JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
PAGES 1479^1510
2014
doi:10.1093/petrology/egu031
Rhythmic Layering Formed by Deposition
of Plagioclase Phenocrysts from Influxes of
Porphyritic Magma in the Cuillin Centre,
Isle of Skye
MARK E. BRANDRISS*, SHARON MASON AND KELSEY WINSOR
DEPARTMENT OF GEOSCIENCES, SMITH COLLEGE, NORTHAMPTON, MA 01267, USA
RECEIVED MARCH 16, 2013; ACCEPTED MAY 20, 2014
ADVANCE ACCESS PUBLICATION JULY 2, 2014
The Outer Bytownite Gabbros of the Cuillin Centre on the Isle of
Skye include a 175 m rhythmically layered sequence consisting of
alternating layers of coarse-grained massive gabbro and finer-grained
laminated gabbro. We infer that the massive layers formed from repeated influxes of plagioclase-phyric magmas, which flowed into an
existing magma chamber and deposited their entrained phenocrysts
on the chamber floor.The phenocrysts accumulated rapidly, trapping
large amounts of pore liquid in the cumulus pile. During the intervals
between influxes, laminated gabbro layers formed by crystallization
of the resident magma; these layers accumulated more slowly, by crystal settling or in situ growth, allowing sufficient time for their pore
liquids to be expelled by compaction or adcumulus crystallization.
The resulting fractions of trapped pore liquid were 510% in laminated gabbros and 20^40% in massive gabbros. High porosity in
the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage, with trapping of pore liquids facilitated
by the confining effects of impermeable laminated cumulates above
and below. Among layers, whole-rock Sr and Nd isotope ratios
reveal variable degrees of contamination by the Archean crust beneath
Skye. In the lower part of the section, influxes of plagioclase-phyric
magma are associated with a shift to more primitive mineral compositions, lower 87Sr/86Sr, and higher 143Nd/144Nd, indicating that the
replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber. In accordance
with this interpretation, massive layers have lower 87Sr/86Sr than
laminated layers, consistent with derivation of the plagioclase phenocrysts from a less contaminated source. In the upper part of the section, the shift toward more primitive and less contaminated
compositions is reversed, suggesting that over time the replenishing
*Corresponding author. Telephone: 413-585-3585. Fax: 413-585-3786.
E-mail: [email protected]
magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion.
These observations and inferences, coupled with published evidence
for other isotopic reversals, support a model for growth of the
Cuillin Centre by frequent replenishment of a small-volume magma
chamber with pulses of magma that underwent complex histories of
fractionation and contamination during their transit through the
Archean crust; periods dominated by assimilation and fractional
crystallization within the feeder system alternated with periods of
vigorous recharge by relatively primitive and less contaminated
magmas. These cycles were superimposed on an overall trend toward
less contamination through time, consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the
Hebridean magmatic province matured.
KEY WORDS:
Skye; Cuillin; gabbro; Sr isotopes; Nd isotopes
I N T RO D U C T I O N
Igneous layering in large intrusions preserves detailed records of the physical and chemical evolution of plutonic
systems. In some cases, such as the Skaergaard Intrusion
in East Greenland, sequences of layers appear to represent
essentially continuous accumulations of crystals formed by
fractionation of magma in a nearly closed system, via
mechanisms such as gravitational settling of crystals, in
situ crystal growth, and postcumulus modification by percolating intercumulus liquids (Wager & Deer, 1939; Wager
ß The Author 2014. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
JOURNAL OF PETROLOGY
VOLUME 55
& Brown, 1968; McBirney, 1996; Irvine et al., 1998). More
commonly, however, layering develops within open magmatic systems, and in some cases layering may actually
form as a consequence of the thermal, physical, and chemical changes caused by new injections of magma into an
existing magma body. Among the many and diverse examples are particular types of layers in the Kap Edvard
Holm Complex of East Greenland (Tegner et al., 1993), the
Bushveld Complex of South Africa (Kruger, 2005), the
Muskox Intrusion of Canada (Irvine, 1980), the Fongen^
Hyllingen Intrusion of Norway (Meyer & Wilson, 1999),
the Rum Complex of the Scottish Hebrides (Emeleus
et al., 1996; Holness & Winpenny, 2009), the Jimberlana
Intrusion of Western Australia (Campbell, 1977), the
Basement Sill in the Dry Valleys of Antarctica (Be¤dard
et al., 2007; Jerram et al., 2010), and the mafic^felsic complexes of the Coastal Maine Magmatic Province (Wiebe,
1993, 1994). In such cases, understanding the mechanisms
of layer formation is crucial to interpreting the record of
open-system behavior preserved in shallow and mid-crustal plutons, and may provide insights into the evolution of
related volcanic systems.
In this study we examine igneous layering in the Outer
Bytownite Gabbros of the Cuillin Centre, a Paleocene
mafic intrusive complex on the Isle of Skye in northwestern
Scotland. Here, layer-to-layer variations in rock texture
and chemistry provide records of magma fractionation, hybridization, and crustal contamination in the magmatic
plumbing system that fed the Cuillin Centre during its
growth. We conclude that (1) formation of rhythmic layers
in this part of the complex resulted from repeated influxes
of phenocryst-laden basaltic magma that deposited their
suspended plagioclase crystals on the floor of an existing
magma chamber, (2) the influxes were of hybrid magmas,
consisting of relatively primitive, plagioclase-phyric midocean ridge basalt (MORB)-like basalt that had mixed
with more evolved and contaminated magmas en route to
the main Cuillin chamber, and (3) the influxes represented
early pulses of the relatively primitive magma that eventually dominated the pluton and related volcanism during
the late stages of mafic magmatism on Skye.
GEOLOGIC A L S ET T I NG
The geology of Skye
The Isle of Skye is dominated by plutonic and volcanic
rocks of late Paleocene to early Eocene age (61^55 Ma),
forming one of several Palaeogene magmatic centres of
the Hebridean Igneous Province in northwestern Scotland
(Bell & Harris, 1986; Emeleus & Gyopari, 1992; Emeleus
& Bell, 2005). Voluminous Hebridean magmatism was
caused by impingement of the proto-Iceland plume during
the early stages of rifting and continental breakup that
eventually led to formation of the North Atlantic ocean
basin (Kent & Fitton, 2000; Bell & Williamson, 2002;
NUMBER 8
AUGUST 2014
Emeleus & Bell, 2005). The Hebridean centres are thus
part of the North Atlantic Igneous Province, which extends
from the British Isles to northeastern Canada (Saunders
et al., 1997).
The geology of Skye has been summarized by Bell &
Harris (1986) and Emeleus & Bell (2005). Most of the
island is covered by basaltic lavas, which are intruded by
gabbros and minor ultramafic rocks of the Cuillin Centre,
by younger granites of the Red Hills, and by swarms of
mafic dikes, cone sheets, and sills and a number of small
gabbroic and ultramafic bodies. The lavas were erupted
onto sedimentary rocks of Proterozoic to Mesozoic age,
which are in turn underlain by Archean basement of the
Lewisian Gneiss Complex. The Lewisian rocks consist
mainly of tonalitic and granodioritic orthogneisses with
lesser volumes of mafic, ultramafic, anorthositic and granitic orthogneisses and metapelitic paragneisses (Weaver &
Tarney, 1980, 1981; Park & Tarney, 1987; Park et al., 2002;
Mendum et al., 2009). Seismic velocity profiles through
northern Britain suggest a transition from mainly amphibolite-grade Lewisian rocks in the upper crust to dominantly granulite-grade Lewisian rocks in the lower crust
(Hall & Simmons, 1979; Hall, 1987).
The Cuillin Centre
The Cuillin Centre is exposed over a roughly circular area
c. 10 km in diameter, and is composed mainly of gabbro
and troctolite with lesser amounts of peridotite (Fig. 1). It
is a composite intrusion, as indicated by numerous examples of intrusive relationships among lithologically distinct units (Bell & Harris, 1986; Emeleus & Bell, 2005).
These units form an arcuate map pattern centered on the
eastern side of the complex, where they are truncated by
younger felsic plutons and breccias. Mineralogical layering
is present in many units and dips consistently toward the
approximate center of the arcuate pattern (Wager &
Brown, 1968; Bell & Harris, 1986). The sequence of intrusion indicates that the Cuillin Centre generally grew
inward from its margins, with the Outer Gabbros representing the earliest phase of intrusion and the Inner
Gabbros representing the latest (Bell & Harris, 1986;
British Geological Survey, 2005; Emeleus & Bell, 2005).
All of the major plutonic units are cut by swarms of mafic
dikes and cone sheets (Mattey et al., 1977; Bell & Harris,
1986; Bell et al., 1994; Tibaldi et al., 2011; Bistacchi et al.,
2012).
Detailed descriptions of the Cuillin Centre have been
provided by Wager & Brown (1968) and Bell & Harris
(1986), who used the lithological nomenclature employed
in early studies of Skye (e.g. ‘Outer Layered Eucrites’ instead of ‘Outer Bytownite Gabbros’). A less detailed but
more recent summary has been provided by Emeleus &
Bell (2005), who used a revised and modernized nomenclature that conforms to the British Geological Survey’s
1:25 000 geological map of the Skye Central Complex
1480
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
5 km
N
SCOTLAND
basaltic
lavas
25
35
30
granite &
pyroclastic rocks
35
45
20
Lo
ch
sk
ui
Co
r
20
C3
25
C2
basaltic lavas &
sedimentary rocks
C1
basaltic
lavas
Rubha
Port Sgaile
Loch Scavaig
45
50
Inner Gabbros & Bytownite Troctolites
Outer Bytownite Troctolites
Outer Bytownite Gabbros
Outer Gabbros
Layered Peridotites
other rocks
35
dip of layering
location of section
Fig. 1. Simplified geological map of the Cuillin Centre, showing zones C1, C2 and C3 of the Outer Bytownite Gabbros [modified from British
Geological Survey (2005)]. Grid coordinates are UK National Grid.
(British Geological Survey, 2005). We have followed this
revised nomenclature.
Relationship to the mafic lavas of Skye
The Cuillin Centre intrudes a thick sequence of mildly alkaline flood basalts, known as the Skye Main Lava Series
(SMLS), which crops out over most of Skye (Anderson &
Dunham, 1966; Williamson & Bell, 1994; Bell &
Williamson, 2002; Emeleus & Bell, 2005). These alkaline
lavas are locally overlain by eroded remnants of tholeiitic
flows, referred to as ‘Preshal More-type’ lavas in reference
to the type locality (Esson et al., 1975; Thompson et al.,
1980; Williamson & Bell, 1994). Petrological and geochemical studies of the Cuillin suggest that most of its parent
magmas were cogenetic with the lavas of Preshal Moretype, and that the Cuillin itself may have been part of a
feeder system for these lavas (Thompson et al., 1972; Esson
et al., 1975; Dickin et al., 1984; Bell et al., 1994).
Crustal contamination of Skye magmas
Strontium, neodymium, and lead isotope studies have
shown that the magmas of Skye underwent variable
1481
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
amounts of crustal contamination as they ascended
through the Lewisian gneissic basement. The tholeiitic
magma series, including most of the Cuillin gabbros, the
mafic cone sheets, and the Preshal More lavas, were contaminated mainly in the amphibolitic upper crust; the
slightly alkaline lavas of the Skye Main Lava Series, and
the Outer Gabbros of the Cuillin, were contaminated
mainly in the granulite-facies lower crust (Carter et al.,
1978; Moorbath & Thompson, 1980; Dickin, 1981;
Thompson et al., 1982; Thirlwall & Jones, 1983; Dickin
et al., 1984, 1987; Morrison et al., 1985; Bell et al., 1994).
F I E L D R E L AT I O N S H I P S A N D
P E T RO G R A P H Y
Our study focuses on a small area of the Cuillin Centre, a
section through part of the layered Outer Bytownite
Gabbros exposed near the southeastern end of Loch
Coruisk (Fig. 1). The study area consists of glacially
smoothed outcrops on the southern flank of a 140 m hill
immediately north of Loch nan Leachd (the peak of the
hill is located at approximately NG 49285 19615). Gabbros
in this area have been studied and described in detail by
Carr (1952) and Weedon (1961).
The Outer Bytownite Gabbros formed during an intermediate stage of development of the Cuillin Centre. They
comprise c. 1600 m of layered gabbro and have been subdivided into three zones (C1, C2, and C3) based on textural,
mineralogical and structural criteria (Carr, 1952; Weedon,
1961; Bell & Harris, 1986). The upward transition from C1
to C2 is associated with a slight increase in the anorthite
content of plagioclase, the appearance of abundant coarse
‘phenocrysts’ of calcium-rich plagioclase, and a pronounced increase in the abundance of xenoliths. These features led Carr (1952) and Weedon (1961) to identify the
transition as marking a vigorous influx of slightly more
primitive magma. Our study focuses on c. 175 m of layered
gabbro section in the lower part of zone C2, along with a
single sample collected from underlying zone C1. The
stratigraphic positions of samples collected for this study,
along with their National Grid coordinates, are provided
in Supplementary Data: Electronic Appendix 1 (all supplementary data are available for downloading at http://
www.petrology.oxfordjournals.org).
Rhythmic layering in zone C2
Zone C2 has rhythmic layering on a scale of meters to tens
of meters, consisting of alternating layers of coarse-grained
massive gabbro and fine- to medium-grained laminated
gabbro (Figs 2 and 3). The cycle is repeated at least 10
times in the measured section (Fig. 4). In the following discussion, single layers within zone C2 are designated by letters E to BB.
Fig. 2. Massive layers overlying laminated layers in Outer Bytownite
Gabbro zone C2 (see Fig. 4 for the series of named layers). (a)
Massive gabbro layer W at upper right, overlying laminated gabbro
layer V at lower left; near [NG 4921 1951]. The contact dips roughly
308 into the hillside. (b) Massive gabbro layer F at upper left, overlying laminated gabbro layer E at lower right; the location is a couple
of hundred meters west of layer F sample SK201. The deformed laminae below the contact are millimeter-scale layers of coarse plagioclase
within the laminated gabbro. The contact dips about 308 into the outcrop, toward the left.
Laminated gabbro layers
The laminated gabbro layers are fine- to medium-grained
(0·5^1·5 mm) hypidiomorphic granular cumulates of
plagioclase, augite, and olivine, with a weak to moderate
lamination parallel to mineralogical layering. Intergranular oxides and orthopyroxene are commonly present but
extremely sparse (1%). The oxides consist mostly of
intergranular magnetite intergrown with ilmenite, or of
magnetite patchily developed at the margins of olivine
grains. The orthopyroxene is present as thin selvedges
along the margins of olivine grains or as minute intergranular crystals between augite grains. The rocks weather to
dark olive-green or greenish-brown.
Many of the laminated gabbros also contain conspicuously larger grains of calcium-rich plagioclase, typically
1482
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Fig. 3. Thin sections of typical laminated and massive gabbros. The numbers are mol % anorthite in plagioclase at the points indicated. (a)
Laminated gabbro sample SK138, in plane-polarized light; ol indicates large (2 mm) olivine grains adhering to the plagioclase phenocryst.
The large olivine grains have compositions of Fo75^76 (analyses 14 and 15 in Electronic Appendix 5), practically identical to the Fo74^75 of olivine
in the groundmass. (b) Massive gabbro sample SK206, in plane-polarized light; bi, biotite; opx, orthopyroxene. (c) Close-up view of laminated
gabbro SK138, showing the zoned margin of the large phenocryst in (a), in cross-polarized light. Typical groundmass is visible at right. (d)
Close-up view of massive gabbro sample SK206, showing zoning in coarse plagioclase crystals at upper left and lower right, in cross-polarized
light. A smaller unzoned plagioclase crystal (An69) is ophitically enclosed by augite at lower left.
2^5 mm in size, that are scattered in the finer-grained
matrix and give the rock a porphyritic appearance (e.g.
Fig. 3a). These are the ‘calcic-phase phenocrysts’ described
by Carr (1952), Weedon (1961), Wager & Brown (1968), and
Bell & Harris (1986). Their distribution is variable (Fig. 5).
