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2.9 Elie Ness

[NT 498 993]

A Livingstone

Introduction

Within the Midland Valley Terrane, Permo–Carboniferous alkali basaltic tuffs and minor intrusions, together with one locality in the NW Scottish Highlands, are eight localities known to contain pyrope garnet-bearing pyroxenite xenoliths and pyrope xenocrysts (Upton et al. 2003) of which Elie Ness is an outstanding example. The Elie Ness diatreme underwent an extensive series of eruptions including explosive types. The xenoliths, xenocrystic suites and lithologies provide an insight into the Elie Ness volcanic history and lower crust and upper mantle beneath Scotland (Gernon et al., 2013, 2016).

The volcanic vents of East Fife, first described by Geikie (1879), of which more than 100 are known, are intruded at various levels into Carboniferous sedimentary rocks. Numerous studies (e.g., Balsillie, 1920, 1923, 1927; Cumming, 1928, 1936; Francis, 1960, 1970; Francis and Ewing, 1961; and Francis and Hopgood, 1970) primarily concentrated on structures, neck emplacement and volcanic processes. Description of the Elie Ness locality and its relationship to other volcanic features in the area have been given by Geikie (1902), Cumming (1928) and Forsyth and Chisholm (1977). More recent work on the xenolithic inclusions of plutonic cumulates, spinel–lherzolites, anorthoclase megacrysts, amphibole, zircon, augite and pyrope by Chapman (1974, 1976) and pyrope by Colvine (1968) reawakened interest in these vents. MacIntyre et al. (1981) established a chronology for the volcanic neck emplacements, which occurred between 318 and 29 Ma. Detailed petrological, geochemical and oxygen isotope studies on xenoliths and xenocrysts were reported by Upton et al. (1999).

Elie Ness is renowned for pyrope garnet, the ‘Elie ruby’ (Farnie, 1860) which has been collected for well over 150 years. Donaldson (1984) studied the pyrope kinetics in ascending basaltic magma and concluded crystallisation occurred in a mantle-derived magma. An eruption from depth penetrated and disrupted early cumulate assemblages carrying nodules and high-pressure pyrope xenocrysts to the surface. From broader, and detailed, studies (Upton et al., 1983) the mineralogical and geochemical nature of the Upper Mantle and Lower Crust beneath Scotland has become clearer.

Description

Early descriptions of the Elie Ness vent, well exposed between Lady’s Tower and the lighthouse, were given by Geikie (1902) and Cumming (1928). The north-east – south-west elongated vent (Fig. 24) occurs over an area measuring 400 × 300 m and comprises greenish basaltic tuff plus agglomerate containing basalt bombs and sedimentary blocks. Partially well-bedded pyroclastic rocks dip centroclinally, decreasing inwards from 50° to 10° towards the presumed centre west of the lighthouse. Minor intrusions, mainly tuffisitic and basanitic dykes cut the vent (Forsyth and Chisholm, 1977). In the agglomerate, glassy alkali basalt fragments form the majority of the igneous suite. Mafic biotite and/or green amphibole plus albite xenoliths (to a few centimetres), and amphibole, pyroxenites plus a megacryst suite, occur with basanitic blocks enclosed in partially-bedded tuff. In the vent kaersutite and/or biotite pyroxenites±olivine are well represented. MacIntyre et al. (1981) additionally reported edenitic amphibole–mica pyroxenites. Elie Ness pyroxenites vary from feldspar-free varieties to those containing albitic plagioclase (Upton et al., 1999). Small anorthoclasite xenoliths also occur and typically display a simple mineralogy comprising anorthoclase, minor zircon and chlorite, the latter deemed to be derived from pyroxene breakdown. Additional to the xenoliths a megacryst suite, predominantly anorthoclase, amphibole, zircon, augite and pyrope, can be subdivided into cumulate-derived xenocrysts and those precipitated as phenocrysts (augite and pyrope) within the magma.

The Elie Ness anorthoclase fractured crystals are small and Mason et al. (1982) presented electron and ion microprobe analyses, with Upton et al. (1999) adding a further analysis. Compared to anorthoclase from other Fife vents the Elie Ness feldspar is intermediate in LREE content: 1–2 ppm La and moderate to high Sr plus Ba contents > 1500 ppm (Upton et al., 1999). MacIntyre et al. (1981) revealed Elie Ness anorthoclase, in common with that from other Fife necks, to possess a high temperature, disordered, unmixed structure. K–Ar dating of anorthoclase from the Elie Ness vent yielded an age of 290± Ma.

At Elie Ness zircons form a prominent minor member of the megacryst assemblage, and may attain 1 cm. Hinton and Upton (1991) reported ion microprobe analyses for zircon occurring as a megacryst, and also in an anorthoclase-rich xenolith. Megacryst zircon exhibits optically visible oscillatory growth zoning which was studied by cathodoluminescence plus back-scattered electron imaging prior to ion micrprobe analysis. The latter revealed that the optically observed zoning resulted from variations in Y, REE, Th and U. Anorthoclase-rich xenolith zircons, though chemically similar to megacryst zircon, lack optically visible zonation. Hinton and Upton (1991) analysed for LREE, HREE and reported up to 22 trace elements. All zircons analysed presented positive Ce anomalies. Hundreds of equant to subhedral zircon grains are accessioned in the National Museums Scotland (G1994.88.1, G1995.35.1 and G1995.131.1).

