Seawater signatures in the supracrustal Lewisian Complex, Scotland

Abstract

Marble in the supracrustal rocks of the Lewisian Complex, Tiree, includes chlorine-bearing amphiboles, chlorine-rich apatite, sulphur-rich scapolite, albite and phlogopite, all of which are regarded as evidence for evaporites in other metamorphosed sequences. Titanite yields U–Pb ages of ∼1.6 Ga, i.e. late Laxfordian, which excludes a younger imprint of sodium metasomatism. Traces of anhydrite, and isotopically heavy pyrite, also indicate deposition from seawater. Elsewhere in the Hebrides, tourmaline in Lewisian Complex marbles may represent seafloor exhalative deposits. Combined, the evidence suggests Lewisian Complex supracrustal marbles formed in an evaporative environment, like other Palaeoproterozoic successions across the North Atlantic region.

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Cite this article: Parnell J, Boyce AJ, and
Næraa T (2022) Seawater signatures in the
supracrustal Lewisian Complex, Scotland.
Geological Magazine 159:1638–1646. https://
doi.org/10.1017/S0016756822000474
Received: 3 March 2022
Revised: 2 May 2022
Accepted: 2 May 2022
First published online: 2 June 2022
Keywords:
Lewisian; Palaeoproterozoic; evaporites;
marble; titanite; scapolite; Scotland; Tiree
Author for correspondence: J. Parnell,
Email: J.Parnell@abdn.ac.uk
© The Author(s), 2022. Published by Cambridge
University Press. This is an Open Access article,
distributed under the terms of the Creative
Commons Attribution licence (http://
creativecommons.org/licenses/by/4.0/), which
permits unrestricted re-use, distribution and
reproduction, provided the original article is
properly cited.
Seawater signatures in the supracrustal
Lewisian Complex, Scotland
J. Parnell1, A.J. Boyce2and T. Næraa3
1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom; 2Scottish Universities
Environmental Research Centre, Rankine Avenue, Glasgow G75 0QF, United Kingdom and 3Department of
Geology, Lund University, Sölvegatan 12, SE-22362 Lund, Sweden
Abstract
Marble in the supracrustal rocks of the Lewisian Complex, Tiree, includes chlorine-bearing
amphiboles, chlorine-rich apatite, sulphur-rich scapolite, albite and phlogopite, all of which
are regarded as evidence for evaporites in other metamorphosed sequences. Titanite yields
U–Pb ages of ~1.6 Ga, i.e. late Laxfordian, which excludes a younger imprint of sodium meta-
somatism. Traces of anhydrite, and isotopically heavy pyrite, also indicate deposition from sea-
water. Elsewhere in the Hebrides, tourmaline in Lewisian Complex marbles may represent
seafloor exhalative deposits. Combined, the evidence suggests Lewisian Complex supracrustal
marbles formed in an evaporative environment, like other Palaeoproterozoic successions across
the North Atlantic region.
1. Introduction
The Lewisian Complex of northern Britain has been very extensively studied for over 200 years
(MacCulloch, 1819; Peach et al.1907; Park & Tarney, 1987; Mendum et al.2009). The complex
consists particularly of tonalitic gneisses of Archaean age, derived from an igneous protolith. In
common with Precambrian basement elsewhere in the North Atlantic region (Fig. 1), the
Lewisian Complex also includes local supracrustal successions of metasediment (Fig. 2). The
supracrustal successions contain a range of chemical sediments, including graphitic schists,
ironstones and marbles, which where dated (Whitehouse & Bridgwater, 2001; Park, 2002)
are mid-Palaeoproterozoic (~1.9–2.0 Ga). Across the region, from North America to Russia
(Fig. 1; Table S1 in the Supplementary Material available online at https://doi.org/10.1017/
S0016756822000474), these successions have evidence of former evaporites in the form of sur-
viving sulphate minerals, metamorphic minerals with a signature of evaporitic seawater, and
sulphur isotope data. To date, however, no such evidence is reported from Britain. Here we
report evidence for signatures that would be consistent with evaporitic facies in the Lewisian
Complex of Scotland. The uniformity of facies assemblage across the region, together with a
setting related to an accretionary plate boundary (Park, 2002), implies that evaporitic facies were
marine rather than lacustrine.