In some layers they are very sparse or absent, whereas in
others they constitute several per cent of the rock
(Fig. 5a^c). They are exceptionally abundant in laminated
layer U (Fig. 4), in which they locally make up roughly
half the rock (Fig. 5e). In some places they are concentrated in distinct layers ranging from millimeters to tens
of centimeters thick and extending for meters or tens of
meters along strike (Figs 2b and 5d). Some of these
layers are sharply defined, whereas others appear as diffuse
decimeter-scale bands in which the phenocrysts gradually
become sparser upward (Fig. 6). The plagioclase phenocrysts commonly clump together to form glomerocrysts
(e.g. Fig. 5d), and in one sample (SK138) we found a glomerocryst of coarse plagioclase and olivine (Fig. 3a). Such
multiphase glomerocrysts appear to be uncommon, however, as we did not find them in other laminated gabbro
samples.
The laminated gabbros contain a great variety of xenoliths (Fig. 7), including fragments of gabbro, peridotite,
troctolite, and diabase, as well as country rock xenoliths
of basaltic lava (Carr, 1952; Weedon, 1961; Bell & Harris,
1986). Some xenoliths appear to be autolithic fragments of
massive gabbro layers, and others are clearly pieces of the
peridotite exposed a kilometer to the west. For most, however, the exact protoliths are difficult to determine.
Xenoliths are common in most of the laminated gabbros
and are particularly abundant in layers T and V (Fig. 4).
Most xenoliths are flattened and elongated parallel to the
lamination and layering, suggesting that they underwent
1483
JOURNAL OF PETROLOGY
25
VOLUME 55
BB
SK216
20
V
L
SK172
155
massive gabbro
laminated gabbro
finely layered massive
& laminated gabbro
laminated gabbro,
strongly porphyritic
AA
71
15
SK174
SK173
75
SK211
AUGUST 2014
160
78
O2
O1
NUMBER 8
unusually abundant
xenoliths
sharp contact
cone
sheet
150
55
SK145
SK201 sample
(Stratigraphic scale in meters)
L
SK212
SK218
SK146
V
Z
SK209
K
10
J
I
SK210
SK208
145
50
SK207
U
H
SK213
100
SK206
5
G3
SK205
45
Z
SK178
SK177
G2
G1
F
0
95
SK204
Y
SK201
SK223E,F
40
SK176
SK175
T
X
SK221 &
SK224
90
W
SK140
SK138
-5
SK202
35
SK214
E
S
SK217
85
R
-10
30
xenolith
P
SK203
-15
to SK220
(-93 m)
V
SK215
80
25
SK219
78
Fig. 4. Stratigraphic section of the layered gabbros of zone C2 in the study area. Single layers within zone C2 are designated by letters (gaps
and subdivisions in the lettering sequence are due to revisions during mapping). A height of 0 m is arbitrarily assigned to the boundary between
layers E and F. Sample SK220 is from zone C1, at 93 m.
1484
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Fig. 5. Variations in the appearance and distribution of plagioclase phenocrysts, as seen in thin sections (27 mm 46 mm). (a) Laminated
gabbro SK218, which lacks phenocrysts and has a nearly uniform grain size of about 1mm. (b) Laminated gabbro SK205, which displays a
slightly greater range of grain sizes but is not distinctly porphyritic in appearance. Several of the larger grains (arrows) are compositionally
zoned, with cores of 78, 86, and 87% An and rims of 71^72% An; groundmass grains have compositions of 74^75% An, except for one anomalous value of 60% An. (c) Laminated gabbro SK138, displaying a distinctly porphyritic texture of the type common in laminated gabbros in
the study area. (d) Laminated gabbro SK216, consisting of porphyritic fine-grained gabbro (groundmass 78^79% An) with a centimeter-scale
layer of coarse plagioclase phenocrysts (cores 81^91% An, mantles 75^82% An, rims 69^71% An). The coarse layer contains abundant ophitic
orthopyroxene, magnetite, and ilmenite, all of which are absent from the fine-grained zone above it. (Note the plagioclase glomerocryst at
upper left.) (e) Laminated gabbro SK213, from layer U, displaying an exceptionally high abundance of coarse plagioclase grains. The texture resembles that of the massive gabbros, but the outcrop pattern is very different, and consists of a diffuse zone of coarse plagioclase within a laminated gabbro sequence; it also lacks the abundant orthopyroxene and oxides characteristic of the massive gabbros. (f) Massive gabbro SK208,
displaying the coarse plagioclase, abundant oxides, and ophitic pyroxenes typical of the massive gabbro layers.
partial melting and plastic deformation during compaction
of the overlying cumulates (Fig. 7a). Conspicuous exceptions include certain xenoliths of gabbro and blocks of
orange-weathering peridotite, which retain equant shapes
that are presumably indicative of rigid behavior owing to
more refractory compositions (Fig. 7b^d). Such blocks
commonly indent the layers beneath them and have layers
draped over their upper surfaces (Fig. 7b), and where they
are in contact with the flattened and deformed xenoliths,
the layers are conspicuously pinched and thinned both
above and below the blocks (Fig. 7c and d).
Massive gabbro layers
The massive gabbro layers are coarse-grained granular cumulates that lack visible lamination. They consist mainly
of blocky euhedral plagioclase crystals (2^8 mm) and
1485
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
solidified. In some places the segregations are developed
patchily along the bases of massive layers and extend into
the underlying laminated gabbros as small dike-like
bodies (Fig. 8b) or dispersed clots. One dike-like body, several meters in length, penetrates into both underlying and
overlying laminated gabbros (Fig. 8a). The coarse-grained
segregations are typically very rich in plagioclase and contain ophitic augite and abundant magnetite. One small
sample was collected and found to contain trace amounts
of biotite and alkali feldspar in thin section.
Gabbro of zone C1
Fig. 6. Sublayering within laminated layer G1, produced by concentrations of coarse plagioclase crystals. Glacial striations run subvertically (upper right to lower left) through the field of view. Location is
[NG 49090 19522].
coarse subophitic grains of augite (Figs 3b and 5f). The
plagioclase is chemically zoned (Fig. 3b and d) and very
abundant, typically constituting 60^75% of the rock.
Granular olivine crystals up to a few millimeters in size
are usually present but are generally sparse (51%); an
exception is a sample taken from the lowermost 2 cm of
massive layer F, which contains about 15% olivine.
Intergranular orthopyroxene and oxides are much more
abundant than in the laminated gabbros. Orthopyroxene
(commonly ophitic, in some places replacing olivine) typically constitutes 1^2% of the rock, whereas oxides (magnetite or magnetite^ilmenite intergrowths) typically
constitute a few per cent. Trace amounts of intergranular
biotite are also present in some samples (Fig. 3b). The
rocks weather to gray, tan, and reddish-brown.
The lower boundaries of massive layers are typically
sharp, whereas the upper boundaries are in most cases
gradational into laminated gabbro. The bases of massive
layers are mostly conformable with fine-scale layering in
the underlying laminated gabbros (e.g. Fig. 2b) but in a
few places are subtly transgressive. In one instance, the
base of a massive layer appears to sink into and deform
fine-scale layering in the laminated gabbro beneath it, producing bulbous structures resembling load casts (Fig. 8a).
Single massive layers are laterally extensive, and some
(e.g. layer G2) can be followed for 200 m or so along
strike, with little variation in thickness, before disappearing beneath soil and vegetation. In exposures further to
the NE, in Coire Dubh, Carr (1952) reported that one
such layer could be followed for nearly half a mile.
Some massive layers contain patchy segregations, typically centimeters to decimeters in size, of very coarse to pegmatitic leucogabbro (Fig. 8a). These have irregular and
slightly diffuse boundaries, suggesting that they segregated
from the massive gabbros while the latter were only partly
A single sample of gabbro from zone C1, below the rhythmically layered sequence, was collected near the middle
of Rubha Port Sgaile (Fig. 1). This sample is petrographically distinct from both gabbro types of zone C2. It is
medium-grained (1^3 mm), laminated, and consists
mainly of tabular plagioclase, subophitic augite, and
granular olivine, with traces of intergranular Fe^Ti oxide
and with orthopyroxene locally replacing olivine at grain
margins. Zoning is present in some plagioclase. This
gabbro is noticeably coarser than the laminated gabbros
of zone C2, consistent with the descriptions of Weedon
(1961).
G E O C H E M I S T RY
Representative samples of gabbro were selected for wholerock major and trace element analysis by X-ray fluorescence spectrometry (XRF). A subset was analyzed for
rare earth elements (REE) by inductively coupled plasma
mass spectrometry (ICP-MS), and for Sr and Nd isotopic
ratios by thermal ionization mass spectrometry (TIMS).
Mineral compositions were determined by energy-dispersive spectrometry using a scanning electron microscope
(SEM-EDS). Details of methods are provided below.
Analytical methods
Prior to crushing and powdering, samples were hammered
and sawn to remove all weathered material, and sawn surfaces were abraded with tungsten carbide paper to prevent
contamination from the saw blade.
Whole-rock concentrations of major and trace elements
were determined by XRF at the University of Massachusetts, Amherst, following methods described by Rhodes
(1996) and Rhodes & Vollinger (2004). Major elements
were measured on samples fused with a La-bearing lithium borate flux and trace elements (other than REE)
were measured on pressed powder pellets. Major element
analyses were performed in duplicate for most samples,
with the averages of the duplicates reported in Table 1.
Samples for which only one analysis was obtained are indicated in the table. Trace element analyses were performed
once per sample.
1486
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Fig. 7. Xenoliths in laminated gabbros; the sizes shown are typical. (a) Xenoliths (pale gray) flattened in the plane of layering in laminated
layer E, near the southeastern tip of Loch Coruisk (the trail to the loch runs through the gully below the outcrop; view is to the west). The xenoliths are mostly fine-grained and plagioclase-rich; possible protoliths include basalt, diabase, gabbro, and troctolite. (b) Peridotite xenolith
(outlined) in laminated layer E, with laminations wrapped around top and bottom; near [NG 4914 1946]. The prominent left-to-right lineations
are glacial striations. (c) Coarse-grained gabbro xenolith (outlined; possibly a massive gabbro autolith) near the base of laminated layer G1,
with a flattened, fine-grained, pale gray xenolith draped over it and another visible at the upper right; near [NG 4910 1950]. (d) Peridotite xenolith (outlined) in layer E, overlying and deforming a pale gray fine-grained xenolith. The layering, barely discernible, is subhorizontal; faint glacial striations trend from upper left to lower right.
Concentrations of REE were determined by ICP-MS at
Activation Laboratories Limited of Ontario, Canada,
using samples fused with a lithium metaborate^tetraborate
flux.
Mineral compositions were measured by SEM-EDS on a
JEOL JSM 6400 scanning electron microscope at Smith
College, using a beam diameter of c. 2 mm, a beam curent
of 2·7 nA, and an accelerating voltage of 20 keV. X-rays
were collected with a solid-state detector and analyzed by
a ZAF quantitative method calibrated on mineral standards. Analysis of samples of known composition (mineral
standards, and samples analyzed elsewhere by wavelength-dispersive electron microprobe analysis) indicated
that analyses were routinely accurate to within 1mol %
for major end-member components in plagioclase and olivine, and 2 mol % for major end-member components in
1487
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
period of the study yielded mean 87Sr/86Sr ¼ 0·710262 0·000012 (2s) for NBS 987 and 143Nd/144Nd ¼ 0·512108 0·000008 (2s) for JNdi-1. Initial ratios were calculated
for an age of 58·9 Ma [the age of a pegmatitic segregation
in the Outer Bytownite Gabbros, as determined by
Hamilton et al. (1998)] using Sr and Rb concentrations
determined by XRF and Nd and Sm concentrations determined by ICP-MS.
Whole-rock compositions
Major elements and normative minerals
Fig. 8. Outcrop features of massive gabbro layers. (a) Bulbous structures at the base of massive layer S, deforming fine-scale layering
within underlying laminated layer R, at [NG 49154 19509]. A patchy
pegmatitic segregation (black and white) is developed in the massive
gabbro at lower right; a thinner dyke-like pegmatitic body, prominent
just below the person’s feet, extends (barely visibly, at arrow) into the
underlying laminated layer and also into overlying laminated layer T
(out of view to the upper right). The contact dips roughly 308 into
the hillside, toward the right. (b) Massive layer L overlying laminated
layer K; a diffuse zone of coarser oxide-rich gabbro is present in the
massive layer (at upper right) and extends into fractures in the underlying laminated gabbro.
augite (Supplementary Data: Electronic Appendices 2 and
3). A small number of additional plagioclase and augite
analyses were carried out by similar methods on an FEI
Quanta 450 SEM, also at Smith College. Analyses of
single grains using both instruments yielded plagioclase
compositions identical to within 1mol % anorthite, and
augite compositions identical to within 2 Mg# [molar
100 Mg/(Mg þ Fe)].
Strontium and neodymium isotope analyses were determined by TIMS at the University of North Carolina,
using methods detailed by Gray et al. (2008). Strontium isotope measurements were normalized to 86Sr/88Sr ¼ 0·1194
and neodymium isotope measurements to 146Nd/144Nd ¼
0·7219. Replicate analyses of standards over the time
Whole-rock concentrations of major and trace elements are
reported inTables 1 and 2. The laminated and massive gabbros have different ranges of major element concentrations
(Fig. 9), owing mainly to the large differences in mineral
modes. Compared with the laminated gabbros, the massive
gabbros tend to be richer in Al, Na, Fe, and Ti, poorer in
Mg and Ca, and have high Fe/Mg ratios, reflecting their
high proportions of plagioclase and iron oxides relative to
olivine and augite. A prominent exception to this general
pattern is laminated gabbro sample SK213, which is from
strongly plagioclase-phyric layer U and has correspondingly high Al and low Mg and Fe. The C1 gabbro is compositionally distinct from both types of C2 gabbro and
does not plot consistently with either type on bivariate
diagrams.
In Table 3, major element analyses have been converted
to normative mineralogies, calculated as volume percentages to best approximate mineral modes. For the massive
gabbros, which contain appreciable amounts of magnetite
and ilmenite, normative minerals were calculated assuming Fe2O3 ¼ 0·15 FeO by weight (Brooks, 1976). For the
laminated gabbros, which contain essentially no visible
oxides, normative oxides were suppressed by calculating
all Fe as FeO and excluding the small amounts (50·4%)
of measured TiO2.
Normative calculations provide good estimates of mineral proportions in the laminated gabbros. This was verified by point-counting a thin section of sample SK204:
800 points yielded modal proportions of 59·5% plagioclase, 33·3% augite, and 7·3% olivine, compared with
normative proportions of 55·6%, 36·0%, and 8·4%,
respectively (calculated assuming normative augite ¼
normative diopside þ orthopyroxene). For the massive gabbros, the relative amounts of normative ferromagnesian
silicates and oxides are strongly dependent on the assumed
proportions of Fe2O3 and FeO, but the amount of normative plagioclase is practically unaffected and is therefore
presumed to be a good approximation of its actual modal
fraction.
