues against large-scale turbulent convection having occurred in the sill, as the establishment of such convection would greatly reduce the cooling rate at the top of the sill (Huppert & Sparks, 1989). It would be even more inconsistent with a margin formed after melt-back of an original chilled selvedge as a result of increased heat flow from vigorous internal convection in the sill, and we can only interpret this upper margin as having formed by rapid cooling of the intruded magma against relatively cold country rock.
The rapid growth of olivine from quickly cooled basaltic or picritic magmas results in skeletal crystals (Drever & Johnston, 1957). Donaldson (1976) showed experimentally that skeletal olivines grow from a basaltic liquid cooled at rates greater than about 2·7°C/h, while subhedral and granular crystals form at slower cooling rates. The euhedral nature of the olivine phenocrysts in the upper margin of the sill and the absence of skeletal crystals suggest that the phenocrysts formed under cooling rates slower (probably much slower) than 2·7°C/h. Clearly, this is inconsistent with their formation at the same time as the groundmass plagioclase. Latypov (2003b) has argued that, as non-skeletal olivine crystals can form at cooling rates up to 2 or 3°C/h, the presence, size and shape of phenocrysts in the chilled margins of intrusions cannot be used to determine whether they formed before or after emplacement. Notwithstanding Latypov's argument, the abundance, size and euhedral habit of the olivine phenocrysts in the upper margin of the sill, together with the fine-grained nature of the groundmass, lead us to conclude that the olivines were present in the magma at the time it chilled against the sediments, i.e. their crystallization pre-dated emplacement. The same is almost certainly true of the plagioclase microphenocrysts.
Hence, while this chilled margin might not represent the bulk composition of the sill, or even of the Olivine Teschenite unit, it does represent the composition of the upper part of the leading edge of the first thin wedge of magma as it propagated along the fracture that was to develop into the sill.
Olivine teschenite–picrite
This boundary is recognized by a sudden downward change to a finer-grained rock accompanied by a more than fourfold increase in modal olivine. The latter is reflected by substantial increases in MgO, Ni and Mg# (Fig. 6), with smaller rises in total Fe and Cr. There are concomitant decreases in Al2O3, CaO (Fig. 5b), SiO2, Na2O and the incompatible elements. Chemically, as well as petrographically, this boundary represents a clear break that can be interpreted as a ‘reversal’ in the sense of a change to a more primitive composition by the intrusion of fresh magma.
Upper discontinuity
In terms of grain size, mineralogy and chemistry, this is the most pronounced break in the sill with medium-feldspar picrite (above) against a very fine-grained facies of the GOPd. Downward across this junction, there are marked increases in Al2O3, CaO (Fig. 5b), Na2O, K2O, Cr and the incompatible elements with decreases in MgO, Ni and Mg# (Fig. 6). It is difficult to see this as anything other than a mechanical (i.e. intrusive) contact across which there was a change to a less primitive magma. Further, the petrographic characteristics of this boundary suggest the overlying picrite was either solid or a semi-solid crystal mush, and significantly cooler than the GOPd magma when the latter was emplaced (Henderson et al., 2000).
GOPd–crinanite
With relatively subtle grain size changes across this junction, it is the most difficult of the internal boundaries to identify petrographically; however, significant changes between the lowermost GOPd and highest crinanite (Table 1) occur for Cr (3500–260 ppm), Ni (500–125 ppm) and Zr (255–70 ppm). Other clear changes occur in CaO, TiO2, Co, Cu and Zn. The Sr and Nd isotope evidence is also consistent with a boundary at this position.
Lower discontinuity
The Lower Discontinuity has been described from the north coast of Garbh Eilean (Fig. 1) (Murray, 1954b), the SE coast of Garbh Eilean (Murray, 1954b; Gibb & Henderson, 1989), the east coast of Eilean an Tighe (Murray, 1954b; Gibb & Henderson, 1989) and Drill-holes 1 and 2 (Fig. 1; Gibb & Henderson, 1996). Breaks in modal mineralogy, grain size and texture occur across the discontinuity and marked local variations, especially in the nature of the picrite immediately below it (Fig. 8), might suggest the presence of a transgressive intrusive contact between the Picrodolerite–crinanite and its host Picrite unit.