2. Geological setting and methods
Samples were collected from supracrustal rocks in the Lewisian Complex on the island of Tiree
(Fig. 2). The supracrustal rocks on Tiree include garnet schists, graphitic schists, sandstones,
ironstones and marbles (Westbrook, 1972; Whitehouse & Russell, 1997). The best exposed sec-
tions are at Vaul (National Grid Reference NM 048490), Balephetrish (NM 014473) and Gott
(NM 044459). They are metamorphosed to amphibolite facies and sheared, but they preserve
detrital mineralogy. Estimated P–Tconditions are 10.5 ±1.5 kbar and 810 ±50 °C
(Cartwright, 1992).
Mineral phenocrysts are especially abundant in the Tiree marbles (Fig. 3). The inclusions
were investigated using scanning electron microscopy, conducted in the ACEMAC facility at
the University of Aberdeen.
The mineralogy of the Lewisian Complex elsewhere in Scotland is affected by sodium meta-
somatism which also affected the Neoproterozoic Moine Supergroup (Sutton & Watson, 1951;
May et al.1993). An imprint by Neoproterozoic or younger fluids would exclude interpretations
of a Lewisian protolith, and must be tested. Accordingly, the assemblage of mineral phenocrysts
was dated using U–Pb analysis of titanite crystals. U–Pb isotope analyses were done using the laser
ablation –inductively coupled plasma –mass spectrometry (LA-ICP-MS) laboratory at Lund
University, where a Teledyne Photon Machines G2 laser is coupled to a Bruker Aurora Elite quad-
rupole ICP-MS. The laser is equipped witha HelEx two-volumesample cell with an energy meter.
Instrument tuning, using NIST612, was aimed at obtaining high and stable signal counts on lead
isotopes, on low oxide production (below 0.5 % monitoring 238U/238U16Oand232Th/232Th16O)

and on Th/U ratios around 1. Standard-sample-standard bracketing
incorporated the natural titanite MKED1 (Spandler et al.2016)as
primary reference material, and natural titanite ONT2 (Spencer
et al.2013) as a secondary standard. Each analysis was made with
300 shots at 10 Hz with a fluence of 1.5 J cm−2. Baseline composi-
tions were measured for 30 s before each measurement, andsubtrac-
tion was done with a step-forward approach. Common Pb was
monitored by measuring 202Hg and mass 204 (204Hg þ204Pb).
Baseline levels on mass 204 were c. 440 cps with a standard error
(SE) around 20 cps (5–6 %). Data reduction was done with iolite
using the X_U_Pb_Geochron4 DSR (Paton et al.2010,2011),
and the common Pb correction was done using the VizualAge
DRS (Petrus & Kamber, 2012). Plotting and final age calculations
were done with Isoplot(R); intercept ages are based on ‘model 1’cal-
culations, and errors confidence levels are 95 % with overdispersion
(Vermeesch, 2018).
For sulphur isotope analysis, pyrite samples were combusted
with excess Cu
2
O at 1075 °C in order to liberate the SO
2
gas under
vacuum conditions. Liberated SO
2
gases were analysed on a VG
Isotech SIRA II mass spectrometer, with standard corrections
applied to raw δ66SO
2
values to produce true δ34S. The standards
employed were the international standard NBS-123, IAEA-S-3 and
SUERC standard CP-1.
3. Criteria
Several criteria have been proposed for the identification of replaced
evaporites in Precambrian successions (Warren, 2016). The criteria
are based particularly on the chemistry of mineral phenocrysts that
developed during metamorphism up to hundreds of millions of years
after sedimentation (Moine et al.1981; Warren, 2016; Hammerli &
Rubenach, 2018).Theevidenceismostconvincingwherethereare
multiple positive criteria. The most direct evidence is:
(i) The survival of evaporite minerals anhydrite, gypsum or halite.
(ii) Pseudomorphs of evaporite minerals, most distinctively gyp-
sum and halite (e.g. Ririe, 1989; Zentmyer et al.2011).
Evidence for former evaporites also includes the growth of spe-
cific minerals containing chlorine or boron derived from seawater:
(iii) Phlogopite mica, which contains a high magnesium content
and traces of chlorine (Schreyer et al.1980; Moine et al.1981).
(iv) Minerals indicative of sodium metasomatism, including albite
and especially scapolite, which contains traces of sulphur and/
or chlorine (Mora & Valley, 1989). High sulphur levels in par-
ticular reflect assimilation of sulphate evaporites (Morrissey &
Tomkins 2020; Zeng et al.2020), while high chlorine levels are
also measured in scapolite from skarn deposits (Mora &
Valley, 1989).