The plagioclase-rich nature of the massive layers, as
observed in outcrop, is confirmed and quantified by normative mineralogies. Normative plagioclase is in the
range of 62^75% for eight of the nine massive samples
1488
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Table 1: Whole-rock major and trace element analyses1
Laminated gabbros (C2)
Sample:
SK138
SK146
SK173
SK174
SK177
SK178
SK202
SK203
SK204
Height (m):2
88·75
148
157·1
158·9
95·2
96
–5
–13
2
Duplicate?:3
N
N
Y
N
Y
Y
Y
Y
Y
SiO2
49·50
48·72
49·96
49·80
50·36
49·58
48·77
49·89
TiO2
0·30
0·25
0·26
0·29
0·36
0·27
0·28
0·26
49·86
0·28
Al2O3
12·79
17·28
15·84
16·39
10·11
12·29
17·51
16·99
16·31
Fe2O3*
6·67
6·06
6·15
6·10
7·19
6·41
5·59
5·58
5·81
MnO
0·14
0·13
0·12
0·13
0·15
0·13
0·11
0·11
0·12
MgO
12·04
9·21
10·21
9·20
12·78
12·15
8·65
9·09
9·47
CaO
17·18
16·36
16·46
16·42
18·23
17·69
16·98
17·02
17·17
Na2O
0·90
1·31
1·60
1·39
1·12
1·10
1·37
1·49
1·42
K2O
0·03
0·04
0·06
0·05
0·04
0·03
0·05
0·05
0·05
P2O5
0·01
0·01
0·01
0·01
0·01
0·00
0·01
0·00
0·01
Total
99·56
99·37
100·67
99·78
100·35
99·65
99·32
100·48
100·50
0·2
0·1
0·2
0·3
6
7
8
Nb
Zr
10
Y
8·1
Sr
98
Rb
6·3
136
0·7
Ga
10
0·4
7
119
1·0
0·1
10
7·4
9·4
126
71
1·0
13
12
13
0·6
9
0·1
0·2
0
0
5
6
4
5
7·3
87
0·4
10
6·8
124
6·7
7·2
141
0·6
13
125
0·7
0·8
13
13
Zn
30
26
27
28
31
26
24
24
25
Ni
131
92
98
90
120
120
73
74
83
Cr
1200
375
346
311
1163
1084
445
397
424
V
139
99
127
129
180
149
118
114
125
Ba
11
15
14
19
9
10
17
14
14
Laminated gabbros (C2)
Sample:
SK205
SK207
SK209
SK210
SK213
SK218
SK219
SK224
Height (m):2
4·5
7·6
11·95
11·05
46·6
148·4
80·3
38
Duplicate?:3
Y
Y
Y
N
Y
N
Y
Y
SiO2
49·58
49·00
50·20
49·87
47·87
49·63
49·67
TiO2
0·31
0·31
0·36
0·34
0·18
0·28
0·26
49·33
0·25
Al2O3
14·93
14·65
12·83
14·17
25·27
17·02
14·04
16·41
Fe2O3*
6·44
6·65
6·25
6·10
4·04
5·76
6·00
5·48
MnO
0·13
0·13
0·13
0·13
0·07
0·12
0·13
0·11
MgO
10·69
10·99
10·79
10·49
5·11
9·20
11·49
9·89
CaO
17·04
16·52
18·32
17·79
16·26
16·63
17·44
17·63
Na2O
1·31
1·18
1·30
1·13
1·68
1·54
1·12
1·13
K2O
0·05
0·04
0·04
0·05
0·06
0·06
0·03
0·03
P2O5
0·01
0·01
0·01
0·01
0·01
0·01
0·00
0·01
Total
100·49
99·48
100·23
100·08
100·55
100·25
100·18
100·27
(continued)
1489
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
Table 1: Continued
Laminated gabbros (C2)
Sample:
SK205
SK207
SK209
SK210
SK213
SK218
SK219
SK224
Height (m):2
4·5
7·6
11·95
11·05
46·6
148·4
80·3
38
Duplicate?:3
Y
Y
Y
N
Y
N
Y
Y
Nb
0·2
0·1
Zr
7
7
Y
Sr
7·3
7·5
109
Rb
0·2
10
9·2
103
0·7
0·8
0·1
0·2
0
0·1
9
4
7
4
4
8·1
91
4
99
0·6
12
0·1
0·9
11
6·9
170
0·8
11
6·8
123
0·9
16
6·3
94
118
0·8
13
0·3
Ga
12
11
12
Zn
29
32
27
26
19
26
26
23
Ni
99
110
82
94
50
87
115
89
Cr
459
444
754
612
351
384
1076
1591
V
138
138
176
154
60
125
129
111
Ba
14
9
13
19
18
20
8
13
Massive gabbros (C2)
C1 gabbro
Sample:
SK140
SK145
SK172
SK176
SK201
SK206
SK208
SK214
SK215
SK220
Height (m):2
89·35
149·9
155·2
94
0·3
5·55
9·65
34·2
27·8
–93
duplicate?:3
N
N
N
N
Y
N
Y
Y
Y
Y
SiO2
47·92
47·36
48·74
48·55
45·98
48·19
48·16
47·37
47·87
TiO2
0·48
0·37
0·55
0·44
0·45
0·55
0·60
0·36
0·78
50·26
0·50
Al2O3
22·96
18·44
16·30
17·34
22·31
20·58
22·14
19·08
20·01
15·54
Fe2O3*
6·02
9·21
8·32
7·81
8·93
7·37
6·75
8·12
8·29
7·33
MnO
0·11
0·16
0·16
0·14
0·14
0·13
0·11
0·14
0·13
0·14
MgO
5·45
9·82
8·92
9·91
7·51
6·88
5·56
8·93
6·68
9·16
CaO
14·85
13·68
14·86
15·05
12·56
14·80
14·99
14·44
14·54
15·72
Na2O
1·64
1·40
1·63
1·53
1·84
1·80
1·79
1·61
1·84
1·81
K2O
0·12
0·08
0·12
0·09
0·12
0·14
0·15
0·09
0·16
0·13
P2O5
0·03
0·02
0·04
0·02
0·02
0·03
0·04
0·02
0·04
0·03
Total
99·58
100·54
99·64
100·88
99·86
100·47
100·29
100·16
100·34
100·62
Nb
Zr
Y
Sr
Rb
Ga
0·7
27
11·1
169
3·0
15
0·5
19
10·3
136
2·4
13
1·0
33
14·5
124
4·2
13
0·4
19
9·6
132
2·7
13
0·7
20
7·0
171
2·4
15
0·8
27
11·5
157
3·5
15
0·8
29
12
161
3·5
15
0·5
16
8·6
141
2·0
14
1·0
32
13·4
149
5·0
15
0·6
23
11
126
2·4
14
Zn
32
50
46
39
47
36
34
42
40
36
Ni
77
122
101
118
89
61
48
106
62
68
Cr
184
184
510
609
33
265
246
404
316
175
V
128
94
148
132
107
148
149
104
190
180
Ba
43
34
41
34
40
41
45
26
42
57
1
Major oxides
2
Stratigraphic
3
in wt %, trace elements in ppm.
height as indicated in Fig. 4.
Major oxide analyses carried out in duplicate are indicated by Y; analyses carried out only once are indicated by N.
1490
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Table 2: Rare earth element analyses (concentrations in ppm)
Laminated
Massive
C1 gabbro
Sample:
SK205
SK213
SK218
SK219
SK224
SK140
SK145
SK201
SK206
SK214
SK220
La
0·59
0·58
0·69
0·35
0·41
2·12
1·45
1·46
1·88
1·26
2·17
Ce
1·77
1·54
1·90
1·14
1·23
5·49
4·00
3·80
5·12
3·50
5·62
Pr
0·28
0·22
0·30
0·21
0·22
0·73
0·56
0·50
0·69
0·51
0·76
Nd
1·81
1·26
1·85
1·46
1·45
3·86
3·17
2·56
3·74
2·85
3·98
Sm
0·72
0·49
0·70
0·65
0·60
1·27
1·12
0·83
1·28
0·95
1·28
Eu
0·395
0·352
0·385
0·335
0·340
0·628
0·563
0·484
0·597
0·526
0·605
Gd
1·09
0·57
0·98
0·98
0·90
1·63
1·50
1·00
1·65
1·30
1·57
Tb
0·23
0·12
0·22
0·21
0·20
0·34
0·31
0·21
0·35
0·27
0·34
Dy
1·54
0·82
1·47
1·43
1·35
2·30
2·10
1·42
2·35
1·79
2·24
Ho
0·32
0·17
0·30
0·30
0·28
0·47
0·42
0·28
0·48
0·36
0·46
Er
0·90
0·48
0·86
0·87
0·81
1·40
1·24
0·84
1·44
1·07
1·35
Tm
0·132
0·070
0·125
0·126
0·117
0·206
0·185
0·131
0·210
0·155
0·194
Yb
0·84
0·46
0·77
0·77
0·73
1·31
1·20
0·86
1·32
1·00
1·25
Lu
0·129
0·071
0·112
0·110
0·105
0·199
0·181
0·133
0·200
0·159
0·188
Fig. 9. Whole-rock major element compositions, with oxides plotted versus MgO.
(with one outlier at 57%) and 43^50% for 15 of the
17 laminated gabbro samples (with outliers at 36%
and 79%, the latter being a phenocryst-rich sample from
layer U).
Trace elements
Concentrations of selected trace and minor elements, normalized to the average of Cuillin cone sheet compositions
reported by Bell et al. (1994), are plotted in Fig. 10.
1491
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
Table 3: Whole-rock mineral norms (vol. %)*
plagy
ne
cpx
opx
ol
ilm
mag
ap
Mg/(Mg þ Fe)
% An in plag
Laminated gabbros (C2)
SK138
43·94
41·29
5·63
9·11
0·02
0·781
79
SK146
58·38
29·61
3·33
8·66
0·02
0·751
78
SK173
55·04
33·55
0·57
10·83
0·02
0·767
71
SK174
56·12
31·90
5·73
6·23
0·02
0·749
75
SK177
36·25
11·48
0·02
0·779
71
SK178
43·32
44·86
0·15
11·67
0·790
74
SK202
59·23
31·53
0·89
8·33
SK203
57·68
32·85
1·46
8·01
SK204
55·59
34·62
1·33
8·44
SK205
51·46
36·92
0·61
10·99
SK207
50·59
35·58
3·18
SK209
44·27
SK210
48·65
SK213
79·16
SK218
58·16
31·58
SK219
48·11
SK224
54·61
0·43
0·74
51·82
0·754
77
0·763
74
0·02
0·763
74
0·02
0·767
74
10·64
0·02
0·766
76
8·60
0·02
0·774
73
7·94
0·02
0·773
76
6·98
0·02
0·714
80
1·31
8·93
0·02
0·760
74
39·78
1·73
10·38
0·791
76
35·18
1·78
8·41
0·02
0·781
79
46·36
40·94
0·28
2·45
13·57
0·02
Massive gabbros (C2)
SK140
74·93
13·34
7·46
3·07
0·56
0·58
0·06
0·642
78
SK145
62·04
17·74
6·92
11·99
0·45
0·92
0·04
0·679
77
71
SK172
57·59
27·30
6·31
7·19
0·68
0·84
0·09
0·680
SK176
59·14
25·14
3·55
10·82
0·53
0·78
0·04
0·715
74
SK201
74·72
7·22
2·42
14·19
0·53
0·88
0·04
0·625
75
SK206
69·19
18·48
3·40
7·50
0·65
0·72
0·06
0·649
74
SK208
73·22
15·95
4·62
4·76
0·70
0·66
0·08
0·620
75
SK214
64·66
19·73
1·90
12·44
0·43
0·81
0·05
0·686
75
SK215
68·29
18·96
3·16
7·77
0·92
0·82
0·09
0·615
72
55·52
32·28
4·53
6·26
0·60
0·73
0·07
0·712
67
C1 gabbro
SK220
*Modified CIPW norms, recalculated to volume per cent using mineral specific gravities in a spreadsheet template
provided by C. K. Hollocher (http://minerva.union.edu/hollochk/c_petrology/norms.htm; accessed 24 January 2014).
Norms for oxide-bearing gabbros (massive gabbros and C1 gabbro) are calculated assuming Fe2O3/FeO ¼ 0·15 (Brooks,
1976). Norms for gabbros lacking appreciable oxides (laminated gabbros) are calculated with all Fe calculated as FeO, and
without TiO2 (to suppress normative magnetite and ilmenite, which would otherwise produce normative oxides totaling
0·6–1·2%).
yNormative orthoclase (up to 1·1%) has been included in the normative plagioclase total, based on the assumption that K
is present mainly as a solid solution constituent in plagioclase.
The laminated and massive gabbros form separate groups,
with the massive gabbros having appreciably higher concentrations of the incompatible elements. These include
the high field strength elements (HFSE; P, Zr, Ti, Y;
Fig. 10a), large ion lithophile elements (LILE; K, Rb, Ba;
Fig. 10a), and REE (Fig. 10b). On an REE plot normalized
to the average cone sheet (Fig. 10b), the massive gabbros
and the C1 gabbro have patterns that are slightly concave
upward, reflecting relative depletion in the middle REE
(MREE), whereas the laminated gabbros typically have
patterns with slightly positive slopes, reflecting relative depletion in the light REE (LREE). All of the gabbros have
positive Eu anomalies, and all are depleted in incompatible
elements relative to the cone sheets, consistent with formation of the gabbros by cumulus processes with plagioclase
as a cumulus phase.
1492
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Fig. 10. Whole-rock trace element compositions, normalized to the
average concentrations in Cuillin cone sheets reported by Bell et al.
(1994). (a) Alkalis, alkaline earths, and high field strength elements.
(b) Rare earth elements.
Mineral compositions
Compositions of plagioclase, olivine, and augite are reported in Supplementary Data: Electronic Appendices
4^6 and are summarized below.
In the laminated gabbros, groundmass plagioclase compositions exhibit distinct stratigraphic trends. Average
anorthite contents increase with stratigraphic height to approximately the middle of the section, at which point the
trend reverses and values gradually decline upward.
Compositions range from An68^75 at the bottom and top
of the section to An76^84 near the middle. Groundmass
plagioclase is typically unzoned, but in a single laminated
gabbro sample (SK223E) the fine-grained groundmass
contains reversely zoned grains (cores of An68^72 and rims
of An73^79). This sample is from the top of laminated layer
E, within a few centimeters of the contact with overlying
massive layer F.
Phenocrysts in the laminated gabbros are very calcic,
with core compositions typically An82^91. In contrast to
the groundmass grains, there is no readily discernible
trend with stratigraphic height. Most phenocrysts are normally zoned in a simple pattern, with Ca-rich cores (commonly with slight reverse zoning) mantled by distinctly
more sodic overgrowths having compositions similar to
the groundmass plagioclase (Fig. 3a and c).
In the massive gabbros, coarse plagioclase grains are
strikingly similar to the phenocrysts in the laminated gabbros. They are very calcic, with core compositions typically
in the range An82^92 (e.g. Fig. 3b), and there is no discernible trend with stratigraphic height. Nearly all are normally zoned, with distinct cores and mantles that follow
the simple compositional zoning pattern observed in the
laminated gabbro phenocrysts (Fig. 3d). Unlike the phenocrysts in laminated gabbros, however, most plagioclase
grains in the massive gabbros have additional thin sodic
rims (An57^72) that overgrow the mantle zones (compare
Fig. 3c and d).
Plagioclase
Olivine
Plagioclase compositions vary considerably within the section, as shown in Fig. 11a. Single grains have been classified
by texture, using separate criteria for the laminated and
massive gabbros.
In the laminated gabbros of unit C2, plagioclase is classified as either groundmass or phenocryst, based on grain
size. In most laminated gabbros, the grain-size distribution
is distinctly bimodal (e.g. Fig. 5c and d) and phenocrysts
are easily recognized. In some samples, grain-size variations are less extreme (e.g. Fig. 5b); in these samples, we
have classified the coarsest grains as ‘phenocrysts’ based
on their close compositional similarity to the distinct
phenocrysts in other laminated rocks and on the appreciable difference in composition between these coarser
grains and the plagioclase in the surrounding finer
matrix. In the massive gabbros of unit C2, in which the
size distribution is essentially continuous, grains are classified as coarse (43 mm), medium (1^3 mm) or fine
(51mm). In the laminated gabbro of unit C1, the plagioclase is almost uniformly medium-grained (1^3 mm).
Olivine compositions are considerably less variable, as
shown in Fig. 11b. In the laminated gabbros, compositions
range from Fo71 to Fo78, and there is no consistent trend
with stratigraphic height. In the massive gabbros, compositions range from Fo66 to Fo75, and the olivines are consistently poorer in magnesium (by several mol % Fo) than
those in laminated gabbros at comparable heights in the
section.
Augite
The augites contain, in addition to the major quadrilateral
components, measurable amounts of chromium. The
Mg# and Cr2O3 contents of augites are plotted versus
stratigraphic height in Figs 11c and 12, respectively.
In the laminated gabbros, single augite grains are compositionally homogeneous, with Mg# ranging from 75 to
82. There is a subtle compositional trend, with average
Mg# increasing upsection to a maximum at c. 40^80 m
and then decreasing thereafter. Chromium concentrations
correlate positively with Mg# and follow a similar
1493
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
Fig. 11. Compositions of plagioclase (a), olivine (b), and augite (c) plotted versus stratigraphic height. In (a) and (c), the trends for groundmass grains in the laminated gabbros are outlined. (Note the break in vertical scale between zones C1 and C2.)
pattern. If whole-rock and mineral data are considered together, it is apparent that essentially all of the Cr in the
laminated gabbros is contained in augite. This is demonstrated in Fig. 12b, which shows Cr2O3 concentrations in
augites calculated from whole-rock Cr contents and mineral modes, assuming that all of the Cr is in augite; the
good agreement with SEM-EDS analyses confirms the validity of this assumption.
