University of Birmingham
Local and regional controls on the lateral
emplacement of the Ben Hiant Dolerite intrusion,
Ardnamurchan (NW Scotland)
Magee, Craig; Stevenson, Carl; O'Driscoll, Brian; Petronis, Michael S.
DOI:
10.1016/j.jsg.2012.03.005
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Citation for published version (Harvard):
Magee, C, Stevenson, CTE, O'Driscoll, B & Petronis, MS 2012, 'Local and regional controls on the lateral
emplacement of the Ben Hiant Dolerite intrusion, Ardnamurchan (NW Scotland)' Journal of Structural Geology,
vol 39, pp. 66-82. DOI: 10.1016/j.jsg.2012.03.005
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Journal of Structural Geology 39 (2012) 66e82
Contents lists available at SciVerse ScienceDirect
Journal of Structural Geology
journal homepage: www.elsevier.com/locate/jsg
Local and regional controls on the lateral emplacement of the Ben Hiant Dolerite
intrusion, Ardnamurchan (NW Scotland)
Craig Magee a, *, Carl T.E. Stevenson a, Brian O’Driscoll b, Michael S. Petronis c
a
School of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK
c
Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, PO Box 9000, Las Vegas NM 87701, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 September 2011
Received in revised form
23 February 2012
Accepted 18 March 2012
Available online 27 March 2012
Sub-volcanic and related satellite intrusions record the delivery, storage and accommodation of magma
before eruption. However, when several volcanic centres are in close proximity, the relationship between
centre and satellites may be ambiguous. Here we examine the structure of the Ben Hiant Dolerite satellite
intrusion, discuss its relationship with the Ardnamurchan Central Complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre, the Mull Central
Complex (w35 km to the southeast). Structural field observations and anisotropy of magnetic susceptibility (AMS) fabric analyses reveal that the Ben Hiant Dolerite was emplaced as a series of lobes in
a sequence of stacked sheets. The AMS fabric data further indicate that the Ben Hiant Dolerite intruded
laterally and may have been sourced from either the Ardnamurchan Central Complex or the Mull Central
Complex. Testing either model and discriminating between these two possible source reservoirs remains
an outstanding challenge. However, the potential implications of the lateral movement of magma over
many 10’s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Anisotropy of magnetic susceptibility
Ardnamurchan
Ben Hiant Dolerite
Magma lobe
Regional dyke
1. Introduction
The emplacement of sub-volcanic intrusions and the development of the magmatic pathways that connect them control the
growth of volcanic edifices and, along with magma composition,
eruptive styles (Gudmundsson, 1995; Annen et al., 2001; Sparks,
2003; Corazzato et al., 2008). Understanding magmatic pathway
dynamics and crustal accommodation of sub-volcanic intrusions, as
well as related host rock deformation, therefore contributes
significantly to the assessment of volcanic hazards (Sparks, 2003;
Tibaldi and Pasquarè, 2008). The generally accepted view of subvolcanic systems is that they consist of a series of vertically
stacked major magma reservoirs (e.g. laccoliths) interconnected by
dykes, inclined sheets (cone sheets and ring dykes) and/or sill
conduits with relatively minor lateral magma transfer involved (e.g.
Gudmundsson, 1990; Geldmacher et al., 1998; Kerr et al., 1999;
Zellmer and Annen, 2008). Many major intrusions, however, are
now recognised as having been assembled from discrete magma
* Corresponding author. Present address: Department of Earth Sciences and
Engineering, Imperial College, London SW7 2BP, UK. Tel.: þ44 (0) 20 7594 9983.
E-mail address: [email protected] (C. Magee).
0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsg.2012.03.005
pulses emplaced incrementally (Pitcher, 1979; Glazner et al., 2004;
Stevenson et al., 2007a; Zellmer and Annen, 2008; Annen, 2009).
Collectively, these studies demonstrate that our current understanding of volcano-tectonic processes both chemically and structurally may not be as simple as often assumed.