Pyrope forms either isolated xenocrysts in the agglomerate matrix or occurs as loose grains in local beach sand. Equant fragments and grains (sub-millimetre to 5+ mm, and rarely 20 mm) show no obvious resorption features or basaltic coatings. The generally inclusion-free optically homogeneous garnet ranges through orange–red colours to crimson. Chemical analyses presented by Heddle (1901), Colvine (1968), Chapman (1976) and Donaldson (1984) reveal a titanian composition close to 67 mol% pyrope. Significantly TiO₂ ranges from approximately 0.4 to 0.6 wt%, CaO around 5 wt% and Cr₂O₃ in the range 0.03–0.04 wt%.

Clinopyroxene phenocrysts within pyroclastic and minor intrusive phases show a wide compositional range of Al₂O₃ versus CaO compared with xenolithic clinopyroxenes from other Scottish provinces and highlight the complexity of the tectonomagmatic processes within the lithosphere under Scotland (Upton et al., 2011).

An extensive suite of several thousand garnet grains, approximately 2–3 mm and greater, is accessioned in the National Museums Scotland (G1992.11.1 and G1994.15.68). Donaldson (1984) reported the garnet to be inclusion free; however, a substantial number separated from the large collection contain inclusions, preliminary examination of which identifies the presence of nickel sulphides.

Interpretation

An important aspect in deciphering events is that pyrope and augite are not found in cumulate assemblages within the Elie Ness vent, nor are basanitic blocks. Chromium- deficient pyrope does not occur in ultrabasic nodules, nor as kimberlitic xenocrysts (Nixon et al., 1963). Because of the unusual pyrope chemistry Colvine (1968) argued that it originated as a primary precipitating phase from a suitable magma at depth, whereas Chapman (1976) proposed pyrope co-precipitated with sub-calcic augite as high-pressure phenocrysts at depths greater than 70 km from a primitive basic alkaline magma at a minimum pressure and temperature of 25 kbar and 1350°C. Alternatively, they may represent fragments from mechanical disintegration of coarse-grained mantle pyroxenite.

Pyrope is unstable at low pressures, either decomposing into other minerals or being resorbed. There is no textural evidence that Elie Ness pyrope either underwent decomposition or resorption. Donaldson (1984) demonstrated that pyrope decomposition to pyroxene plus melt, and resorption reactions, at 1300–1400°C are extremely rapid. He proposed that the transporting magma was considerably cooler and pyrope possibly crystallised at 1000°C in a magma where H₂O was significant.

The various volcanic lithofacies at Elie Ness possess REE patterns that highlight a continuous increase from HREE to LREE and high Zr/Y values that imply retention of garnet in a mantle source (Gernon et al., 2016).

Rhythmic zoning in the Elie Ness zircon suggests variations in REE, Th and U contents during parent magma evolution may be related to physical-chemical changes rather than changes in absolute levels of REEs. Zircon megacryst formation around 31 Ma favours co-crystallisation with cumulate phases, unlike younger anorthoclase crystallisation at 29 Ma associated with neck emplacement.

In the broader context the Fife volcanic necks, including that at Elie Ness, display individual differences, yet collective evidence presents a picture of low-pressure accumulate minerals crystallising at shallow crustal levels. Following accumulate crystallization, a deep-seated undersaturated magma at mantle level, bearing high-pressure megacrysts, broke through the cumulate fraction and continued to surface eruption. Helium isotopic studies of garnet megacrysts by Kirstein et al. (2004) support the view that the Permo–Carboniferous suite of upper mantle xenoliths were contaminated by a lower mantle plume under Scotland. However, advanced seismographic techniques clearly indicate that the Icelandic mantle plume has an irregular shape with radial fingers extending outwards of which one occurs underneath Scotland's thin crust (Schoonman et al., 2017).

Gernon et al. (2013, 2016) proposed that the REE patterns signify that the garnet was retained in a mantle source when peridotite melted in the mantle garnet–lherzolite facies. Furthermore, the subhedral and unresorbed nature of the pyrope suggests a rapid rise through the lithosphere. The wide range of clinopyroxene compositions indicate changing pressures during magma fractionation plus magma mixing in the lower crust around 22–28 km depths. Gernon et al. (2016) from phenocrystic, megacrystic and autolithic data concluded that the Elie Ness diatreme originated from magma mixing, rapid ascent and explosive eruptions.

Conclusions

Of the 100 or so East Fife volcanic vents, that at Elie Ness is renowned for the occurrence of pyrope garnet. This red, semi-precious gem has been collected for over 150 years either from vent rocks or local beach sand. Its origin has been the subject of much study.

At shallow crustal levels in the volcanic chamber crystals developed in the magma and under the influence of gravity sank to accumulate a crystal mush which then consolidated. A second deep-seated magma in which pyrope garnet crystallized, ascended, broke through and fragmented the accumulate, and rose to the surface bearing gas-charged magma.

Glossary

Basanite: Volcanic rock with calcium-rich plagioclase plus olivine and augite with a feldspathoid (analcime, leucite or nepheline).

Diatreme: A volcanic vent created in sedimentary rocks by the explosive forces of a gas-charged magma.

Kaersutite: Member of the amphibole group containing sodium , aluminium, calcium, magnesium and titanium.

Lithofacies: A term used to cover various sedimentary rocks in the area.

Megacryst: Crystal larger than those in a fine-grained matrix, which are visible to the unaided eye.

Xenocryst: Crystal foreign to the igneous rock in which it occurs.

Xenolith: Rock fragment foreign to the igneous rock in which it occurs.