In the massive gabbros, augite compositions are more
variable, both within single samples and over the entire
section. Coarse grains are commonly zoned, with Mg#
and Cr content decreasing from core to rim; variations in
Mg# of up to 13 have been measured within a single
grain. The cores of grains have Mg# ranging from 74 to
87 and Cr2O3 concentrations ranging from 0·0 to 0·9%.
There is no consistent stratigraphic trend, but unusually
magnesian and chromian augites are conspicuously present in a few samples in the lower part of the section.
The rims of zoned grains, together with small intergranular grains, are more iron-rich than the augites in laminated
gabbros at similar stratigraphic levels (Fig. 11c and 12).
Sr and Nd isotopes
Whole-rock strontium and neodymium isotope analyses
have been recalculated to initial ratios for an age of 58·9
Ma (Table 4) and plotted on an Nd^Sr isotope diagram
(Fig. 13), along with published values for other mafic igneous rocks from Skye and average values for granulitegrade and amphibolite-grade gneisses of the Lewisian
crust. The Cuillin gabbros plot as an extension of a negatively sloping trend that includes the Preshal More basalts
and Cuillin cone sheets, and that can be projected back
toward mid-ocean ridge basalt. The gabbros extend the
trend to lower 143Nd/144Nd and higher 87Sr/86Sr, with a
slight steepening of the slope. The Cuillin trend is distinct
from that of the Skye Main Lava Series, which is displaced
1494
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
87
Sr/86Sr is consistently 0·0002 lower in the massive gabbros than in adjacent laminated gabbro layers.
Nd isotope stratigraphy
Initial Nd isotope ratios of whole-rocks also vary with
stratigraphic height (Fig. 14b). The general trend is nearly
the mirror image of that for Sr isotopes: the upward transition from zone C1 to zone C2 is marked by a substantial increase in 143Nd/144Nd, and the shift is partly reversed in
zone C2, in which 143Nd/144Nd reaches a maximum at
about 35 m and then gradually decreases upward to the
top of the section. Unlike the case with Sr, however, there
is no consistent difference between the Nd isotope compositions of laminated and massive gabbros. Instead, the two
types of gabbro form a single Nd isotope trend, with
143
Nd/144Nd increasing upward to c. 35 m and then consistently decreasing throughout the upper part of the section.
DI SCUSSIONçOR IGI N OF T H E
R H Y T H M I C L AY E R I N G
Fig. 12. Chromium contents of augites versus stratigraphic height. (a)
Concentrations of Cr2O3 in augite in the massive gabbros and C1
gabbro. (b) Concentrations of Cr2O3 in augite in the laminated gabbros and C1 gabbro; SEM-EDS measurements are compared with
concentrations calculated from whole-rock analyses and modal mineral proportions, assuming all Cr is contained in augite (see text).
to lower 87Sr/86Sr. These features are consistent with contamination of the Cuillin magmas mainly during transit
through the amphibolite-grade Lewisian upper crust, as
proposed by Dickin et al. (1984) and Bell et al. (1994), although the slight steepening of the slope suggests the possibility of additional contamination in the granulite-grade
lower crust (e.g. Font et al., 2008).
Sr isotope stratigraphy
Initial Sr isotope ratios of whole-rocks vary with stratigraphic height (Fig. 14a). The upward transition from
zone C1 to zone C2 is marked by a substantial decrease in
87
Sr/86Sr, similar to the shift measured by Moorbath &
Thompson (1980) for two samples straddling the C1^C2
boundary nearby. This shift is partly reversed in unit C2,
in which 87Sr/86Sr gradually increases upward to the top
of the section. The laminated and massive gabbros in zone
C2 follow parallel trends with stratigraphic height, but
Based on the features described above, we infer that the
massive layers in the Outer Bytownite Gabbros formed
from repeated influxes of plagioclase-phyric basaltic
magma, which flowed into an existing magma chamber
and deposited their entrained phenocrysts on the floor to
produce coarse-grained plagioclase-rich cumulates. The
accumulation of phenocrysts occurred rapidly, trapping
high proportions of intercumulus liquids within the pore
spaces of the aggrading cumulus pile. During the intervals
between influxes, laminated gabbro layers formed by
cotectic crystallization of plagioclase, augite and olivine
from the magma residing in the chamber. These laminated
layers accumulated more slowly, by crystal settling or in
situ growth, allowing sufficient time for their pore liquids
to be expelled by compaction or adcumulus crystallization.
The initial influxes of plagioclase-phyric magma were
more primitive and less contaminated than the magma already in the chamber, but as time passed, the influxes
gradually became more evolved and contaminated as the
replenishing magmas underwent greater degrees of hybridization, contamination, and fractional crystallization en
route to the Cuillin. This episodic replenishment of the
Cuillin magma chamber produced the distinctive patterns
of textural, chemical, and isotopic variation observed
within the layered gabbro sequence. The field, petrographic, and chemical evidence for these inferences is discussed below.
Structures in the layering
Various outcrop features suggest that the layers formed by
accumulation of crystals at the floor of a magma body.
The locally transgressive bases of some massive layers,
and the basal structures resembling load casts (Fig. 8a),
are consistent with gravitational deposition of coarse
1495
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
Table 4: Whole-rock Sr and Nd isotope analyses
Sample
87
Sr/86Sr*
Rb
Sr
(ppm)
(ppm)
87
Rb/86Sr
(87Sr/86Sr)iy
143
Nd/144Nd*
Sm
Nd
(ppm)
(ppm)
147
Sm/144Nd
(143Nd/144Nd)iy
Laminated
SK205
0·704766 10
0·7
109
0·02
0·704750
0·512659 21
0·72
1·81
0·24
0·512566
SK218
0·705002 8
0·9
123
0·02
0·704984
0·512543 11
0·70
1·85
0·23
0·512455
SK219
0·704846 10
0·8
94
0·02
0·704825
0·512662 8
0·65
1·46
0·27
0·512558
SK224
0·704788 11
0·3
118
0·01
0·704782
0·512666 10
0·60
1·45
0·25
0·512570
SK140
0·704667 11
3·0
169
0·051
0·704624
0·512574 10
1·27
3·86
0·199
0·512497
SK145
0·704832 10
2·4
136
0·051
0·704789
0·512479 24
1·12
3·17
0·214
0·512397
SK201
0·704595 10
2·4
171
0·041
0·704561
0·512624 9
0·83
2·56
0·20
0·512548
SK206
0·704650 11
3·5
157
0·064
0·704596
0·512657 9
1·28
3·74
0·207
0·512577
SK214
0·704575 11
2·0
141
0·041
0·704541
0·512704 8
0·95
2·85
0·20
0·512626
0·705237 10
2·4
126
0·055
0·705191
0·512293 26
1·28
3·98
0·194
0·512218
Massive
C1 gabbro
SK220
*Uncertainties are 2s values in the final decimal places.
yInitial ratios are calculated for an age of 58·9 Ma (Hamilton et al., 1998).
Measurements were normalized to 86Sr/88Sr ¼ 0·1194 and 146Nd/144Nd ¼ 0·7219.
This study:
Preshal More
basalts
laminated gabbro (zone C2)
massive gabbro (zone C2)
gabbro (zone C1)
3
Other data:
North
Atlantic
MORB
other Cuillin Centre gabbros
and troctolites
Cuillin cone sheets
M
ye
Sk
ain
2
L
av
aS
er
ie
s
to avg. Lewisian
granulite-grade
gneiss
to avg. Lewisian
amphibolite-grade
gneiss
1
Fig. 13. Initial Nd and Sr isotopic compositions in the Outer Bytownite Gabbros, with other mafic igneous rocks of Skye for comparison. Ratios
are age-corrected to 58·9 Ma, the U^Pb age of zircon from a pegmatite in the Outer Bytownite Gabbros (Hamilton et al., 1998). Data from
other Cuillin Centre rocks analyzed by Dickin et al. (1984) are numbered as follows: 1, Outer Gabbro; 2, Outer Bytownite Troctolite; 3, Inner
Bytownite Troctolite. Other sources of data: Skye Main Lava Series and Preshal More basalts, Moorbath & Thompson (1980), Dickin et al.
(1987); Cuillin cone sheets, Bell et al. (1994); average Lewisian gneisses, Dickin (1981); North Atlantic MORB is Reykjanes MORB and
Kolbeinsey data of Mertz & Haase (1997).
1496
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Coarse Ca-rich plagioclase
Fig. 14. Initial isotopic ratios of Sr (a) and Nd (b) plotted versus
stratigraphic height.
plagioclase crystals atop a deformable cumulus mush. In
the laminated layers, some amount of gravitational accumulation is implied by the abundance and diversity of
xenoliths, which were clearly derived from different sources
and transported to their present locations (Carr, 1952;
Weedon, 1961). Similarly, the anomalously calcic phenocrysts dispersed through the laminated gabbros imply
physical transport and deposition rather than exclusively
in situ growth. Accumulation of xenoliths, autoliths, and
phenocrysts in the laminated layers may have occurred
simultaneously with some degree of in situ growth of cumulus crystals (e.g. Campbell, 1978), but the presence of these
diverse foreign bodies nevertheless indicates that gravitational settling delivered at least some material to the floor
of the magma chamber during accumulation of laminated
layers throughout the section.
The coarse plagioclase grains in massive layers are essentially identical in composition to the plagioclase phenocrysts in laminated layers, suggesting a common origin for
the two crystal populations. Their anomalously calcic compositions indicate derivation from an external source that
was chemically distinct from the magma residing in the
Cuillin chamber prior to the onset of rhythmic layering in
zone C2. Given the typical zoning pattern (a calcium-rich
core with an abrupt transition to a more sodic mantle),
the simplest inference is that the crystals grew as phenocrysts in relatively primitive magmas that mixed with
more evolved magmas en route to, or within, the main
Cuillin magma body. This interpretation is consistent
with the work of previous investigators, who referred to
the coarse plagioclase crystals as ‘calcic-phase phenocrysts’
and inferred that they were carried into the Cuillin by
magmas that episodically replenished the intrusion (Carr,
1952; Weedon, 1961; Wager & Brown, 1968; Wadsworth,
1982; Bell & Harris, 1986). The appreciable thickness of
sodic mantles in some phenocrysts suggests that, after
mixing, the crystals underwent a significant period of
growth before being deposited on the magma chamber
floor. This in turn suggests that mixing and hybridization
occurred in subsidiary bodies (perhaps sills, dykes, or
other plutons) before the magma flowed into the main
Cuillin intrusion. In view of their derivation from outside
the pluton, such phenocrysts would technically have
become ‘antecrysts’ upon addition to the resident magma
(e.g. Davidson et al., 2007), but for simplicity we will
retain the term ‘phenocryst’ to describe the crystals both
before and after their addition to the Cuillin.
Among the minor intrusions and lava flows contemporaneous with the Cuillin are porphyritic basalts with
Ca-rich plagioclase phenocrysts essentially identical in
composition to the phenocrysts in the Outer Bytownite
Gabbros (An82^92). These include subsets of the Preshal
More lavas of the volcanic sequence (Esson et al., 1975;
Font et al., 2008), the tholeiitic cone sheets of the Cuillin
Centre (Bell et al., 1994), and the tholeiitic dykes of the
Skye Dyke Swarm (Anderson & Dunham, 1966;
Donaldson, 1977; Mattey et al., 1977). The latter include a
group of gabbroic anorthosite dykes choked with plagioclase phenocrysts that are remarkably similar to those in
the Cuillin gabbros: the dyke phenocrysts constitute up to
60^70% of the dyke rocks, are typically equant in shape,
1·5^5 mm in size, have homogeneous cores of An87^93, and
are zoned to labradoritic mantles (Anderson & Dunham,
1966; Donaldson, 1977). This zoning was interpreted by
Donaldson (1977) as evidence for mixing of compositionally distinct pulses of magma in a shallow crustal magma
system. Given the striking similarities between phenocrysts
in the gabbroic anorthosite dykes and coarse plagioclase
in the gabbros, we suggest that these dykes may represent
1497
JOURNAL OF PETROLOGY
VOLUME 55
hybrid magmas of the type that episodically replenished
the Cuillin chamber during formation of the Outer
Bytownite Gabbros. Although the phenocrysts are especially abundant in zone 2 of the Outer Bytownite
Gabbros, they are present at least sparingly in all major
layered gabbro units of the Cuillin Centre (Carr, 1952;
Wager & Brown, 1968; Wadsworth, 1982), suggesting persistent contributions from this magma source during
much of the Cuillin Centre’s growth.
The common occurrence of calcic plagioclase phenocrysts as thin subsidiary layers in laminated gabbros suggests that, in addition to the discrete pulses of magma that
produced massive layers, there was either semi-continuous
minor replenishment of the chamber or frequent reworking
of unconsolidated cumulates. Possible evidence for reworked cumulates includes the plagioclase^olivine glomerocryst in Fig. 3a. Reworking is also suggested by the
occurrence of massive gabbro blocks as cognate xenoliths,
with the equant shapes of these blocks implying derivation
from existing layers that had already solidified almost
completely.
Evidence for trapped pore liquids in
massive gabbros
Petrographic, mineralogical and chemical evidence indicates that large amounts of pore liquid were trapped
within the massive layers, whereas very little pore liquid
was trapped within the laminated layers.
Minor minerals
Orthopyroxene, magnetite, ilmenite and biotite all form
relatively late during the crystallization of tholeiitic
magmas (e.g. Wager & Brown, 1968; Dick et al., 2000; Thy,
2003), so their abundance in the massive gabbros provides
strong evidence for extensive trapping and crystallization
of evolved pore liquid in the massive gabbro cumulates
(e.g. Morse, 1979; Thy, 2003; Bernstein, 2006; Borghini &
Rampone, 2007). In the laminated gabbros, in contrast,
the extreme scarcity of orthopyroxene and oxides and the
almost complete absence of biotite imply low fractions of
trapped pore liquid, perhaps owing to exclusion during
adcumulus growth or to expulsion by compaction of the
cumulus mush. Features in the laminated gabbros that are
compatible with compaction include development of the
lamination itself (Hunter, 1996), wrapping of laminae
around both the tops and bottoms of xenoliths (Hunter,
1996; Meurer & Boudreau, 1996), and the pronounced flattening of xenoliths (Brandriss et al., 1996). Especially notable is the pinching and thinning of flattened xenoliths
above rigid blocks (Fig. 7c), implying post-depositional
compaction by the weight of overlying crystals. None of
these features are apparent in the massive gabbros.
NUMBER 8
AUGUST 2014
Mineral compositions
In the massive gabbros, the normal zoning of augite and
plagioclase is consistent with formation of overgrowths
derived from evolved pore liquids, with the thin and
distinctly sodic rims of many plagioclase grains being particularly noteworthy (e.g. Fig. 3d). In the laminated gabbros, in contrast, the lack of zoning in augite and
plagioclase (with the exception of plagioclase phenocrysts)
reflects a paucity of trapped liquids during final
crystallization.
The olivines, despite their lack of compositional zoning,
also preserve a record of pore liquid crystalllization. The
relatively low Mg content of olivines in the massive gabbros is a trapped liquid effect: the olivine in massive layers
re-equilibrated with evolved Fe-rich pore liquids, whereas
the olivine in laminated layers (which lacked abundant
pore liquids) did not. The olivines behaved differently
from the augites owing to comparatively rapid diffusion of
Mg and Fe through the olivine structure; at magmatic temperatures, the Mg^Fe interdiffusion coefficient is roughly
two orders of magnitude greater for olivine than for clinopyroxene (Dimanov & Wiedenbeck, 2006; Dohmen &
Chakraborty, 2007), and as a result, the closure temperatures for Fe^Mg interdiffusion during slow cooling are
about 3008C lower for olivine (Costa et al., 2008).
Postcumulus re-equilibration thus occurred in the olivines
but not in the augites, and only in the augites was compositional zoning preserved. The trapped liquid effect in the
olivine, combined with the antecryst origin of the plagioclase, provides an explanation for the anomalous combination of Fe-rich olivine and Ca-rich plagioclase in the
massive gabbros of the study area.