(v) Tourmaline, which is the major reservoir of boron in meta-
evaporitic rocks (Henry et al.2008; Riehl & Cabral, 2018).
Tourmaline is often restricted to beds containing scapolite
assumed to be derived from saline fluids (Mora &
Valley, 1989).
Corroborative evidence includes:
(vi) Chlorine-rich apatite, which could be derived from a mag-
matic source or seawater, and which is positive evidence
where a magmatic input is lacking (Mao et al.2016).
(vii) Sulphur isotope compositions of pyrite, which are compa-
rable to the heavy composition of evaporites/seawater rather
than the near-zero composition of magmatic sulphur
(Golani et al.2002; Johnston et al.2006).
(viii) High-salinity fluids, especially in ore deposits that formed
during or shortly after sedimentation (e.g. Conliffe
et al.2013).
4. Results
Mineral phenocrysts in the Tiree marbles (Figs 3and 4) are domi-
nated by pyroxenes (enstatite, diopside), amphiboles (tremolite),
olivine (forsterite), micas and feldspars, and also scapolite, titanite
(sphene), apatite, epidote, pyrite and quartz.
Fig. 1. (Colour online) Map of North Atlantic region,
reconstructed for early Proterozoic time (after Park,
2002), showing inferred occurrences of evaporites and
types of evidence. Details in Table S1, in the
Supplementary Material available online at https://doi.
org/10.1017/S0016756822000474.
Seawater signatures in the supracrustal Lewisian Complex 1639

Minerals present in trace amounts include the sulphates anhy-
drite and barite. Anhydrite occurs as crystal fragments up to 15 μm
size, in a calcite-rich groundmass. Barite occurs as a disseminated
overprint on phenocrysts of pyroxene and amphibole.
Measurements of the sulphur isotope composition of five pyrite
crystals in marble at Gott yielded closely clustered values of
11.7, 11.7, 11.7, 11.9 and 12.3 ‰(Fig. 5).
Sodium metasomatism is represented by replacive albite and
scapolite. Scapolite has been recognized in the Tiree marble since
the first petrographic studies (Coomaraswamy, 1903; Hallimond,
1947), and the Lewisian supracrustal rocks are conspicuously richer
in scapolite than other rocks in Britain (Flett, 1907). The anion chem-
istry of scapolite includes variable combinations of −Cl, −F, −Sand
−OH, where proportions of −Cl and −S are interpreted to suggest
Fig. 2. Map of NW Scotland, showing distribu-
tion of Lewisian supracrustal inliers.
1640 J Parnell et al.

relative contributions from replaced halite and gypsum (Warren,
2016). Sulphur and chlorine were measured up to 2.30 % and
0.93 % respectively in the scapolite (Table S2,inthe
Supplementary Material available online at https://doi.org/10.1017/
S0016756822000474). The higher sulphur contents occur in scapolite
with the lower chlorine contents, as expected when both contribute to
the same atomic sum. The marble at Gott, and to a lesser extent in
otherTireemarble,ispartially altered to masses of albite.
Phlogopite occurs pervasively through the supracrustal marbles
on Tiree and in other Scottish localities. The phlogopite
Fig. 3. (Colour online) Phenocryst-bearing marble, Palaeoproterozoic, Tiree. (a) Gott, rich in olivine (arrowed), pyroxenes, amphiboles, K-feldspar and streaks of graphite; (b)
Balephetrish, rich in pyroxenes (arrowed) and amphiboles. Pink colour due to talc groundmass.
Fig. 4. (Colour online) Backscattered electron images of mineral phases in marble, Gott, Tiree. (a) Multi-phase phenocryst pf pyroxene (light grey, P) with potassium feldspar
(grey, K) and titanite (bright, T), all coated with ven eer of quartz (black, Q). (b) Abundant small barite (bright, B) within potassium feldspar an d pyroxene. (c) Two crystals of titanite
(T) within calcite, and abundant small pyrite (bright). (d) Crystal of anhydrite (A), showing characteristic cubic cleavage.
Seawater signatures in the supracrustal Lewisian Complex 1641

consistently contains 0.15 to 0.25 wt % chlorine. Apatite crystals
are chlorine-bearing, up to 2.8 wt % where measured (Table S3,
in the Supplementary Material available online at https://doi.
org/10.1017/S0016756822000474). The marble is not spatially
related to any magmatic deposits. Tourmaline is not recorded in
Tiree marble, but it does occur in marbles in several other
Lewisian supracrustal outcrops in Harris (Coward et al.1969),
Iona (Rock, 1987) and Gairloch –Loch Maree (Robertson
et al.1949).