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Whole-rock chemical profiles across the Lower Discontinuity are available from the well-exposed locality on Garbh Eilean (Gibb & Henderson, 1989
, fig. 3) and Holes 1 and 2. For 5–6 m on either side of the Lower Discontinuity in Hole 1, there is a very irregular distribution of Ni and Cu that contrasts with the relatively systematic variation in Hole 2. These anomalously high concentrations reflect the localized occurrence of pentlandite and chalcopyrite that could have formed by introduction of hydrothermal S-bearing fluids along the discontinuity (Gibb & Henderson, 1996; Henderson et al., 2000). The contrast between Cr distributions within the picrites immediately below the Lower Discontinuity in Holes 1 and 2 (Fig. 8) may be related to grain size, petrographic and mineralogical differences in the picrites. In Hole 1, the large-feldspar picrite, some 5 m below the discontinuity, grades upwards through medium-feldspar picrite to small-feldspar picrite at the discontinuity, whereas in Hole 2, a similar gradation to small-feldspar picrite occurs but then reverses so large-feldspar picrite reappears below the Lower Discontinuity. Differences in the augite (0·5% Cr2O3) and chrome spinel (20–25% Cr2O3) contents of the picrite variants are the likely cause of the disparities in the Cr profiles.
Across the Lower Discontinuity (from picrodolerite to picrite) in Hole 2, Gibb & Henderson (1996, fig. 5) recorded a small increase in the Mg# of the olivine (2%Fo) and a decrease in olivine Ni content. However, no corresponding changes were observed in the compositions of the plagioclase and augite. Gibb (1973) and Gibb & Henderson (1996) attributed these features to post-intrusive equilibration of interstitial liquids across the discontinuity while both picrite and picrodolerite were crystal mushes.
We conclude that changes in chemical composition across the Lower Discontinuity confirm the field and petrographic evidence that it is an intrusive contact (possibly transgressive) formed by the emplacement of the Picrodolerite–crinanite unit into the earlier Picrite.
Picrite–olivine teschenite
Petrographic identification of this boundary, which is recorded only from Hole 2 and Drill-hole DJ2 (Fig. 1), is hampered by the increasing alteration of the rock towards the bottom of the sill. However, it is more clearly defined by the breaks in element concentrations and element ratio profiles. These breaks are essentially inverted versions of those across the olivine teschenite–picrite boundary just below the top of the sill (Figs. 5 and 6).
Lower margin of the sill
The lower margin of the sill has been studied from isolated, poorly exposed outcrops along the north coast of Garbh Eilean (Drever & Johnston, 1959), from Drill-hole DJ2 (Drever & Johnston, 1965) and from our Hole 2. The rock from the north coast, described by Drever & Johnston as the lower margin, consists of about 15% of small, mainly euhedral or subhedral, phenocrysts of olivine and sporadic microphenocrysts of plagioclase in a very fine-grained groundmass. It appears to be almost identical to the upper margin of the sill described above [cf. Fig. 7 and plate XVI-D of Drever & Johnston (1959)]. Unfortunately, no chemical analysis of this rock was given. The lowermost 1·3 m of the sill recovered from Drill-hole DJ2 was referred to by Drever & Johnston (1965) as "fine-grained olivine teschenite". Close to the lower margin of the sill, the rock is very heavily altered but was described as "comparable with ... the lower contact ... at the north end of Garbh Eilean". This hydrothermal alteration can be attributed to the activity of convectively circulating, heated groundwater, initially trapped beneath the sill, gaining access to the lower reaches of the sill through contraction fractures formed during post-solidification cooling (Gibb & Henderson, 1978, 1996; Dickin et al., 1984).
The lowest sill sample analysed (SC770) is from 15 cm above the contact where the alteration is less severe. Compositionally SC770 is almost indistinguishable from SC988 just below the upper margin of the sill and any differences are attributable to post-solidification hydrothermal alteration. Thus, the chemistry is consistent with both the upper and lower margins of the sill being formed by the rapid chilling of the leading portion of the first batch of magma intruded into the sill.
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MULTIPLE INTRUSION MODEL |
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The occurrence of paired (top and bottom), inverted breaks in the chemical profiles and the coincidence of these with unit boundaries identified from field and petrographic observations is unequivocal evidence of multiple intrusion (Fig. 4). Further support comes from the chemical coherence of the now separated parts of the earliest intrusive units. This is best seen by restoring the chemical profiles of the sill to what they would have been before the third magma was emplaced to form the Picrodolerite–crinanite unit, i.e. removing everything between the Upper and Lower Discontinuities (Fig. 9). For all modal, mineralogical and chemical parameters, the fit of the internally differentiated Upper and Lower Picrite units is striking. These correspondences are all the more remarkable as (a) the two sections being fitted together are
400 m apart laterally, and (b) the subsequent intrusion of the crinanitic magma may have been slightly transgressive with respect to the picrite. The restored fits are equally good with respect to Sr and Nd isotope ratios (Foland et al., 2000, figs 4 and 7). Further, this suggests that very little of the picrite was displaced or swept away by the subsequent intrusions.