Incompatible elements
High concentrations of incompatible elements in the massive gabbros provide further evidence for crystallization of
evolved pore liquids within the massive cumulates.
Furthermore, given reasonable constraints on the compositions of pore liquids and cumulus mineral assemblages,
whole-rock trace element concentrations provide a means
of estimating the trapped liquid fraction. Let us consider,
for example, the whole-rock concentrations of Ba and Zr,
two elements that are almost perfectly incompatible in all
the major cumulus minerals. Concentrations of these elements in the gabbros are shown in Fig. 15a, together with
concentrations in the Cuillin cone sheets (Bell et al., 1994)
and the Preshal More lavas (Thompson, 1982). The cone
sheets are probably consanguineous with the Cuillin
gabbros (Bell et al., 1994) and began intruding the complex before the gabbros were fully consolidated
(Tibaldi et al., 2011). They are therefore plausible proxies
for liquids that replenished and resided within the Cuillin
chamber. Using this assumption, the line in Fig. 15b
shows calculated mixing proportions for two hypothetical
1498
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
end-members: (1) a pure plagioclase cumulus assemblage,
representing accumulated phenocrysts in the massive gabbros; (2) a pore liquid trapped in the cumulus pile, with a
hypothetical composition calculated as the average of
cone sheet compositions reported by Bell et al. (1994). Also
shown is a hypothetical solid gabbroic cumulus assemblage
consisting of 50% plagioclase, 40% augite, and 10% olivine, approximating the cumulus mineralogy of the laminated gabbros. All gabbro samples cluster along the
hypothetical mixing line between cumulates and liquid,
with calculated trapped liquid contents of 0^10% in the
laminated gabbros and 20^40% in the massive gabbros.
These are reasonable values (Shirley, 1986; Hunter, 1996;
Jerram et al., 1996), similar to the fractions of trapped
liquid calculated for low-porosity and high-porosity cumulates of the Skaergaard Intrusion (Tegner et al., 2009) and
the Main Zone of the Bushveld Complex (Lundgaard
et al., 2006).
Although the calculations in Fig. 15b are rough approximations, with precision greatly limited by uncertainties in
liquid composition, the general result is fairly robust: for
most plausible liquids (i.e. the large majority of single
cone sheet and Preshal More lava compositions), the gabbros cluster around hypothetical mixing lines, with calculated trapped liquid contents of the order of tens of per
cent for the massive gabbros and considerably less for the
laminated gabbros. Similar results are obtained when
other incompatible elements are used for the calculations.
Whole-rock Fe/Mg
Fig. 15. (a) Ba and Zr concentrations in the Outer Bytownite
Gabbros, with Cuillin cone sheets and Preshal More basaltic lavas
for comparison. Sources of data: Cuillin cone sheets, Bell et al. (1994);
Preshal More lavas, Thompson (1982). (b) Magnified view of (a),
showing the cumulate^liquid mixing model described in the text.
Values for a hypothetical pure plagioclase phenocryst cumulate (0%
liquid, indicated by the circled P) are calculated by assuming equilibrium elemental partitioning between plagioclase and a liquid of
Preshal More basalt composition. Values for a hypothetical solid gabbroic cumulate of 50% plagioclase, 40% augite and 10% olivine
(indicated by the circled G) are calculated by assuming equilibrium
partitioning between minerals and a liquid of average cone sheet
The characteristic effects of pore liquid crystallization are
also apparent in whole-rock major element compositions.
As the fraction of trapped pore liquid increases, the Fe/
Mg ratio of the bulk crystal^liquid mixture, and ultimately of the solidified rock, should also increase and the
Mg# should decrease (Barnes, 1986). This is seen in
Fig. 16, in which the Mg# of the gabbros is plotted versus
the normative abundances of ferromagnesian minerals.
The massive gabbros, as expected, have appreciably lower
Mg# than the laminated gabbros, owing to their higher
proportions of trapped liquid. Furthermore, for each type
of gabbro, the Mg# declines as the abundance of ferromagnesian minerals decreases; this is also an effect of
trapped liquid crystallization, because the smaller the
amount of ferromagnesian minerals in the original cumulate, the greater the shift in bulk Fe/Mg caused by a given
amount of pore liquid crystallization. Another interesting
Fig. 15. Continued
composition. The mixing line for G is very similar to that for P, and is
therefore omitted for clarity. Mineral^liquid partition coefficients
(for basaltic liquid) are 0·048 and 0·1 for Zr in plagioclase and
augite, respectively, and 0·23 and 0·026 for Ba in plagioclase and
augite, respectively (Rollinson, 1993).
1499
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 8
AUGUST 2014
consistent with trapping of a few tens of per cent liquid
(Barnes, 1986).
The simplifying assumptions are somewhat more valid
for the laminated gabbros, which lack oxides, lack strong
compositional zoning, and are dominated by cumulus minerals. The equilibrium requirement is still not met, however, as olivine and augite compositions are poorly
correlated in parts of the section. Nevertheless, it is worth
noting that the range of variation in Fe/Mg among the
laminated gabbros (about 0·04 for whole-rock Mg#,
excluding the exceptionally plagioclase-rich sample
SK213) is comparable with the range of shifts produced by
crystallization of 10% or less trapped pore liquid
(Barnes, 1986), and is thus in general agreement with estimates from incompatible trace element concentrations.
Factors controlling trapped liquid content
Fig. 16. Whole-rock Mg# vs normative abundance of mafic minerals
(pyroxene þ olivine þ oxides).
feature is the greater variability of Mg# in the massive
gabbros, which implies greater variations in trapped
liquid content. This is exactly as expected, given their
greater spread in incompatible element abundances
(Fig. 15).
Barnes (1986) presented a method for quantifying the
effect of trapped liquid on Fe/Mg ratios of cumulus olivines
and pyroxenes, assuming that all Fe and Mg in the
trapped liquid was incorporated into silicates during crystallization and that Fe^Mg exchange equilibria were maintained among all phases. These conditions, however,
clearly were not met in the gabbros studied here. In the
massive gabbros, the presence of abundant intergranular
magnetite and ilmenite demonstrates that much of the Fe
in the liquid was consumed by formation of oxides, and
the persistence of chemical zoning in large augite grains,
and the poor correlation of olivine and augite compositions
in most of the section (Fig. 11b and c), indicates that Fe^
Mg exchange equilibria were not achieved among and
within the silicates. Furthermore, the trapped liquid calculation requires knowledge of mineral proportions in the
original cumulus assemblages, but determining these proportions in the massive gabbros is problematic owing to
the difficulty of distinguishing between cumulus and postcumulus ferromagnesian silicate grains. Accurate calculations of trapped liquid effects are therefore not feasible for
the massive gabbros, although the shifts of up to 0·2 in
Mg# relative to the laminated gabbros are broadly
The propensity of the massive layers to trap high proportions of pore liquid can be explained by two factors that
controlled the rate and extent of compaction: (1) the relatively low density of the plagioclase-dominated cumulus
mineral assemblage, which minimized the gravitational
driving force for compaction; (2) the high rate of accumulation of the phenocrysts as they settled from suspension,
which outpaced the rate at which pore liquids could be
expelled from the rapidly aggrading cumulus pile. The
crucial effects of cumulate density and accumulation rate
have been documented by Tegner et al. (2009), who used incompatible trace element modeling to examine the proportions of trapped liquids in cumulate rocks of the
Skaergaard Intrusion. They found that the highest proportions of trapped liquid (30^52%, similar to our estimates
for massive layers in the Cuillin) occurred in troctolitic cumulates of the Lower Zone; they attributed this to the low
density of the solid cumulus assemblage, coupled with
high accumulation rates caused by rapid cooling and crystallization near the floor of the intrusion. Throughout the
Skaergaard, they found that layers with low-density
(plagioclase-rich) cumulus assemblages contained appreciably more trapped pore liquid than layers with highdensity (plagioclase-poor) cumulus assemblages. Physical
models of cumulus compaction predict that rapid accumulation will greatly diminish the efficiency of compaction
and pore liquid expulsion (e.g. Shirley, 1986), so it is not
surprising that, in the massive layers of the Cuillin, the
combination of a low-density cumulus assemblage and a
high rate of accumulation resulted in the trapping of large
fractions of pore liquid.
Sr and Nd isotopic trends
The isotopic differences between laminated and massive
gabbro layers indicate different histories of crustal contamination. Plagioclase is the only mineral in the rocks that
contains appreciable amounts of strontium, so the different
Sr isotopic compositions of laminated and massive gabbros
1500
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
imply isotopically distinct sources for the plagioclase in
these two rock types. This is consistent with formation of
the massive layers by accumulation of phenocrysts from
influxes of magma that were chemically and isotopically
distinct from those already present in the Cuillin chamber.
Low 87Sr/86Sr ratios in the massive gabbros (Figs 13
and 14a) indicate derivation of the phenocrysts from
magmas that were less contaminated than the resident
magmas, and the high anorthite contents of phenocrysts
indicate derivation from magmas that were relatively
primitive. Taken together, these properties suggest
magmas of Preshal More type as a potential source. The
Preshal More lavas, and some of the related Cuillin cone
sheets, contain Ca-rich plagioclase phenocrysts and are
among the least contaminated igneous rocks on Skye,
with some having chemical and isotopic compositions similar to MORB (Fig. 13; Esson et al., 1975; Moorbath &
Thompson, 1980; Thompson et al., 1980; Dickin, 1981;
Thirlwall & Jones, 1983; Dickin et al., 1987; Bell et al.,
1994). Hybridization between these magmas and more
contaminated differentiates could produce the Nd^Sr
array in Fig. 13, in which the Outer Bytownite Gabbros
plot along an extension of the trend for Preshal More
lavas and Cuillin cone sheets. The Outer Bytownite
Gabbros are themselves divisible into two distinct but parallel subtrends, with the massive gabbros offset to lower
87
Sr/86Sr owing to their accumulations of isotopically
anomalous phenocrysts.
The proportion of phenocrysts required to produce the
observed Sr isotopic shift can be estimated from the strontium isotope compositions of plagioclase phenocrysts in
Preshal More lavas. The phenocrysts analyzed by Font
et al. (2008) have Ca-rich cores (An88^89) and initial
87
Sr/86Sr of 0·7031^0·7039. If such phenocrysts were mixed
with a gabbro similar to the sample from zone C1
(SK220, initial 87Sr/86Sr ¼ 0·7052), then such crystals
would have to contribute roughly 30^50% of the total
strontium to produce a rock with 87Sr/86Sr initial ratios
resembling those of the massive gabbros in the lower part
of zone C2 (initial 87Sr/86Sr ¼ 0·7045^0·7046). This approximation, although very crude, is consistent with the
presence of tens of per cent of phenocrysts in the massive
gabbro layers.
Neodymium isotope ratios provide further support for
the proposed mechanism of layer formation. Neodymium
is concentrated preferentially in augite and apatite; the
neodymium content of plagioclase is negligible by comparison, so 143Nd/144Nd in the gabbros should be practically unaffected by the accumulation of plagioclase
phenocrysts. As predicted, there is no consistent difference
between the neodymium isotope ratios of laminated and
massive gabbros (Fig. 14b). Instead, the two types of layers
together define a single stratigraphic trend, with
143
Nd/144Nd increasing upward from zone C1 to zone C2,
then reversing direction and decreasing through the
uppermost 100 m of section. This is essentially a mirror
image of the strontium isotope trend, with both isotopic
systems reflecting the same general pattern of changing
contamination through time.
An especially interesting feature of the neodymium isotope stratigraphy is seen in the upper part of the section,
where massive layers are distinctly more contaminated
than the laminated layers immediately beneath them
(Fig. 14b). The shifts, although small, are abrupt, implying
that these massive layers must have been deposited from
replenishing magmas that underwent hybridization and
crustal contamination prior to their addition to the
Cuillin chamber. Otherwise, the addition of relatively uncontaminated magma would have shifted the ratio in the
opposite direction. Thus the neodymium data, like the
compositional zoning profiles in plagioclase phenocrysts,
imply that new influxes of magma had already hybridized
with more evolved (and apparently more contaminated)
magmas before flowing into the Cuillin intrusion.
Trends in mineral compositions
If the laminated gabbros consist largely of crystals that
formed within the Cuillin chamber, then their cumulus
mineral compositions should record the chemical evolution of the resident magma. Comparing the compositional
variations in plagioclase and augite (Figs 11 and 12) with
the stratigraphic column (Fig. 4) reveals correlations that
are consistent with formation of the massive layers from
influxes of primitive magma: increasingly primitive mineral compositions (indicated by increasing An content of
plagioclase and Mg# and Cr content of augite) are associated with the appearance of abundant and voluminous
massive layers in the lower part of zone C2, whereas increasingly evolved compositions (as indicated by decreasing An content of plagioclase and Mg# and Cr content
of augite) are associated with a decline in the proportion
of massive layers in the uppermost 100 m of the section. In
addition, the presence of reversely zoned plagioclase in
the uppermost few centimeters of laminated layer E
(sample SK223E) can be attributed to postcumulus
growth in the presence of pore liquid components that
infiltrated or diffused from the more primitive magma of
overlying massive layer F. The influence of primitive porphyritic magmas is also apparent in laminated layer U
(at 47 m), which is exceptionally rich in plagioclase phenocrysts and also contains some of the most primitive
groundmass plagioclase and augite in the entire section.
The trend toward more evolved feldspars and pyroxenes
in the upper part of the section, despite the continued
(albeit reduced) influx of phenocryst-laden magmas, suggests that fractional crystallizationçeither within the
Cuillin itself, or in the source reservoirs that fed itç
became a dominant process during the latter stages of accumulation in this part of the intrusion. Above the 75 m
1501
JOURNAL OF PETROLOGY
VOLUME 55
level, the upward progression toward more evolved mineral compositions is accompanied by acceleration of the
trend toward greater isotopic contamination, suggesting
that fractionation was coupled with assimilation of country
rocks. Whether assimilation occurred in situ, or within the
crustal plumbing system that supplied magma to the
Cuillin, will be discussed more extensively in a later
section.
Alternative mechanisms for layering
We have examined several alternative mechanisms for
layering development and found them to be unsatisfactory.
Let us consider, for, example, whether the plagioclase
phenocrysts could have been derived by magmatic erosion
or slumping of plagioclase-rich cumulates from earlier
units within the Cuillin itself, the most obvious source
being the Outer Bytownite Troctolites (Hutchison, 1968;
Hutchison & Bevan, 1977). The eight Outer Bytownite
Troctolite samples analyzed by Dickin et al. (1984) had initial 87Sr/86Sr ranging from 0·70476 to 0·70744, with all but
one sample 0·70580. These values are higherçin most
cases substantially soçthan those of nearly all the massive
gabbros analyzed in our study. The single exception is massive gabbro SK145, with a whole-rock 87Sr/86Sr of 0·70479
that overlaps slightly with the value of 0·70476 for a single
troctolite sample. If this massive gabbro includes a component of more radiogenic Sr derived from trapped pore liquids (as implied by mineral and trace element data),
then the 87Sr/86Sr of the phenocryst fraction would be
lower than that of the whole-rock, in which case there
would be no overlap at all between the isotopic compositions of the troctolites and those of the aggregated plagioclase phenocrysts. This makes it very unlikely that the
phenocrysts could have been derived by reworking of
these older troctolitic cumulates.
The isotopic data also preclude mechanisms of massive
layer formation that rely on intracumulus migration of
pore liquids, or emplacement of sills into an existing
layered sequence. Migration of silicate pore liquids would
presumably transport and redistribute Nd as well as Sr, so
there is no obvious way by which advection, circulation,
or percolation of pore liquids could produce massive
layers with anomalous Sr isotope ratios while leaving Nd
isotope ratios nearly unchanged (compare Fig. 14a and b).