Analyses for dating of the titanite in marble from Gott, Tiree,
were made in situ on 13 titanite grains in polished rock slabs
(Fig. 4), ranging in size from c. 100 ×50 μm to 700 ×200 μm. A
total of 30 spots were analysed; the largest grain was targeted with
seven analyses. No age differences are observed between the differ-
ent grains. All analysed domains are common-Pb (PbC)-rich, with
206Pb/204Pb ratios ranging from 500 to 18 000. On a Wetherill
Concordia diagram, uncorrected data fan out from being 90 %
to 102 % concordant, with a larger spread along the Concordia.
Pb corrected data are from 95 % to 103 % concordant, and plot
on a relatively well-defined Discordia line (Fig. 6) with a lower
intercept at 330 ±157 Ma and an upper intercept at 1593 ±7
Ma (n=30; MSWD =5.4). This indicates that the applied PbC
works well, and age data are therefore obtained from the PbC data.
Deselecting all reverse discordant analyses yields an upper inter-
cept age of 1593 ±11 Ma (n=17; MSWD =3.0) (Fig. 6), and
selecting only 100 % concordant analyses yields a Concordia age
of 1586 ±7(n=4; MSWD =2.2). The best age estimate for the
titanite crystallization is 1593 ±11 Ma (MSWD =3.0).
5. Discussion
5.1 Sulphates and sulphides
The anhydritein Tiree is the first recorded in the Lewisian Complex.
In Palaeoproterozoic supracrustal successions in the North Atlantic
region, anhydrite is preserved in Greenland (Horn et al.2019),
Sweden (Martinsson et al.2016) and Russia (Serdyuchenko,
1975). Together with well-preserved pseudomorphs after gypsum
in many regions including Sweden (Lager, 2001) and Canada
(Bell & Jackson, 1974; Zentmyer et al.2011;Hodgskisset al.
2019), there is extensive evidence for sulphate-bearing seawater,
at ~2.0–1.9 Ga. In several cases, sulphur isotope data are available
and are strongly positive, in accord with an evaporative origin for
the anhydrite. The occurrence of anhydrite in the Lewisian
Complex is therefore not anomalous, and rather is consistent with
the global picture of widespread evaporites in the Palaeoproterozoic.
Evidence for pseudomorphs in Tiree may be obscured by shearing
focused on the metasediments.
The temporal relationship between anhydrite and barite cannot
be proven, but the overprinting pattern of the barite suggests that it
is most likely to be later, in which case the barite sulphur could have
been remobilized from the anhydrite and precursor gypsum. In the
Hudson Bay region, pseudomorphs of ~2.0 Ga gypsum are simi-
larly overprinted by barite (Hodgskiss et al.2019).
The sulphur-bearing scapolite from Tiree suggests derivation from
a sulphate-rich sedimentary environment (Morrissey & Tomkins
2020;Zenget al.2020). Several other Palaeoproterozoic successions
contain scapolite attributed to metamorphism of evaporites, but only
one of five data sets has a sulphur content as high as the range for the
Tiree scapolite (Table S2, in the Supplementary Material available
online at https://doi.org/10.1017/S0016756822000474).
Globally, sulphide deposits of the mid-Palaeoproterozoic that
have been characterized by sulphur isotopic composition fall into
two main groups. Volcanic massive sulphides derived from mag-
matic-hydrothermal fluids have a composition of ~0 ‰, while sul-
phides attributed to derivation from seawater sulphate have a
heavy (positive δ34S) composition. This could include volcanic
massive sulphides in which the hydrothermal fluids were recycled
from seawater rather than purely magmatic. Mid-
Palaeoproterozoic (1.9–1.8 Ga) diagenetic pyrite is characterized
by positive δ34S values, reflecting derivation from seawater with
a relatively limited sulphate content, notwithstanding the occur-
rence of sulphate evaporites (Scott et al.2014). In the North
Atlantic region, Palaeoproterozoic volcanic massive sulphides
and diagenetic sulphidic shales have distinct compositions
(Fig. 5). Sulphur isotope data from sulphides in the Lewisian supra-
crustal inliers have hitherto been limited to the volcanic massive
sulphide deposit at Kerry Road, Gairloch, are tightly grouped
Fig. 5. (Colour online) Sulphur isotope compositions
of sulphide deposits of mid-Palaeoproterozoic (1.9 to
1.8 Ga) age, comparing data from NW Scotland with
data from the wider North Atlantic region (Table S4,
in the Supplementary Material available online at
https://doi.org/10.1017/S0016756822000474). Data
show dominance of heavy (positive) values. Kerry
Road deposit plots near-zero like other VMS deposits.