Several studies have highlighted that lateral emplacement of
stacked laccolithic and sill-like satellite intrusions 25 km from
a central complex (i.e. a sub-volcanic intrusive network) source
may occur (Hyndman and Alt, 1987; Petronis et al., 2004; Horsman
et al., 2005; Stevenson et al., 2007b; Morgan et al., 2008). For
example, the Trachyte Mesa, Black Mesa and Maiden Creek satellite
intrusions were emplaced 8e10 km from the associated Mt Hillers
central complex in the Henry Mountains, Utah (Horsman et al.,
2005; Morgan et al., 2008). Satellite intrusions may thus be
spatially removed from the subsequent volcanic processes active
within central complexes. Therefore, studying the emplacement of
satellite intrusions may provide important information about the
controls on magma flow within sub-volcanic systems, final
emplacement position, magma accommodation and intrusion
geometry (see Glazner et al., 2004; Annen, 2009).
Here we present field observations and structural measurements that are integrated with new anisotropy of magnetic
susceptibility (AMS) data to re-assess the emplacement of the Ben
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Hiant Dolerite (Ardnamurchan, NW Scotland). The Ben Hiant
Dolerite is a satellite intrusion located 2 km to the SE of the main
Ardnamurchan Central Complex, part of the British and Irish
Palaeogene Igneous Province (BIPIP), from which it is considered to
originate (Fig. 1; Geikie, 1897; Richey and Thomas, 1930; Gribble,
1974). Previous emplacement models have suggested that the
satellite intrusion is either a single, homogenous mass (Richey and
Thomas, 1930), an amalgamation of incrementally emplaced
inclined sheets (Geikie, 1897; Gribble, 1974) or a series of lava flows
(Judd, 1874). Although these models account for the geometry and
mapped relationships of the Ben Hiant Dolerite, the actual mechanism of magma emplacement is poorly understood. The aim of
this study is to develop an emplacement model for the Ben Hiant
Dolerite in order to constrain the possible impact and contribution
of this satellite intrusion to volcanic edifice construction. Our
results suggest that the Ben Hiant Dolerite may have been fed from
the Mull Central Complex (Fig. 1), implying that discrete volcanic
centres may influence the growth and evolution of laterally adjacent edifices.
2. Geological setting
The central complexes of the BIPIP, which formed at ca.
61e55 Ma in relation to the opening of the North Atlantic (Emeleus
and Bell, 2005), represent the deeply eroded roots of ancient
volcanic edifices and typically preserve the magmatic pathways
and reservoirs that fed this period of volcanism (Fig. 1a). Studies
resulting from this direct access to the complex sub-volcanic
intrusive network in the BIPIP have, over the last w120 years,
provided many observations and emplacement models (e.g. the
cone sheet and ring dyke emplacement model; Harker, 1904; Bailey
et al., 1924; Richey, 1928; Richey and Thomas, 1930; Anderson,
1936; Stevenson et al., 2007a; Tibaldi et al., 2011) fundamental to
our current understanding of volcano-tectonic processes.
The Ardnamurchan Central Complex (w58 Ma), an archetypal
area for cone sheet intrusion, consists predominantly of numerous
major gabbroic intrusions emplaced as either laccoliths, lopoliths
67
or stacked sheet intrusions into Proterozoic Moinian metasedimentary rocks, well-bedded Mesozoic metasedimentary rocks and
Early Palaeogene (61e59 Ma) basalt lavas and poorly consolidated
volcaniclastic breccias (Fig. 1b; Richey and Thomas, 1930; Day,
1989; Emeleus and Bell, 2005; Brown and Bell, 2006, 2007;
O’Driscoll et al., 2006; O’Driscoll, 2007). Cross-cutting relationships
and the estimated foci of associated cone sheet swarms, traditionally interpreted to originate from a central source at depth,
have been used to suggest the Ardnamurchan Central Complex can
be sub-divided into three temporally and spatially distinct phases
of intrusive activity (Centres 1, 2 and 3 respectively) (Fig. 1b; Richey
and Thomas, 1930; England, 1988; Emeleus and Bell, 2005).