Advective mechanisms that depend primarily on transport
by hydrous fluids, whether late magmatic or subsolidus,
are similarly unworkable; hydrous fluids could potentially
transport Sr while leaving Nd unaffected, but it seems implausible that such processes could consistently shift Sr
ratios by almost exactly the same amount at every point
in the section. Alternatively, emplacement of sills into the
layered sequence could potentially create isotopically
anomalous massive layers, but there is no reason to expect
that the Nd isotope ratios in sills and their host cumulates
(hypothetically represented by massive and laminated
NUMBER 8
AUGUST 2014
gabbro layers, respectively) would combine to produce a
single smooth trend with stratigraphic height, as is
observed (Fig. 14b). The problems with these alternative
hypotheses cannot be resolved by appealing to selective
transport by diffusion; in pyroxenes, feldspars, and basaltic
liquids, Sr diffuses faster than Nd (Lesher, 1994;
Cherniak, 2010; Cherniak & Dimanov, 2010; Zhang et al.,
2010), so redistribution via diffusion should homogenize Sr
isotope ratios more thoroughly than Nd isotope ratios,
which is the opposite of the observed pattern. In summary,
the mechanism of massive layer formation that we have
proposedçinvolving the accumulation of isotopically
anomalous plagioclase phenocrystsçseems by far the simplest explanation for the isotopic characteristics of gabbros
in the section.
M AJ O R E L E M E N T C O N S T R A I N T S
ON C U M U LUS A N D
P O S T C U M U L U S P RO C E S S E S I N
M A S S I V E L AY E R S
A more detailed model for the formation of massive layers
can be developed from constraints imposed by major element chemistry and mineral modes. The simplest possible
model, based on the preponderance of cumulus plagioclase
and the estimated fractions of trapped liquid (Fig. 15b), is
deposition of a plagioclase cumulate followed by closedsystem crystallization of 20^40% liquid trapped in the
pore spaces. The observed mineral proportions, however,
are inconsistent with such a simple process; in particular,
there is too little plagioclase and an overabundance of
ferromagnesian minerals. To demonstrate this, we consider
a hypothetical cumulate consisting of 60^80% plagioclase
crystals with basaltic liquid trapped in the interstices.
Crystallization of basaltic liquid produces roughly 50^
55% plagioclase by volume, so crystallization of 20^40%
liquid in a framework of plagioclase should produce a
rock with more than 80% feldspar. The massive gabbros,
in contrast, contain only about 60^75% feldspar (Table 3).
Alternative models are therefore required to account for
the excess ferromagnesian minerals.
One possibility is that the original cumulus assemblage
contained mafic silicates in addition to plagioclase. This
would be consistent with the anomalously magnesian and
Cr-rich cores of some augite grains (Figs 11c and 12),
which may have been primitive phenocrysts carried into
the Cuillin along with the calcic plagioclase. Another possibility is that the mechanism of pore liquid crystallization
was more complex than simple closed-system solidification;
pore liquids from the overlying melt body, or from underlying cumulates, may have percolated or circulated through
the cumulate pile during crystallization, or some fraction
of pore liquid may have been expelled during compaction
of the partially solidified cumulus mush. These hypotheses
1502
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
can be tested by applying equilibrium and mass-balance
constraints imposed by Fe/Mg ratios in the gabbros and
their constituent minerals, and by examining the results
in the context of observable field relationships relevant to
the potential mobility of pore liquids.
Constraints imposed by whole-rock
FeO/MgO
We can begin by examining the Fe/Mg ratio of the main
body of magma in the Cuillin chamber, inferring that this
magma representsçto a first approximationçthe initial
composition of the pore liquid at each level in the section.
If we assume that the laminated gabbros crystallized as cumulates in equilibrium with the resident magma, then we
can use Fe/Mg ratios of cumulus minerals to calculate the
Fe/Mg ratio of this magma, using well-established relationships describing the partitioning of Fe and Mg among minerals and melts. The assumption of equilibrium between
laminated gabbro minerals and the main body of melt is
justifiable if fractions of trapped liquid in laminated layers
were very small (i.e. 510%, as suggested by incompatible
element concentrations), in which case the trapped liquid
shifts in Fe/Mg ratios of minerals would have been slight
(Barnes, 1986). Using this assumption, we can calculate
the molar FeO/MgO ratio of the resident liquid during
crystallization of each laminated gabbro layer using the
relationship
KD ¼ FeO=MgO augite = FeO=MgO liquid ¼ 0 21
in which the value of 0·21 for KD is calculated from an empirical equation provided by Barnes (1986):
FeO=MgO augite =0 78 ¼ 0 88 FeO=MgO olivine
coupled with the experimentally determined olivine^melt
partitioning function determined by Roeder & Emslie
(1970):
KD ¼ FeO=MgO olivine = FeO=MgO liquid ¼ 0 30:
Using these equations, we calculated FeO/MgO for the
liquid in equilibrium with the average composition of
augite in each laminated gabbro sample; results ranged
from 1·10 to 1·43. The ratios of total iron to magnesium in
the liquids will have been higher, as the liquids certainly
contained some amount of Fe2O3 in addition to FeO. The
ratio Fe2O3/FeO in fresh tholeiitic basalts is typically close
to 0·15 by weight (Brooks, 1976), so if we add the corresponding amount of Fe2O3 to FeO for each liquid and then
recalculate all Fe as FeO (the total being denoted FeOT),
then the range of FeOT/MgO is 1·24^1·62.
If the initial pore liquids had the compositions of liquids
residing within the Cuillin chamber, and if they underwent closed-system crystallization within a cumulus framework consisting entirely of plagioclase grains, then
Fig. 17. Whole-rock molar FeOT/MgO ratios in massive gabbros,
compared with ratios in liquids calculated to be in equilibrium with
augite in the laminated gabbros. (See text for discussion.)
practically all Fe and Mg in the massive layers would
have been derived from pore liquids, and the FeOT/MgO
ratios of massive gabbros and calculated liquids should be
approximately the same. This is clearly not the case, as
shown in Fig. 17. The massive layers consistently have
lower FeOT/MgO (and higher Mg#) than the resident liquids from which the laminated gabbros crystallized. In
other words, the ferromagnesian minerals in the massive
layers are considerably more primitive than would be expected for bulk crystallization of trapped liquid in a
plagioclase cumulate. We can envision at least three plausible mechanisms, perhaps acting together, that could account for this discrepancy.
First, the cumulus assemblage might have contained
mafic silicates in addition to plagioclase, a hypothesis consistent with the anomalously magnesian and chromian
cores of some augite grains. In this case, the high Mg/Fe
ratios of the massive gabbros would be at least partially attributable to accumulation of Mg-rich phenocrysts of
augite and possibly olivine, perhaps carried by the same
1503
JOURNAL OF PETROLOGY
VOLUME 55
pulses of primitive magma that deposited the plagioclase
phenocrysts. Given the low mineral^melt partition coefficients for Nd and Sr in olivine and augite, accumulation
of these ferromagnesian phenocrysts would have had little
effect on isotopic ratios, provided that augite phenocryts
did not constitute the bulk of augite in the final rock.
Second, the initial pore liquid fraction might have been
substantially greater than 20^40%, with some amount of
evolved, high-Fe/Mg liquid having been expelled by compaction during crystallization. In this case, major elements
and highly incompatible trace elements would have been
decoupled from each other; some amount of Fe and Mg
would have crystallized as ferromagnesian silicates while
the system was still open, whereas highly incompatible
elements would have crystallized only from the liquid that
was ultimately trapped. Incompatible elements would
then provide reasonable estimates of the trapped liquid
fractions, whereas Mg/Fe ratios would not.
Finally, the initial stages of postcumulus crystallization
within massive layers may have occurred while liquid
from the main body of magma convected through the
porous, loosely packed cumulus pile (Tait et al., 1984; Kerr
& Tait, 1985, 1986). This would have provided a continuous
supply of unfractionated liquid to intercumulus crystals as
they grew, thereby suppressing evolution toward higher
FeO/MgO. Such a process could have continued until porosity and permeability decreased sufficiently to stop melt
percolation and thereby end convective exchange with the
overlying body of magma. Kerr & Tait (1986) and
Campbell (1987) have pointed out that this mechanism of
adcumulus growth would be most efficient near the top of
the cumulus pile, and could thus produce an impermeable
cap that would trap residual liquids in the underlying cumulates. Such a mechanism could help explain the retention of trapped liquids in massive layers: rapid
accumulation of plagioclase phenocrysts would have been
followed by slower adcumulus growth, concentrated near
the top of the pile and in the overlying laminated layers,
thereby sealing pore liquids in the massive gabbro underneath. This mechanism, like the one involving compaction
and expulsion of liquids during crystallization, would
have decoupled whole-rock major element ratios from incompatible trace element concentrations.
Mobility of pore liquids
Field relationships suggest that if pore liquids escaped from
the massive layers, they could not have percolated and
redistributed material through thick overlying piles of partially molten cumulates. The extensive trapping of liquids
in massive layers is itself indicative of physical barriers to
liquid percolation over distances greater than the scale of
rhythmic layering (i.e. meters). The geometries of gabbroic
pegmatite bodies (Fig. 8) support this inference. As discussed previously, the pegmatites nucleated as diffuse
bodies within massive layers and then propagated into
NUMBER 8
AUGUST 2014
overlying and underlying laminated layers via brittle fractures, suggesting that by the time the pegmatites formed,
the laminated layers were already largely solidified and
impermeable to diffuse intergranular flow. It therefore follows that if liquids escaped from the massive cumulates by
intracumulus percolation, then they probably did so soon
after the cumulates formed, while they were still very
near the interface with the main body of magma.
Eventually, reduction of porosity at the tops of the layers
would have created impermeable caps that trapped pore liquids in the massive layers beneath them (e.g. Campbell,
1987).
Stratigraphic trends in mineral compositions, when considered in detail, provide further evidence against largescale migration of pore liquids. For example, if we examine
the lower part of the section (from about 0 to 50 m), the
upward increases in Mg# of augite and %An in plagioclase (Fig. 11) could potentially be construed as evidence
for large-scale, upward percolation of evolved pore liquids
expelled from the cumulates below; in the case of a new
influx of primitive magma, such a process would upwardly
displace the shift toward more primitive mineral compositions, to a stratigraphic level above that at which the new
magma actually arrived (e.g. Irvine, 1980). This explanation, however, is inconsistent with the observed trend in
chromium contents of augites. Chromium partitions
strongly into pyroxene and should therefore be immobile
in the cumulus pile, yet its upward increase in concentration (Fig. 12b) parallels the changes in Mg# and %An.
This implies that the trend toward increasingly primitive
compositions cannot be attributed to postcumulus liquid
migration, but must instead reflect gradual changes in the
resident magma composition, most probably owing to
magmatic replenishment. The survival of this signature
implies that postcumulus liquid percolation did not greatly
modify rock and mineral compositions at scales of tens of
meters or more, and that the salient chemical and isotopic
trends reflect, for the most part, primary cumulus
processes.
M A G M AT I C P L U M B I N G S Y S T E M
A N D E VO L U T I O N O F T H E
CUILLIN CENTRE
Contamination processes and the supply of
magma
Numerous studies have shown that the Palaeogene
magmas of Skye were contaminated to various degrees
during their transit through the Archean gneissic crust
(Moorbath & Thompson, 1980; Thompson et al., 1982;
Dickin et al., 1984, 1987; Morrison et al., 1985; Bell et al.,
1994; Fowler et al., 2004; Font et al., 2008). Isotopic evidence
indicates that contamination of the Cuillin gabbros and
cone sheets, and of the Preshal More basalts, took place
1504
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
mainly in the amphibolite-facies upper crust (Moorbath &
Thompson, 1980; Dickin, 1981; Dickin et al., 1984; Bell
et al., 1994). In the cone sheets and Preshal More lavas,
strong correlations between isotopic and chemical trends
show that assimilation and contamination were coupled
with fractional crystallization in upper crustal reservoirs
(Moorbath & Thompson, 1980; Bell et al., 1994). In the
main intrusive units of the Cuillin Centre, in contrast, Sr
and Pb isotope ratios do not correlate well with chemical
indicators of fractionation (Dickin et al., 1984), indicating
that contamination and fractionation were decoupled
during growth of the plutonic complex. This led Dickin
et al. (1984) to conclude that the Cuillin was fed by a complex magmatic plumbing system in which numerous small
batches of magma interacted with a heterogeneous felsic
crust, with some magmas having encountered host-rocks
that were easily assimilated whereas others encountered
host-rocks that were relatively refractory owing to earlier
partial melting and extraction of the most fusible
components.
In the section studied here, as in much of the Cuillin,
trends in plagioclase and augite compositions (Fig. 11) are
partially decoupled from trends in Sr and Nd isotope
ratios (Fig. 14). Passing upward from zone C1 to zone C2,
the shift to appreciably lower 87Sr/86Sr and higher
143
Nd/144Nd is not accompanied by major changes in mineral compositions. Ascending through the lower part of
unit C2 (to c. 50^75 m), isotopic ratios in the laminated
gabbros indicate a reversion to slightly greater degrees of
contamination, but plagioclase and augite compositions
grow more primitive. Only in the upper part of the section
do more evolved mineral compositions correlate with
increasing degrees of isotopic contamination. These variations are consistent with replenishment from crustal
magma reservoirs that were themselves sites of mixing
among variably contaminated magmas, with the outputs
from these reservoirs changing as particular inputs waxed
and waned over time (Dickin et al., 1984). In the Outer
Bytownite Gabbros studied here, vigorous recharge of the
source reservoirs by more primitive magmas may have
been the dominant process during formation of the lower
part of the section, whereas assimilation and fractional
crystallization within the source reservoirs may have been
dominant in the upper part. The observed chemical and
isotopic trends would then reflect magma evolution and
hybridization mainly within deeper crustal reservoirs
rather than within the Cuillin itself.
In support of this hypothesis, assimilation of the
Cuillin’s host-rocks cannot readily account for increasing
degrees of contamination in the upper part of the section.
The Outer Bytownite Gabbros were emplaced into
Palaeogene lavas and plutonic rocks with isotopic compositions similar to those of the gabbros themselves, so it
seems unlikely that the necessary contamination could
have occurred in situ. Let us consider, for example, the
xenoliths as potential contaminants. The xenoliths have
not been analyzed, but rough constraints are imposed by
the fact that they are of Palaeogene plutonic rocks and
lavas from the Skye igneous centre, nearly all of which
have isotopic compositions of 87Sr/86Sr 0·703^0·707
(Moorbath & Thompson, 1980; Thompson et al., 1982;
Dickin et al., 1984, 1987; Bell et al., 1994). Where they are
most abundant, xenoliths constitute about 20% of the outcrop over hundreds of square meters of exposed area. If
we consider an extreme case within these parametersçassimilation of, say, one part melt with 87Sr/86Sr 0·707 by
four parts magma with 87Sr/86Sr 0·705çthen, assuming
roughly equal concentrations of Sr in both components,
such contamination could shift 87Sr/86Sr of the magma by
about 0·0004, which is comparable with the range of variation within our section. This extreme scenario seems unlikely, however, if considered in the context of a few
simple observations. In a detailed study of the xenoliths in
zone C2, Carr (1952) determined that a large proportion
were either cognate inclusions derived from within the
Outer Bytownite Gabbros, or fine-grained granular hornfelses of basaltic lava or diabase. The cognate xenoliths
would presumably have been isotopically similar to the
host gabbros, whereas the great majority of the basaltic
lavas and mafic dikes of Skye have lower 87Sr/86Sr than
the gabbros in our section. Extensive in situ contamination
by high 87Sr/86Sr xenoliths would therefore require incorporation of a very unusual assemblage of Palaeogene country rocks as well as extremely efficient anatexis and melt
extraction. This problem is exacerbated by the plagioclase-rich nature of many of the xenoliths, which suggests
that Sr would have been retained in the solid residua
rather than extracted in anatectic melts. Given these limitations, it seems much more likely that the inferred coupling of fractionation and crustal assimilation took place
in deeper magma reservoirs, located within the isotopically distinct Lewisian gneiss complex, and that these reservoirs were episodically tapped to supply magmas to the
main Cuillin magma body. Such an inference is consistent
with numerous isotopic studies of the lavas of Skye and
other igneous centres in the Hebrides (Thompson et al.,
1982; Dickin et al., 1984, 1987; Morrison et al., 1985; Bell
et al., 1994; Fowler et al., 2004; Font et al., 2008).