Tiree data plot with other diagenetic pyrite representing
seawater sources.
1642 J Parnell et al.

around 0 ‰and probably represent fluids of magmatic-hydrother-
mal origin (Drummond et al.2020). In contrast, the pyrite mea-
sured here from the Tiree marble is markedly positive (mean
11.9 ‰), comparable to those of mid-Palaeoproterozoic sulphides
attributed to an origin in seawater (Fig. 5).
5.2 Phenocryst assemblage
The phenocryst assemblage in the Tiree marble is found in many
other Lewisian Complex marbles of northern Scotland, from South
Harris to Scardroy (Fig. 2). A core assemblage of pyroxene,
amphibole, olivine, mica, titanite, epidote and quartz phenocrysts
is consistent across the region (Rock, 1987). The uniformity
implies that the marble-hosted phenocrysts represent the meta-
morphic overprint on the original mineralogy, rather than local
effects.
The ~1600 Ma dates for the Tiree titanite record mineral
growth in the Palaeoproterozoic–Mesoproterozoic, and show no
contribution from the much younger episodes of sodium metaso-
matism found elsewhere in northern Scotland. The sodium, chlo-
rine and sulphur recorded in the mineral assemblage can therefore
be confidently attributed to the chemistry of the depositional envi-
ronment of the marble in the Palaeoproterozoic.
The titanite dates are comparable with the younger ages deter-
mined for reworking of the Lewisian Complex. They show no evi-
dence of the 2500–2000 Ma reworking ages (Crowley et al.2015)
determined for Archaean gneisses in the bulk of the Lewisian
Complex. The age is also younger than the ~1.8–1.7 Ga date
ascribed to the main phase of Laxfordian deformation and meta-
morphism that widely affects the Lewisian Complex in NW
Scotland (Goodenough et al.2013). However, there is increasing
evidence for an event in Scotland in the range 1.6–1.55 Ga,
described by some workers as ‘Late Laxfordian’. This includes a
~1.6 Ga ‘cooling’date for hornblende in a shear zone (Sherlock
et al.2008), a 1.55 Ga Re–Os date for copper mineralization
(Holdsworth et al.2020), a major magnetizing event 1.7 to 1.5
Ga (Piper, 1992) and a 1.6–1.4 Ga age for cooling of granite on
the Stanton Banks west of Tiree (Scanlon & Daly, 2001).
Holdsworth et al.(2020) point out that an event of this age in
Scotland links activity in Canada and Scandinavia at the time,
which saw the later stages of the Labradorian and Gothian orog-
enies respectively.
5.3 Chlorine-bearing phases
Apatite in the Tiree marble commonly contains above 1 wt % chlo-
rine, and up to 2.8 wt % (Table S3, in the Supplementary Material
available online at https://doi.org/10.1017/S0016756822000474).
These contents are higher than those in many other marbles
(Table S3), or apatite in granites and iron deposits which mostly
contain <0.5 % (Ishihara & Moriyama, 2015). The Tiree apatite
is thus considered to be chlorine-rich. This would be consistent
with a seawater origin for the apatite, and there are no associated
magmatic rocks which might indicate an alternative origin.
However, phosphatic rocks were widespread globally at c. 1.9
Ga (Papineau, 2010), and we regard the chlorine-rich apatite as
supporting rather than critical evidence.
The chlorine content of the phlogopite in the marble is not
exceptional, but is comparable with the content in micas from
other suspected metamorphosed evaporites (Moine et al.1981;
Fig. 6. (Colour online) Wetherill Concordia diagram
with Pb corrected data. Data points plot along a rela-
tively well-defined Discordia line. Data points shown as
green ellipses outline the selection of data used for
the final age calculation. Error ellipses are shown with
a 95 % confidence level.
Seawater signatures in the supracrustal Lewisian Complex 1643

Mora & Valley 1989; Opletal et al.2007). The role of micas as a
residence for seawater-derived chlorine is in their relative abun-
dance rather than the content per mineral.