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Going back one stage further to before the intrusion of the Picrite unit and attempting a similar re-fit for the Upper and Lower Olivine Teschenites does not work quite so well. It appears that some of the original teschenite intrusion might have been removed during the emplacement of the Picrite.
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OVERALL CHEMICAL VARIATIONS IN THE SILL |
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We do not present CIPW normative data for the analysed samples; however, it is worth noting that most of the sill rocks are only slightly silica-undersaturated, with about 3% ne in the norm. The topmost olivine teschenite (SC988, Table 1) and the lowest two picrites analysed (SC758, SC760) have
3% hy and rocks from the Lower Olivine Teschenite (SC765L, SC770) have 11% hy. All of these hy-normative rocks exhibit extensive hydrothermal alteration and the inference is that their major element compositions were disturbed by the post-magmatic processes (Gibb & Henderson, 1996; Foland et al., 2000). The GOPd unit is notably heterogeneous with some rocks (e.g. SC1040, SC1060T) having >10% hy but lacking obvious signs of hydrothermal alteration. In these cases, the suggestion is that their hy content is a magmatic feature.
Plots of whole-rock major and trace element data against wt % MgO as a differentiation index (Fig. 10) show the overall variations within the sill. The picrites and picrodolerites define a tight linear trend along an olivine control line towards Fo83, the coexisting olivine composition (see arrows in Fig. 10). The picrodolerites with the lowest MgO contents overlap with the compositional field of the crinanites. The analysed pegmatites have markedly lower Al2O3 contents than the crinanites because of their high contents of augite and Ti-magnetite. The olivine teschenites tend to be slightly less aluminous than picrodolerites with similar MgO contents. CaO and Na2O (Fig. 10c and e) exhibit similar trends along olivine-control lines. Mg# (Fig. 10g) shows a smooth trend with a steeper decrease below 15% MgO, reflecting the increased Fe enrichment in the olivine teschenites, GOPds and especially the more evolved picrodolerites, crinanites and pegmatites. By contrast, Ca# (Fig. 10i) shows only a small decrease until the crinanites and pegmatites. This reflects the fact that albite enrichment in plagioclase became dominant only during the latest stages of sill solidification.
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As expected, Ni decreases sympathetically with MgO (Fig. 10b), but is somewhat variable at the MgO-rich end of the trend because of the occurrence of Ni-bearing sulphides in picrodolerites and picrites close to the Lower Discontinuity. The GOPds lie significantly above the trend and the same is true for Cr (Fig. 10h), where the data are even more scattered. This is because of the variable contents of Cr-rich spinel in the GOPds. Both large ion lithophile (e.g. Ba, Th: Fig. 10d and j) and high field strength (e.g. Zr: Fig. 10f) incompatible element contents increase as MgO decreases, with the strongest enrichment in the crinanites and pegmatites.
A plot of Mg# against Ca# (Fig. 11) allows the effects of variation in the content of (introduced) olivine and post-intrusive fractionation of the liquid to be distinguished. All the large-feldspar picrites and some of the medium- and small-feldspar picrites have Mg#s around 0·80, but Ca# varies from 0·79 to 0·66. The olivine in this group of rocks has a composition close to Fo83 (Gibb & Henderson, 1996) and the relatively constant Mg# indicates that this olivine (which was mainly present as crystals at the time of emplacement) is the dominant ferromagnesian mineral. [A table of mineral compositions for the analysed rocks (Electronic Appendix 1 is available at http://www.petrology.oupjournals.org).] On the other hand, the Ca, alkalis, Al, etc. are contributed largely by minerals that crystallized in situ from the co-existing liquid phase; the range shown by Ca# reveals that the composition of this liquid was variable through the Picrite unit with the less evolved rocks (higher Ca#) being located in the centre of the unit. This could reflect an original feature of the intruded magma or subsequent migration of interstitial liquid during in situ differentiation of the Picrite, or possibly both (see later). The remaining medium- and small-feldspar picrites form a second group with Mg#s between 0·70 and 0·78 and Ca# varying from 0·65 to 0·73. These latter picrites are more evolved than the first group and reflect their lower contents of suspended olivine crystals. It is significant that all the medium- and small-feldspar picrites in the first group are from just below the Lower Discontinuity, whereas those in the second group are from the more marginal parts of the unit.