On Ardnamurchan, the major intrusions of Centre 1 are not well
exposed and include the Glas Bheinn Porphyritic Dolerite, interpreted as a dyke-like intrusion; the steeply inclined intrusive
sheets of quartz gabbro and granophyre west of Fascadale; the
composite, gently inclined Beinn nan Leathaid sheet intrusion and
the Ben Hiant Dolerite, the focus of this study (Fig. 1b; Richey and
Thomas, 1930; Emeleus and Bell, 2005). The lithology of the
largely equigranular Ben Hiant Dolerite ranges from olivinedolerite to quartz-dolerite. From the coast, the intrusion margins
can be traced vertically upward to altitudes of 120 m (NW margin)
and w200e300 m (SE margin) where they then become subhorizontal, resulting in an apparent mushroom-shaped geometry
(Fig. 2; Richey and Thomas, 1930). This geometry and lateral
extension of the margins is well demonstrated by the outlier of Ben
Hiant Dolerite at Sròn Mhor and Stallachan Dubha which rests subhorizontally on the Early Palaeogene volcaniclastic host rock
(Fig. 2). To the NE, the Ben Hiant Dolerite becomes more irregular
(Fig. 2) and has been considered to be an amalgamation of multiple
inclined sheets, gently-dipping NW (Geikie, 1897; Richey and
Thomas, 1930).
Various interpretations of the geometry of the Ben Hiant
Dolerite have led to three proposed emplacement mechanisms:
1. From the prevalent well developed columnar jointing and
stepped topography of the Ben Hiant Dolerite, Judd (1874)
Fig. 1. (a) Location map of Ardnamurchan within the British and Irish Palaeogene Igneous Province detailing the positions of central complexes and the NW-SE regional dyke
swarms (after Emeleus and Bell, 2005). The central complexes are labelled; St, St Kilda; S, Skye; R, Rum; M, Mull; B, Blackstones; Ar, Arran; NI, Ireland and Northern Ireland igneous
centres. (b) Geological overview of the Ardnamurchan Central Complex highlighting the positions of the proposed intrusive centres and the relative positions of the individual
Centre 1 intrusions: a) Glas Bheinn Porphyritic Dolerite, b) steep, sheet intrusions west of Fascadale, c) Beinn nan Leathaid composite intrusion and d) the Ben Hiant Dolerite
(redrawn from Richey and Thomas, 1930; Emeleus, 2009).
68
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Fig. 2. Geology of the Ben Hiant Dolerite and the surrounding host rock, combining new observations and previous work by Richey and Thomas (1930) and Emeleus (2009); grid
references refer to the British National Grid co-ordinates. See also Supplementary Geospatial data. The absence of Sheets 1 and 3 in the cross-section at Stallachan Dubha suggest the
defined sheets are laterally restricted and adjacent to each other. This is reflected in the mapped occurrence of Sheet 1 at Sròn Mhor only.
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
proposed the summit of Ben Hiant represents a volcanic
intrusive plug from which a series of lava flows radiate.
2. Geikie (1897) identified that the Ben Hiant Dolerite consists of
several sills, inclined approximately 20 to the N or NW. Some of
these transgress up through the Proterozoic country rock and
exhibit variable grainsizes (Geikie, 1897). From the mineralogical similarity of the Ben Hiant Dolerite with thinner adjacent
dykes and sills (later identified as cone sheets; Richey and
Thomas, 1930), Geikie (1897) proposed that magma from NESW oriented fissures fed both the Ben Hiant sills, in which
magma was principally concentrated, and the surrounding
inclined sheets. Petrographic analyses do not reveal a systematic progression in the compositional evolution of the Ben Hiant
Dolerite from the base to the top of the intrusion but rather
define a series of compositional ‘steps’ of variable thickness,
which has been used to further suggest that the Ben Hiant
Dolerite consists of a series of separate sheets (Gribble, 1974).
The latter work further suggested the Ben Hiant Dolerite sills
were actually amalgamated cone sheets emplaced within the
Ben Hiant volcanic vent, which provided space for inflation and
coalescence. This model implies that magmatic fabrics within
the Ben Hiant Dolerite will be sub-parallel the sheet boundaries
and therefore dip or plunge moderately to the NW.
3. Contrary to Geikie (1897), Richey and Thomas (1930) described
a distinct absence of intrusive contacts. Coupled with the
overall mushroom-shaped geometry, Richey and Thomas
(1930) suggested that the Ben Hiant Dolerite is a single intrusion emplaced vertically (from a dyke) into the pre-existing
Ben Hiant volcanic vent. The along strike extensions of intrusive sheets to the NE of the Ben Hiant summit were suggested
to have been produced by some of the magma being diverted
laterally into un-intruded cone sheet fractures (Richey and
Thomas, 1930). Magmatic fabrics resulting from this emplacement model should therefore be sub-vertical along the coast
and then rotate to sub-horizontal where the Ben Hiant Dolerite
is observed to overly the host rocks.