The assemblage of chlorine-bearing mineral phases, in particu-
lar scapolite and phlogopite, is typical of metamorphosed evapor-
ites (Moine et al.1981; Mora & Valley, 1989; Warren, 2016). The
combination of scapolite and phlogopite is encountered in several
Palaeoproterozoic supracrustal successions in the North Atlantic
region, including in Bergslagen, Sweden (Oen & Lustenhouwer,
1992), Finland (Reinikainen, 2001) and Baffin Island, Canada
(Belley et al.2017), and in each case has been interpreted as evi-
dence of former evaporites. It would therefore be consistent to
interpret the Lewisian Complex geochemistry as further evidence
of Palaeoproterozoic evaporite deposition.
Saline fluids from chlorine-bearing mineral phases are impor-
tant for transport of metals and creation of ore deposits (Yardley &
Graham, 2002; Riehl & Cabral, 2018; Morrissey & Tomkins, 2020).
Coeval supracrustal rocks in adjacent Greenland and Scandinavia
contain ore deposits in which evaporites are implicated as a source
of mineralizing fluids (Frietsch et al.1997; Horn et al.2019).
Metalliferous ores and mineral showings in the Lewisian supra-
crustals (Coats et al.1997; Drummond et al.2020; Parnell et al.
2021) suggest that there may also be potential deposits in
Scotland, to which evaporite-derived fluids could have
contributed.
5.4 Sodium metasomatism and tourmaline
The albite in Tiree marbles is further evidence of sodium metaso-
matism. The albite is part of the mineral assemblage which dates to
the latest Palaeoproterozoic –early Mesoproterozoic. This distin-
guishes it from episodes of sodium metasomatism in other parts of
northern Scotland, which affect Mesoproterozoic and
Neoproterozoic metasediments (Sutton & Watson 1951;May
et al.1993; Van de Kamp & Leake, 1997), and which must be of
younger age. Albitization occurs in Palaeoproterozoic rocks con-
tiguous to Scotland, in Greenland and Scandinavia. This albitite
in the North Atlantic region is argued to be derived from evaporites
or seawater (Kalsbeek, 1992; Frietsch et al.1997; Gleeson & Smith,
2009), and there is not a clear alternative in Scotland that would
preclude a similar origin.
The lack of recorded tourmaline in the Tiree marble may reflect
the relatively quiescent nature of Palaeoproterozoic sedimentation
there. While tourmaline is increasingly recognized as evidence for
metamorphosed evaporite sequences, in many cases the environ-
ments included exhalative brines on the sea floor (e.g. Oen &
Lustenhouwer, 1992; Jiang et al.1997). Where tourmaline is
reported most abundantly in the Scottish marbles, at Loch
Maree (Robertson et al.1949), the marbles occur in a section that
also hosts exhalative sulphide mineralization and iron formation
(Drummond et al.2020). The tourmaline in Lewisian marble thus
conforms to evidence in other successions for an association with
meta-exhalites. Tourmaline-bearing metasediments across the
North Atlantic region of c. 1.9 Ga age, from Quebec (Chown,
1987) to Hudson Bay (Ricketts, 1978), Greenland (Thomassen,
1992) and Sweden (Hellingwerf et al.1994), are all attributed to
former evaporites.
6. Conclusions
Combinations of criteria have contributed to a picture of evapor-
ites in Palaeoproterozoic supracrustal successions across the North
Atlantic region (Fig. 1; Table S1 in the Supplementary Material
available online at https://doi.org/10.1017/S0016756822000474).
The aspects that are consistent with an evaporitic facies in the
Lewisian Complex of Tiree include anhydrite, scapolite, phlogo-
pite, chlorine-rich apatite, and pyrite with a positive sulphur iso-
tope signature. The attribution is not definitive, but it represents
multifaceted evidence in support of a role for fluids derived from
seawater. The evidence of pseudomorphs is currently lacking.
Nonetheless, it seems likely that the supracrustal rocks of the
Lewisian Complex included evaporitic facies, like their counter-
parts in many other parts of the North Atlantic.
Supplementary material. To view supplementary material for this article,
please visit https://doi.org/10.1017/S0016756822000474
Acknowledgements. J. Bowie, C. Brolly, J. Armstrong and J. Johnston pro-
vided skilled technical support. Electron microscopy was performed with the
help of J. Still in the ACEMAC Facility at the University of Aberdeen.
Scapolite was analysed on a sample from the Hunterian Museum, Glasgow
(no. 134720), loaned courtesy of J. Faithfull. The work was supported in part
by UK Natural Environment Research Council grant NE/M010953/1.
Careful review by A.A. Cabral helped to improve the manuscript.
Conflict of interest. The authors declare none.
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