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The more olivine-rich picrodolerites have Mg#s between 0·73 and 0·80, with Ca#s between 0·68 and 0·74, and define a trend with a gentle positive slope, indicating that some in situ fractionation was superimposed on the physical redistribution of olivine. The remaining picrodolerites from higher in the unit have Mg#s from 0·54 to 0·73 and Ca#s from 0·65 to 0·72, defining a steeper trend representative of a stronger crystal–liquid fractionation with less of a contribution from olivine accumulation. Most of the crinanites lie on a continuation of this trend to lower Mg#s with a faster decrease in Ca#. Two of the more "crinanitic" pegmatites—SC1066 and SC1115 (see Table 1 for locations of numbered specimens)—extend the trend to the most-evolved rock compositions analysed.
Most of the olivine teschenites in Fig. 11 overlap with the more evolved picrodolerites but two from the heavily altered bottom part of the sill have anomalously high Ca#s as a consequence of the hydrothermal leaching of alkalis.
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DIFFERENTIATION WITHIN UNITS |
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The fractionation trends are best shown in ternary chemical variation diagrams plotted in atomic proportions rather than wt %, making it straightforward to locate and define the effects of the various minerals from their end-member stoichiometries.
Olivine teschenite
On a Ca–Mg–(Fe + Mn) diagram (Fig. 12a), the Upper and Lower Olivine Teschenites define an approximately linear trend that is consistent with evolution through fractionation of olivine Fo83 (the composition of the most Mg-rich olivine found in the rocks). The lack of displacements towards the compositions of the plagioclase or clinopyroxene in these rocks (Fig. 12a) indicates that these minerals were not significant at this stage of the sill formation. There is slightly more scatter in terms of Ca–(Mg + Fe + Mn)–(Na + K) in Fig. 12b, with small variable displacements towards the Na + K apex reflecting the presence of analcime in the hydrothermally altered marginal rocks.
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The normalized REE patterns for the Upper and Lower Teschenites (Fig. 13f) are sub-parallel and are characterized by a hump-shaped pattern indicating enrichment in the middle REE relative to the HREE, and a slight enrichment in LREE relative to the HREE. There appear to be small positive Eu anomalies superimposed on the pattern. The Upper Olivine Teschenite (e.g. SC1000) has higher concentrations of REE (Fig. 13f), and incompatible trace elements (Fig. 14f) than the Lower Olivine Teschenite (e.g. SC765L). This may be largely due to the higher olivine content in the latter. The parallel nature of the trends in Figs 13f and 14f reflects the immobility of the REE and most incompatible elements during hydrothermal alteration, which was greater in the Lower than the Upper Olivine Teschenite. The incompatible element diagram (Fig. 14f) indicates depletions in Th and Nb and a characteristic depletion in P. There also appear to be enrichments in Ba, Rb and K but little significance is attached to these, as Ba, Rb and K may not have remained completely immobile during the hydrothermal stage that affected the Sr distribution in the sill (although there is no evidence of this).
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There can be little doubt that the olivine teschenite was emplaced as a suspension of
14% olivine phenocrysts (often with small inclusions of Cr-spinel) and sparse plagioclase microphenocrysts (Fig. 7). Variations in modal mineralogy and whole-rock chemistry suggest that the existing Upper and Lower Olivine Teschenite units represent only part of the original olivine teschenite intrusion, the missing central part having been swept away by the subsequent intrusion of the picrite.
Picrite
The picrites define a strong linear olivine (Fo83) control trend in Fig. 12 extending from the olivine-rich, large-feldspar picrites through medium-feldspar picrites to the small-feldspar picrites found near the top and bottom of the Upper and Lower Picrite units. However, a significant number of medium- and small-feldspar picrites plot at the high-Mg end of the trend along with the large-feldspar picrites. These are the picrites from the internal part of the unit and contrast with the first group of similar grain size, but olivine-poorer, picrites from closer to the margins. Clearly, position in the sill is more relevant than petrographic type to the chemistry of the rock. The fact that the picrites plot on tight linear trends with no displacements towards plagioclase or clinopyroxene (Fig. 12a) shows that these phases were still not significant at this stage of the evolution. The magma intruded to form the Picrite unit was thus a suspension of olivine crystals (Fo83) in a liquid of slightly variable composition. The liquid composition must lie on a continuation of the picrite trend towards the Ca–(