All three emplacement models imply that the Ben Hiant Dolerite
was sourced from the Ardnamurchan Central Complex and was
intruded into a volcanic vent, originally inferred from the moderately to steeply inwardly inclined contacts and the dominance of
acidic igneous clasts in the Early Palaeogene volcaniclastics (cf.
Richey and Thomas, 1930). Brown and Bell (2006) re-interpreted
the volcaniclastics as high-energy debris flows deposited in
a topographic depression after recognising a much greater
proportion of country rock clasts to igneous clasts than previously
recorded, the presence of pollen spores and various sedimentary
structures. This latter work provides additional evidence that the
Ben Hiant Dolerite was emplaced at a relatively shallow depth
(<3 km).
3. Field relationships and petrography
Field and petrographical observations are described here with
respect to their geographical location, following a similar distinction made by Richey and Thomas (1930). We follow this approach
due to the variation observed between the different areas and the
importance of correlating both observation sets. In broad terms, the
equigranular Ben Hiant Dolerite exhibits a medium-grained
cumulate texture comprising plagioclase, clinopyroxene, ilmenite
and occasionally olivine and apatite (Richey and Thomas, 1930).
Interstitial magnetite, alkali feldspar, quartz and calcite also occur
(Richey and Thomas, 1930). New observations are incorporated
with those recorded by Richey and Thomas (1930) in their memoir
that are pertinent to the present study.
69
3.1. South-west of the Ben Hiant summit
The margins between the Ben Hiant Dolerite and the host rock
are poorly exposed. However, at the SE end of the coastal outcrop,
towards Maclean’s Nose, the Ben Hiant Dolerite displays no chilled
margin and an irregular contact, steeply inclined to the NW, with
the exposed thermally baked volcaniclastic country rock (Fig. 1b
and Fig. 2; Richey and Thomas, 1930). Richey and Thomas (1930)
reported that minor felsic veins cross-cut the Ben Hiant Dolerite
close to the SE margin, although their relationship to the contact is
uncertain. The inferred contact zone can be traced topographically
upwards from the exposures of Ben Hiant Dolerite and volcaniclastics. Similarly, adjacent exposures of Proterozoic Moine metasedimentary rocks and Ben Hiant Dolerite in the cliff section at
Rubha Ailein distinguish the NW margin. The contact is likely
steeply inclined to the SE based on the coastal exposure; although
the contact itself is not visible (Fig. 2). Throughout the cliff face,
including adjacent to the inferred steeply inclined contacts,
columnar jointing is sub-vertical.
Approximately 10 m to the southeast of the NW margin [NM
52234 62816; British National Grid co-ordinates], a contact
between the Ben Hiant Dolerite and the Proterozoic Moine metasedimentary rocks is oriented 062/50 NW (strike and dip; Fig. 3a),
sub-parallel to a minor inclined sheet intrusion (068/45 NW;
strike and dip). At the contact with the Proterozoic Moine metasedimentary rock (Fig. 3a), the Ben Hiant Dolerite is chilled (typically <1 mm grainsize) and contains plagioclase laths (40 vol.%) and
heavily altered subhedral olivine (<0.5 mm; 20 vol.%) often
enclosed within coarse (<3 mm) ophitic clinopyroxene (20 vol.%)
(Fig. 3b). Interstitial magnetite is commonly elongated along
cumulus plagioclase and clinopyroxene grain boundaries (Fig. 3c).
A gradational increase from a medium-grained chill zone to the
coarser (Fig. 3d) interior is observed in the Ben Hiant Dolerite over
the marginal 2 m. The relatively coarse Ben Hiant Dolerite is only
observed along the coast and consists of normally zoned plagioclase laths (<7 mm; 50 vol.%), subhedral clinopyroxene (<3 mm;
40 vol.%) and anhedral magnetite and ilmenite (<3 mm; 10 vol.%).
Compared to the chill zone and the majority of the Ben Hiant
Dolerite, the coarse-grained lithology along the coast is relatively
well equilibrated (Fig. 3d).