Support for this model is provided by neodymium isotope trends in the lower part of the section. Following the
major isotopic shift at the transition from zone C1 to zone
C2, the isotopic compositions of the laminated gabbros
change only slightly during accumulation of the next 75 m
of section, despite continued influxes of magma. If these
influxes did not change the isotopic composition of the resident magma appreciably, then the resident and recharge
magmas must have been isotopically very similar. This
suggests that, following the first appearance of new
1505
JOURNAL OF PETROLOGY
VOLUME 55
magma near the base of zone C2, the new influxes
quickly dominated the system. This has interesting implications for the timing of hybridization: if the laminated
gabbros crystallized from resident liquids that were dominated by the new influxes, whereas the massive gabbros
accumulated from their entrained plagioclase phenocrysts, then the consistent isotopic difference between
laminated and massive gabbros implies that the phenocrysts were isotopically distinct from the liquids that carried them into the magma chamber. The phenocrystbearing magmas must therefore have hybridized within
the crust prior to flowing into the Cuillin magma chamber, probably by mixing of a primitive, plagioclasephyric Preshal More-type magma with a more evolved
and more contaminated differentiate.
Geometry of the Cuillin magma chamber
The rapidity with which the resident magma was chemically overwhelmed by replenishments suggests that the
volume of the existing magma body was small. As an alternative, it could be argued that influxes of chemically distinct magma simply ponded at the bottom of a much
larger chamber without mixing appreciably. This, however,
would be difficult to reconcile with the neodymium isotope
stratigraphy. Assuming that neodymium in the rocks is
hosted almost entirely by augite or apatite, and that these
minerals crystallized from liquids within the chamber,
then layers representing isotopically anomalous ponded
magmas (i.e. massive layers) should have neodymium isotopic compositions that deviate sharply from the main
trend. This is not the case. Instead, the neodymium isotope
ratios in massive and laminated layers together produce a
single consistent trend through the section, suggesting that
each new influx mixed with, or volumetrically overwhelmed, the resident magma. It follows that if influxes of
magma representing no more than tens of meters of section
were sufficient to change the isotopic composition of the
system appreciably, then the volume of magma in the
chamber at a particular time must have been small relative
to the roughly 1·6 km of Outer Bytownite Gabbro section
[as measured by Carr (1952) and tabulated by Wager &
Brown (1968)].
The continuity of single layers suggests that the chamber, although small in volume, was laterally extensive. It
may have resembled the sheet-like bodies that have been
inferred for magma chambers in the Eastern Layered
Intrusion of Rum (Holness & Winpenny, 2009). As in the
Cuillin, fresh influxes of magma in the Rum layered gabbros are commonly associated with marked shifts in strontium isotope ratios, implying incremental growth by
repeated additions of variably contaminated magma
(Palacz, 1985; Palacz & Tait, 1985; Renner & Palacz, 1987;
Holness & Winpenny, 2009).
NUMBER 8
AUGUST 2014
Growth of the Cuillin intrusion
At a larger scale within the Cuillin Centre, multiple cycles
of waxing and waning contamination were documented
by Dickin et al. (1984), whose 35 samples from throughout
the centre (including nine from the Outer Bytownite
Gabbros) revealed erratic variations in 87Sr/86Sr during
growth of the complex (Fig. 18). The overall trend is
toward less contamination through time, probably owing
to the Archean crust being gradually depleted of its most
fusible components during repeated episodes of crustal assimilation (Dickin et al., 1984). The multiple reversals
toward higher 87Sr/86Sr ratiosçsuch as the one documented in this study, and at least eight more identified by
Dickin et al. (1984)çrepresent short-lived deviations from
this overall trend. By analogy with our findings, each
such reversal may represent a pause in the delivery of
primitive batches of magma to the network of crustal
magma reservoirs that fed the Cuillin. After accumulating
in these reservoirs, the primitive magmas stalled and
underwent fractional crystallization, contamination, and
hybridization while being episodically tapped to provide
magmas to the main Cuillin chamber, thus producing temporary reversions to more evolved and contaminated compositions. The section studied here provides a detailed
record of one of many such fluctuations in crustal
Fig. 18. Strontium isotope stratigraphy for the Cuillin Centre; data
from Dickin et al. (1984). The vertical scale is the age sequence based
on intrusive relationships and layering. Samples from this study are
located stratigraphically by comparison with sample locations provided by Dickin et al. (1984).
1506
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
contamination within the plutons and lava sequences of
Skye, lending support to a model of a complex crustal
plumbing system in which batches of magma evolved separately, but mixed frequently, en route to the Cuillin and
its overlying lava fields (Dickin et al., 1984; Morrison et al.,
1985).
Pluton growth by incremental and frequent replenishment has been documented in other layered intrusions of
the North Atlantic Igneous Province, including the Rum
Intrusion of Scotland and the Kangerlussuaq and Kap
Edvard Holm intrusions of East Greenland (Bernstein
et al., 1992; Tegner et al., 1993; Riishuus et al., 2008).
Layered sequences such as these may be the plutonic
equivalents of volcanic sequences that record frequent episodes of magma replenishment and withdrawal from shallow crustal magma systems, as commonly revealed by
complex chemical and isotopic zoning profiles in phenocrysts in lavas (Davidson et al., 2007; Streck, 2008). Similar
grain-scale isotopic heterogeneities have been documented
in other layered mafic intrusions, where they have been interpreted as records of magma replenishment, hybridization, wall-rock assimilation, and mingling of crystals
derived from different parental magmas (Mathez &
Waight, 2003; Tepley & Davidson, 2003; Seabrook et al.,
2005; Davidson et al., 2008; Yang et al., 2013). By linking isotopic discontinuities associated with transitions in layering
to the mingling of isotopically distinct crystals, such studies
provide new insights into the histories and mechanisms of
magma recharge during the growth and evolution of composite intrusions. Our documentation of layer-by-layer isotopic variations in the Cuillin contributes to this growing
body of literature, and in view of the likely genetic relationships among plutons, dikes, and lavas of Skye may provide an opportunity to establish direct links between
open-system behavior in intrusive and extrusive systems.
S U M M A RY A N D C O N C L U S I O N
In the Outer Bytownite Gabbros of the Cuillin Centre, episodic influxes of magma carried abundant phenocrysts of
calcium-rich plagioclase, which were deposited on the
floor of the magma chamber to produce cumulus layers of
massive coarse-grained gabbro. Rapid accumulation of
the crystals trapped large fractions of pore liquid that crystallized within the cumulus pile. The abundance of massive
layers implies that the complex grew by repeated additions
of magma, which were small in volume yet changed the
composition of the resident magma substantially. This
chemical sensitivity implies that the magma chamber
itself was small, with the volume of liquid at a given time
being only a minor fraction of the entire complex.
Variations in mineral chemistry and strontium and neodymium isotopic ratios indicate that the replenishing
magmas underwent varying degrees of fractional crystallization and crustal assimilation during their transit
through the crust, with the section studied here representing only one of many cycles that recorded the waxing and
waning influence of contaminated magmas in the Cuillin.
These inferences are consistent with earlier studies of the
Cuillin gabbros, their associated cone sheets, and the
lavas of Skye, which concluded that crustal contamination
occurred in a complex subvolcanic plumbing system of
sills, dikes, and other minor intrusions in which small
batches of magma underwent diverse histories of fractionation, hybridization, and crustal interaction within the
Archean basement (Thompson et al., 1982; Dickin et al.,
1984, 1987; Morrison et al., 1985; Bell et al., 1994; Fowler
et al., 2004; Font et al., 2008). The characteristics of the replenishing magmas in the Outer Bytownite Gabbrosçin
particular, their calcic plagioclase phenocrysts and relatively primitive isotopic compositionsçsupport models in
which the bulk of the Cuillin Centre formed from
magmas of Preshal More type and their differentiates.
More generally, the evidence for frequent replenishment
in a major subvolcanic pluton suggests that layered gabbros
such as the Cuillin, and other composite intrusions of the
North Atlantic Igneous Province, can provide detailed records of magma transport, mixing, and hybridization in
the crustal plumbing systems that supply magmas to
major flood basalt provinces.
AC K N O W L E D G E M E N T S
We thank Dennis Bird for suggesting the Cuillin Centre as
a promising subject for research. We are very grateful to
Drew Coleman and Mike Lester for the isotopic analyses,
to Tony Caldanaro for help with sample preparation, and
to Mike Vollinger and Mike Rhodes for help with the
XRF analyses. The Broadford Post Office staff was extremely helpful in managing transport to and from the
field area. Scottish Natural Heritage and The John Muir
Trust kindly gave permission to collect samples, thereby
making this study possible. The manuscript benefited
greatly from careful and insightful reviews by Stephen
Barnes, C. H. Emeleus, and Frank Tepley, and we thank
them for their detailed and thoughtful critiques.
FU N DI NG
This work was supported by Smith College and Five
Colleges Incorporated, Inc.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
R EF ER ENC ES
Anderson, F. W. & Dunham, K. C. (1966). The Geology of Northern
Skye: Explanation of the Portree (80) and Parts of the Rubha
1507
JOURNAL OF PETROLOGY
VOLUME 55
Hunish (90), Applecross (81) and Gairloch (91) Sheets. Geological
Survey of Great Britain.
Barnes, S. J. (1986). The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contributions to
Mineralogy and Petrology 93, 524^531.
Be¤dard, J. H., Marsh, B. D., Hersum, T. G., Naslund, H. R. &
Mukasa, S. B. (2007). Large-scale mechanical redistribution of
orthopyroxene and plagioclase in the Basement Sill, Ferrar dolerites, McMurdo Dry Valleys, Antarctica: Petrological, mineralchemical and field evidence for channelized movement of crystals
and melt. Journal of Petrology 48, 2289^2326.
Bell, B. R. & Harris, J. W. (1986). An Excursion Guide to the Geology of the
Isle of Skye. Geological Society of Glasgow.
Bell, B. R. & Williamson, I. T. (2002). Tertiary igneous activity. In:
Trewin, N. H. (ed.) The Geology of Scotland, 4th edn. Geological
Society, London, pp. 371^407.
Bell, B. R., Claydon, R. V. & Rogers, G. (1994). The petrology and
geochemistry of cone-sheets from the Cuillin Igneous Complex,
Isle of Skye: Evidence for combined assimilation and fractional
crystallization during lithospheric extension. Journal of Petrology 35,
1055^1094.
Bernstein, S. (2006). In situ fractional crystallization of a mafic pluton:
Microanalytical study of a Palaeogene gabbronorite plug in East
Greenland. Lithos 92, 222^237.
Bernstein, S., Keleman, P. B. & Brooks, C. K. (1992). Evolution of the
Kap Edvard Holm Complex: a mafic intrusion at a rifted continental margin. Journal of Petrology 37, 497^519.
Bistacchi, A., Tibaldi, A., Pasquare', A. F. & Rust, D. (2012). The
association of cone-sheets and radial dikes: Data from the
Isle of Skye (UK), numerical modelling, and implications for
shallow magma chambers. Earth and Planetary Science Letters
339^340, 46^56.
Borghini, G. & Rampone, E. (2007). Postcumulus processes in oceanic-type olivine-rich cumulates: the role of trapped melt crystallization versus melt/rock interaction. Contributions to Mineralogy and
Petrology 154, 619^633.
Brandriss, M. E., Bird, D. K., O’Neil, J. R. & Cullers, R. L. (1996).
Dehydration, partial melting, and assimilation of metabasaltic
xenoliths in gabbros of the Kap Edvard Holm Complex, East
Greenland. AmericanJournal of Science 296, 333^393.
British Geological Survey. (2005). Skye Central Complex, Scotland, Bedrock.
1:25,000 British Geology Series. Keyworth, British Geological Survey.
Brooks, C. K. (1976). The Fe2O3/FeO ratio of basalt analyses: an
appeal for a standardized procedure. Bulletin of the Geological Society
of Denmark 25, 117^120.
Campbell, I. H. (1977). A study of macro-rhythmic layering
and cumulate processes in the Jimberlana Intrusion, Western
Australia. Part I: the Upper Layered Series. Journal of Petrology 18,
183^215.
Campbell, I. H. (1978). Some problems with the cumulus theory. Lithos
11, 311^323.
Campbell, I. H. (1987). Distribution of orthocumulate textures in the
Jimberlana Intrusion. Journal of Geology 95, 35^53.
Carr, J. M. (1952). An investigation of the Sgurr na Stri^Druim Hain
sector of the Basic Igneous Complex of the Cuillin Hills, Isle of
Skye, D.Phil. thesis, University of Oxford.
Carter, S. R., Evensen, N. M., Hamilton, P. J. & O’Nions, R. K.
(1978). Neodymium and strontium isotope evidence for crustal contamination of continental volcanics. Science 202, 743^747.
Cherniak, D. J. (2010). Cation diffusion in feldspars. In: Zhang, Y. &
Cherniak, D. J. (eds) Diffusion in Minerals and Melts. Mineralogical
Society of America and Geochemical Society, Reviews in Mineralogy and
Geochemistry 72, 691^733.
NUMBER 8
AUGUST 2014
Cherniak, D. J. & Dimanov, A. (2010). Diffusion in pyroxene, mica
and amphibole. In: Zhang, Y. & Cherniak, D. J. (eds) Diffusion in
Minerals and Melts. Mineralogical Society of America and Geochemical
Society, Reviews in Mineralogy and Geochemistry 72, 641^690.
Costa, F., Dohmen, R. & Chakraborty, S. (2008). Time scales of magmatic processes from modeling the zoning patterns of crystals. In:
Putirka, K. D. & Tepley, F. J., III (eds) Minerals, Inclusions and
Volcanic Processes. Mineralogical Society of America and Geochemical
Society, Reviews in Mineralogy and Geochemistry 69, 545^594.
Davidson, J. P., Morgan, D. J., Charlier, B. L. A., Harlou, R. &
Hora, J. M. (2007). Microsampling and isotopic analysis of igneous
rocks: Implications for the study of magmatic systems. Annual
Review of Earth and Planetary Sciences 35, 273^311.
Davidson, J. P., Font, L., Charlier, B. L. A. & Tepley, F. J., III (2008).
Mineral-scale Sr isotope variation in plutonic rocksça tool for
unravelling the evolution of magma systems. Transactions of the
Royal Society of Edinburgh: Earth Sciences 97, 357^367.
Dick, H. J. B., Natland, J. H., Alt, J. C., Bach, W., Bideau, D., Gee, J.
S., Haggas, S., Hertogen, J. G. H., Hirth, G., Holm, P. M.,
Ildefonse, B., Iturrino, G. J., John, B. E., Kelley, D. S.,
Kikawa, E., Kingdon, A., LeRoux, P. J., Maeda, J., Meyer, P. S.,
Miller, D. J., Naslund, H. R., Niu, Y.-L., Robinson, P. T., Snow, J.,
Stephen, R. A., Trimby, P. W., Worm, H. U. & Yoshinobu, A.
(2000). A long in situ section of the lower ocean crust: results of
ODP Leg 176 drilling at the Southwest Indian Ridge. Earth and
Planetary Science Letters 179, 31^51.
Dickin, A. P. (1981). Isotope geochemistry of Tertiary igneous rocks
from the Isle of Skye, N.W. Scotland. Journal of Petrology 22, 155^189.
Dickin, A. P., Brown, J. L., Thompson, R. N., Halliday, A. N. &
Morrison, M. A. (1984). Crustal contamination and the granite
problem in the British Tertiary Volcanic Province. Philosophical
Transactions of the Royal Society of London, Series A 310, 755^780.
Dickin, A. P., Jones, N. W., Thirlwall, M. F. & Thompson, R. N.
(1987). A Ce/Nd isotope study of crustal contamination processes affecting Palaeocene magmas in Skye, Northwest Scotand.
Contributions to Mineralogy and Petrology 96, 455^464.
Dimanov, A. & Wiedenbeck, M. (2006). (Fe,Mn)^Mg interdiffusion
in natural diopside: effect of pO2. European Journal of Mineralogy 18,
705^718.
Dohmen, R. & Chakraborty, S. (2007). Fe^Mg diffusion in olivine II:
point defect chemistry, change of diffusion mechanisms and a
model for calculation of diffusion coefficients in natural olivine.
Physics and Chemistry of Minerals 34, 409^430.
Donaldson, C. H. (1977). Petrology of anorthite-bearing gabbroic anorthosite dykes in northwest Skye. Journal of Petrology 18, 595^620.