Along the coast, between the two steeply inclined lateral
contacts, the Proterozoic Moine metasedimentary rock is restricted
to one outcrop (Fig. 2 and Fig. 3a). In Fig. 2, the Proterozoic Moine
metasedimentary rock has been extrapolated w200 m along the
bay on the basis that where the Ben Hiant Dolerite is exposed along
coast it forms headlands (e.g. Rubha Ailein) (see also Emeleus,
2009). This implies there is a difference in erosion between the
Ben Hiant Dolerite and the country rocks. Within the middle of the
coastal extent of Ben Hiant Dolerite [NM 52558 62380], subhorizontal mineral layering (040/16 W; strike and dip), defined
by the relative proportions of coarse-grained clinopyroxene and
plagioclase phenocrysts, is developed on a decimetre scale.
At higher altitudes on the coastal cliff section, the NW and SE
contacts with the host rocks become sub-horizontal (at 120 m and
w200e300 m, respectively). Along the NW margin the subhorizontal Ben Hiant Dolerite apparently exploits an unconformity between the Mesozoic metasedimentary rocks and the Early
Palaeogene olivine-basalt lavas (Fig. 2).
3.2. Stallachan Dubha and Sròn Mhor
The mapped and topographical expression of the Ben Hiant
Dolerite outlier of Sròn Mhor and Stallachan Dubha suggests that
the basal contact dips gently eastward and is sub-parallel to a preexisting intrusive contact between the volcaniclastics and sub-
70
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Fig. 3. (a) Host rock contact between the chilled Ben Hiant Dolerite and Proterozoic Moine psammite observed along the coast [NM 52234 62816]. (b) Plane polarised light
photomicrograph of the chilled Ben Hiant Dolerite observed in Fig. 3a. (c) Reflected light photomicrograph showing that titanomagnetite shape is controlled by the grain boundaries
of primary silicate framework. (d) Plane polarised light photomicrograph of the coarse Ben Hiant Dolerite observed >2 m from the contact depicted in Fig. 3a. The black arrows
highlight dihedral angles approaching 120 ; characteristic of advanced textural equilibration.
horizontal sheets of porphyritic dolerite and pitchstone (Fig. 2).
Moderately to steeply-inclined columnar jointing (w1 m wide) is
well developed throughout this locality. The Stallachan Dubha
portion of the Ben Hiant Dolerite and the small hill outcrop to its NE
are apparently uniform in both composition and texture
throughout (Fig. 2). In contrast, wide variation is observed on Sròn
Mhor where two gullies (w10 m apart), oriented w147e327 and
defined by a prominent topographic step on their SE side, containing heavily weathered and fractured Ben Hiant Dolerite separate distinct lithologies (Figs. 2 and 4).
The Ben Hiant Dolerite observed at Localities 1 and 2 (Fig. 2 and
Fig. 4a; [NM 53635 62198] and [NM 53646 62208]), is typically
medium to coarse-grained and contains cumulus tabular plagioclase (w60 vol.%; <3 mm), clinopyroxene (w15 vol.%; <4 mm),
heavily altered olivine (w13 vol.%) and fineemedium opaque
crystals (w8 vol.%) (Fig. 4b). At Locality 3 (Fig. 2 and Fig. 4a; [NM
53615 62186]) the columnar joints are narrowly spaced (10 cm) and
contain vugs (<20 mm) infilled with quartz. Petrographically, the
Ben Hiant Dolerite at Locality 3 displays a porphyritic texture
consisting of a variolitic groundmass of very fine acicular clinopyroxene, plagioclase and opaque minerals and some coarsegrained phenocrysts (<2 mm; <1 vol.%) of euhedral plagioclase
and anhedral clinopyroxene. Numerous amygdales (<1.5 mm)
infilled with cryptocrystalline silica, probably of similar origin to
the infilling vuggy quartz, are also observed (Fig. 4c). Within the
two gullies separating Localities 1, 2 and 3 (Fig. 2 and Fig. 4a), the
equigranular Ben Hiant Dolerite is much finer-grained (<0.5 mm)
and contains spherulitic amygdales (<0.3 mm) of very fine radiating acicular chlorite (Fig. 4d).