Emeleus, C. H. & Bell, B. R. (2005). British Regional Geology: The
Palaeogene Volcanic Districts of Scotland. British Geological Survey.
Emeleus, C. H. & Gyopari, M. C. (1992). British Tertiary Volcanic
Province, Geological Conservation Review Series, No. 4. Chapman & Hall.
Emeleus, C. H., Cheadle, M. J., Hunter, R. H., Upton, B. G. J. &
Wadsworth, W. J. (1996). The Rum Layered Suite. In:
Cawthorn, R. G. (ed.) Layered Intrusions. Elsevier, pp. 403^439.
Esson, J., Dunham, A. C. & Thompson, R. N. (1975). Low alkali, high
calcium olivine tholeiite lavas from the Isle of Skye, Scotland.
Journal of Petrology 16, 488^497.
Font, L., Davidson, J. P., Pearson, D. G., Nowell, G. M., Jerram, D. A.
& Ottley, C. J. (2008). Sr and Pb isotope micro-analysis of plagioclase crystals from Skye lavas: an insight into open-system processes in a flood basalt province. Journal of Petrology 49, 1449^1471.
Fowler, S. J., Bohrson, W. A. & Spera, F. J. (2004). Magmatic evolution
of the Skye Igneous Centre, Western Scotland: Modelling of assimilation, recharge, and fractional crystallization. Journal of Petrology
45, 2481^2505.
1508
BRANDRISS et al.
PLAGIOCLASE-RICH LAYERS, CUILLIN
Gray, W., Glazner, A. F., Coleman, D. S. & Bartley, J. M. (2008).
Long-term geochemical variability of the Late Cretaceous
Tuolumne Intrusive Suite, central Sierra Nevada, California. In:
Annen, C. & Zellmer, G. F. (eds) Dynamics of Crustal Magma
Transfer, Storage and Differentiation. Geological Society, London, Special
Publications 304, 183^201.
Hall, J. (1987). Physical properties of Lewisian rocks: implications for
deep crustal structure. In: Park, R. G. & Tarney, J. (eds) Evolution
of Lewisian and Comparable Precambrian High Grade Terrains. Geological
Society, London, Special Publications 27, 185^192.
Hall, J. & Simmons, G. (1979). Seismic velocities of Lewisian metamorphic rocks at pressures to 8 kbar: relationship to crustal layering in North Britain. Geophysical Journal of the Royal Astronomical
Society 58, 337^347.
Hamilton, M. A., Pearson, D. G., Thompson, R. N., Kelley, S. P. &
Emeleus, C. H. (1998). Rapid eruption of Skye lavas inferred from
precise U^Pb and Ar^Ar dating of the Rum and Cuillin plutonic
complexes. Nature 394, 260^263.
Holness, M. B. & Winpenny, B. (2009). The Unit 12 allivalite, Eastern
Layered Intrusion, Isle of Rum: a textural and geochemical study
of an open-system magma chamber. Geological Magazine 146,
437^450.
Hunter, R. H. (1996). Texture development in cumulate rocks. In:
Cawthorn, R. G. (ed.) Layered Intrusions. Elsevier, pp. 77^101.
Hutchison, R. (1968). Origin of the White Allivalite, Western Cuillin,
Isle of Skye. Geological Magazine 105, 338^347.
Hutchison, R. & Bevan, J. C. (1977). The Cuillin layered igneous complexçevidence for multiple intrusion and former presence of a picritic liquid. ScottishJournal of Geology 13, 197^210.
Irvine, T. N. (1980). Magmatic infiltration metasomatism, doublediffusive fractional crystallization, and adcumulus growth in the
Muskox Intrusion and other layered intrusions. In: Hargraves, R.
B. (ed.) Physics of Magmatic Processes. Princeton University Press,
pp. 325^383.
Irvine, T. N., Andersen, J. C. . & Brooks, C. K. (1998). Included
blocks (and blocks within blocks) in the Skaergaard intrusion:
Geologic relations and the origins of rhythmic modally graded
layers. Geological Society of America Bulletin 110, 1398^1447.
Jerram, D. A., Cheadle, M. J., Hunter, R. H. & Elliott, M. T. (1996).
The spatial distribution of grains and crystals in rocks.
Contributions to Mineralogy and Petrology 125, 60^74.
Jerram, D. A., Davis, G. R., Mock, A., Charrier, A. & Marsh, B. D.
(2010). Quantifying 3D crystal populations, packing and layering
in shallow intrusions: A case study from the Basement Sill, Dry
Valleys, Antarctica. Geosphere 6, 537^548.
Kent, R. W. & Fitton, J. G. (2000). Mantle sources and melting dynamics in the British Palaeogene Igneous Province. Journal of
Petrology 41, 1023^1040.
Kerr, A. C. & Tait, S. R. (1985). Convective exchange between
pore fluid and an overlying reservoir of dense fluid: a post-cumulus
process in layered intrusions. Earth and Planetary Science Letters 75,
147^156.
Kerr, A. C. & Tait, S. R. (1986). Crystallization and compositional
convection in a porous medium with application to layered igneous
intrusions. Journal of Geophysical Research 91, 3591^3608.
Kruger, F. J. (2005). Filling the Bushveld Complex magma chamber:
lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti^V^magnetite deposits. Mineralium Deposita 40, 451^472.
Lesher, C. E. (1994). Kinetics of Sr and Nd exchange in silicate liquids:
theory, experiments, and applications to uphill diffusion, isotopic
equilibration, and irreversible mixing of magmas. Journal of
Geophysical Research 99, 9585^9604.
Lundgaard, K. L., Tegner, C., Cawthorn, R. G., Kruger, F. J. &
Wilson, J. R. (2006). Trapped intercumulus liquid in the Main
Zone of the eastern Bushveld Complex, South Africa. Journal of
Petrology 151, 352^369.
Mathez, E. A. & Waight, T. E. (2003). Lead isotopic disequilibrium
between sulfide and plagioclase in the Bushveld Complex and the
chemical evolution of large layered intrusions. Geochimica et
Cosmochimica Acta 67, 1875^1888.
Mattey, D. P., Gibson, I. L., Marriner, G. F. & Thompson, R. N.
(1977). The diagnostic geochemistry, relative abundance, and spatial distribution of high-calcium, low-alkali olivine tholeiite dykes
in the Lower Tertiary regional swarm of the Isle of Skye, NW
Scotland. Mineralogical Magazine 41, 273^285.
McBirney, A. R. (1996). The Skaergaard Intrusion. In: Cawthorn, R.
G. (ed.) Layered Intrusions. Elsevier, pp. 147^180.
Mendum, J. R., Barber, A. J., Butler, R. W. H., Flinn, D.,
Goodenough, K. M., Krabbendam, M., Park, R. G. &
Stewart, A. D. (2009). Lewisian, Torridonian and Moine Rocks of
Scotland, Geological Conservation Review Series, No. 34. Joint Nature
Conservation Committee.
Mertz, D. F. & Haase, K. M. (1997). The radiogenic isotope composition of the high-latitude North Atlantic mantle. Geology 25, 411^414.
Meurer, W. P. & Boudreau, A. E. (1996). Compaction of density-stratified cumulates: Effect on trapped-liquid distribution. Journal of
Geology 104, 115^120.
Meyer, G. B. & Wilson, J. R. (1999). Olivine-rich units in the Fongen^
Hyllingen Intrusion, Norway: implications for magma chamber
processes. Lithos 47, 157^179.
Moorbath, S. & Thompson, R. N. (1980). Strontium isotope geochemistry and petrogenesis of the Early Tertiary lava pile of the Isle of
Skye, Scotland, and other basic rocks of the British Tertiary
Province: an example of magma^crust interaction. Journal of
Petrology 21, 295^321.
Morrison, M. A., Thompson, R. N. & Dickin, A. P. (1985).
Geochemical evidence for complex magmatic plumbing during development of a continental volcanic center. Geology 13, 581^584.
Morse, S. A. (1979). Kiglapait geochemistry II: Petrography. Journal of
Petrology 20, 591^624.
Palacz, Z. A. (1985). Sr^Nd^Pb isotopic evidence for crustal contamination in the Rhum intrusion. Earth and Planetary Science Letters 74,
35^44.
Palacz, Z. A. & Tait, S. R. (1985). Isotopic and geochemical investigation of unit 10 from the Eastern Layered Series of the Rhum
Intrusion, Northwest Scotland. Geological Magazine 122, 485^490.
Park, R. G. & Tarney, J. (1987). The Lewisian Complex: a typical
Precambrian high-grade terrain? In: Park, R. G. & Tarney, J.
(eds) Evolution of the Lewisian and Comparable Precambrian High Grade
Terrains. Geological Society, London, Special Publications 27, 13^25.
Park, R. G., Stewart, A. D. & Wright, D. T. (2002). The Hebridean terrane. In: Trewin, N. H. (ed.) The Geology of Scotland, 4th edn.
Geological Society, London, pp. 45^80.
Renner, R. & Palacz, Z. A. (1987). Basaltic replenishment of the
Rhum magma chamber: evidence from unit 14. Journal of the
Geological Society, London 144, 961^970.
Rhodes, J. M. (1996). Geochemical stratigraphy of lava flows sampled
by the Hawaii Scientific Drilling Project. Journal of Geophysical
Research 101, 11729^11746.
Rhodes, J. M. & Vollinger, M. J. (2004). Composition of basaltic lavas
sampled by phase-2 of the Hawaii Scientific Drilling Project:
Geochemical stratigraphy and magma types. Geochemistry,
Geophysics, Geosystems 5, doi:10.1029/2002GC000434.
Riishuus, M. S., Peate, D. W., Tegner, C., Wilson, J. R. & Brooks, C. K.
(2008). Petrogenesis of cogenetic silica-oversaturated and -
1509
JOURNAL OF PETROLOGY
VOLUME 55
undersaturated syenites by periodic recharge in a crustally contaminated magma chamber: the Kangerlussuaq Intrusion, East
Greenland. Journal of Petrology 49, 493^522.
Roeder, P. L. & Emslie, R. F. (1970). Olivine^liquid equilibrium.
Contributions to Mineralogy and Petrology 29, 275^289.
Rollinson, H. (1993). Using Geochemical Data: Evaluation, Presentation,
Interpretation. Longman.
Saunders, A. D., Fitton, J. G., Kerr, A. C., Norry, M. J. & Kent, R. W.
(1997). The North Atlantic Igneous Province. In: Mahoney, J. J. &
Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic, and
Planetary Flood Volcanism. American Geophysical Union, Geophysical
Monograph 100, 45^93.
Seabrook, C. L., Cawthorn, R. G. & Kruger, J. F. (2005). The
Merensky Reef, Bushveld Complex: Mixing of minerals not
mixing of magmas. Economic Geology 100, 1191^1206.
Shirley, D. N. (1986). Compaction of igneous cumulates. Journal of
Geology 94, 795^809.
Streck, M. J. (2008). Mineral textures and zoning as evidence for open
system processes. In: Putirka, K. D. & Tepley, F. J., III (eds)
Minerals, Inclusions and Volcanic Processes. Mineralogical Society of
America and Geochemical Society, Reviews in Mineralogy and Geochemistry
69, 545^594.
Tait, S. R., Huppert, H. E. & Sparks, R. S. J. (1984). The role of compositional convection in the formation of adcumulate rocks. Lithos
17, 139^146.
Tegner, C., Wilson, J. R. & Brooks, C. K. (1993). Intraplutonic quench
zones in the Kap Edvard Holm layered gabbro complex, East
Greenland. Journal of Petrology 34, 681^710.
Tegner, C., Thy, P., Holness, M. B., Jakobsen, J. K. & Lesher, C. E.
(2009). Differentiation and compaction in the Skaergaard
Intrusion. Journal of Petrology 50, 813^840.
Tepley, F. J., III & Davidson, J. P. (2003). Mineral-scale Sr-isotope
constraints on magma evolution and chamber dynamics in the
Rum layered intrusion, Scotland. Contributions to Mineralogy and
Petrology 125, 628^641.
Thirlwall, M. F. & Jones, N. W. (1983). Isotope geochemistry and contamination mechanics of Tertiary lavas from Skye, northwest
Scotland. In: Hawkesworth, C. J. & Norry, M. J. (eds) Continental
Basalts and Mantle Xenoliths. Shiva, pp. 186^208.
Thompson, R. N. (1982). Magmatism of the British Tertiary Volcanic
Province. ScottishJournal of Geology 18, 49^107.
Thompson, R. N., Esson, J. & Dunham, A. C. (1972). Major element
chemical variation in the Eocene lavas of the Isle of Skye,
Scotland. Journal of Petrology 13, 219^253.
Thompson, R. N., Gibson, I. L., Marriner, G. F., Mattey, D. P. &
Morrison, M. A. (1980). Trace-element evidence of multistage mantle
fusion and polybaric fractional crystallization in the Palaeocene
lavas of Skye, NW Scotland. Journal of Petrology 21, 265^293.
NUMBER 8
AUGUST 2014
Thompson, R. N., Dickin, A. P., Gibson, I. L. & Morrison, M. A.
(1982). Elemental fingerprints of isotopic contamination of
Hebridean Palaeocene mantle-derived magmas by Archaean sial.
Contributions to Mineralogy and Petrology 79, 159^168.
Thy, P. (2003). 2. Igneous petrology of gabbros from Hole 1105A: oceanic magma chamber processes. In: Casey, J. F. & Miller, D. J.
(eds) Proceedings of the Ocean Drilling Program, Scientific Results, 179.
Ocean Drilling Program, pp. 1^76.
Tibaldi, A., Pasquare', A. F. & Rust, D. (2011). New insights into the
cone sheet structure of the Cuillin Complex, Isle of Skye,
Scotland. Journal of the Geological Society, London 168, 689^704.
Wadsworth, W. J. (1982). The major basic intrusions. In:
Sutherland, D. S. (ed.) Igneous Rocks of the British Isles. Chichester:
John Wiley, pp. 415^425.
Wager, L. R. & Brown, G. M. (1968). Layered Igneous Rocks. Edinburgh:
Oliver & Boyd.
Wager, L. R. & Deer, W. A. (1939). Geological investigations in East
Greenland, Pt. III. The petrology of the Skaergaard Intrusion,
Kangerdlugssuaq, East Greenland. Meddelelser om Grnland 105, 1^352.
Weaver, B. L. & Tarney, J. (1980). Rare earth geochemistry of
Lewisian
granulite-facies
gneisses,
northwest
Scotland:
Implications for the petrogenesis of the Archaean lower continental
crust. Earth and Planetary Science Letters 51, 279^296.
Weaver, B. L. & Tarney, J. (1981). Lewisian gneiss geochemistry and
Archaean crustal development models. Earth and Planetary Science
Letters 55, 171^180.
Weedon, D. S. (1961). Basic igneous rocks of the southern
Cuillin, Isle of Skye. Transactions of the Geological Society of Glasgow
24, 190^212.
Wiebe, R. A. (1993). The Pleasant Bay layered gabbro^diorite, coastal
Maine: ponding and crystallization of basaltic injections into a silicic magma chamber. Journal of Petrology 34, 461^489.
Wiebe, R. A. (1994). Silicic magma chambers as traps for basaltic
magmas: the Cadillac Mountain Intrusive Complex, Mount
Desert Island, Maine. Journal of Geology 102, 423^437.
Williamson, I. T. & Bell, B. R. (1994). The Palaeocene lava field of
west^central Skye, Scotland: Stratigraphy, palaeogeography and
structure. Transactions of the Royal Society of Edinburgh: Earth Sciences
85, 39^75.
Yang, S. H., Maier, W. D., Lahaye, Y. & O’Brien, H. (2013).
Strontium isotope disequilibrium of plagioclase in the Upper
Critical Zone of the Bushveld Complex: evidence for mixing
of crystal slurries. Contributions to Mineralogy and Petrology 166,
959^974.
Zhang,Y., Ni, H. & Chen,Y. (2010). Diffusion data in silicate melts. In:
Zhang, Y. & Cherniak, D. J. (eds) Diffusion in Minerals and Melts.
Mineralogical Society of America and Geochemical Society, Reviews in
Mineralogy and Geochemistry 72, 311^408.
1510