No brittle deformation was observed associated with the finergrained Ben Hiant Dolerite. It is therefore probable that the
reduced grainsizes observed within the gullies represent chilled
margins delineating separate, thin sheets of Ben Hiant Dolerite. No
palaeosol horizons or other evidence of sub-aerial exposure were
observed between individual sheets. The orientation of the gullies
and columnar jointing suggests that the separate sheets dip
moderately to the SW (Fig. 4a). An intrusive contact, is observed
midway down the south-eastern gully [NM 53607 62217], oriented
at 064/12 SE (strike and dip), suggesting that the Ben Hiant
Dolerite of Locality 3 is part of a cross-cutting later sheet. The
contact is defined by the decreasing width and increasing curvature
of the columnar jointing, particularly above the contact (Fig. 5), and
a decrease in grainsize of the Ben Hiant Dolerite towards the
contact.
3.3. Northern margin
The northern boundary is irregular with finger-like extensions
of Ben Hiant Dolerite protruding into the Proterozoic and Mesozoic
host rock (Fig. 2). North of the Ben Hiant summit there are
numerous isolated outcrops of Mesozoic metasedimentary rocks
(predominantly exposed in stream sections), olivine-basalt lava or
an older porphyritic dolerite within the Ben Hiant Dolerite (Fig. 2).
Their relationship with the topography suggests sub-vertical N-S
boundaries between the host rock screens and Ben Hiant Dolerite
(Fig. 2). One well exposed example occurs along a stream valley
[NM 535 643 to NM 536 638] immediately to the east of Beinn na hUrchrach and shows a w10e50 m wide outcrop of Mesozoic
limestone and shale, with sub-vertical striking contacts, extending
from an altitude of w270 m (i.e. the base of the valley) to an
elevation of w310 m (i.e. the top of the valley) (Fig. 2). At the top of
the stream valley, the Mesozoic host rock is unconformably overlain
by a thin, shallowly SE dipping lens of olivine-basalt lava (Fig. 2).
Towards the base of the stream valley, the host rock outcrop is
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
71
Fig. 4. (a) View of Sròn Mhor depicting locality positions and the intrusive contacts (dashed lines) between individual sheets. Strike and dip bars indicate the approximate dip
direction of each sheet. (b) Photomicrograph, taken under cross-polars, of Ben Hiant Dolerite from Locality 1 showing coarse subhedral plagioclase (Plag) laths and the rounded
olivines (Ol) with altered rims. Clinopyroxene (Cpx) is also present. (c) Variolite photomicrograph, taken under cross-polars, from Locality 3 detailing an amygdale infilled with
cryptocrystalline silica. (d) Plane polarised light photomicrograph of fine acicular plagioclase (Plag), within a medium ophitic clinopyroxene (Cpx), and an amygdale of radiating
chlorite (Chl) and a calcite (Cal) crystal from the gully between Localities 1 and 2.
interrupted by a sub-horizontal sheet of Ben Hiant Dolerite connecting the two masses on either side (Fig. 2). The topographic
relationships with the outer contacts of the Ben Hiant Dolerite on
Beinn na h-Urchrach is suggestive of a w70 m thick, gently SE
dipping sheet bounded at the top by the olivine-basalt lava screen
(Fig. 2). The base of the sheet occurs close to the unconformity
between the Proterozoic and Mesozoic metasedimentary rocks
(Fig. 2) and is inferred from the extrapolation of the sub-horizontal
contact observed at the NW margin of the cliff exposure (see
Section 3.1).
3.4. Overall structure of the Ben Hiant dolerite
Although several inferences are made here, defining the intrusive structure provides an important context for description of the
anisotropy of magnetic susceptibility results. Internal contacts (i.e.
boundaries between separate dolerite sheets) within the Ben Hiant
Dolerite have been identified from the curvature and thinning of
columnar jointing, grainsize reductions (chilled margins) and
amygdale development. Within the main body of the intrusion,
three major intrusive sheets (Sheet 1, Sheet 2 and Sheet 3; Fig. 2)
>70 m thick can be inferred, which may be composed of smaller,
thinner sheets. The base of Sheet 1, or a component sill of Sheet 1,
may be represented by the underlying Proterozoic Moine metasedimentary country rock observed at Camas nan Clacha’ Mora
(Fig. 2 and Fig. 3a). An inferred contact dividing Sheet 1 from Sheet
2 is placed at the transition where the NW outer margin changes
from sub-vertical to sub-horizontal observed towards the top of the
cliff section to the SW of the Ben Hiant summit (Fig. 2). The Sheet 2
and Sheet 3 contact is partially constrained to the NW of the Ben
Hiant summit by the presence of a shallowly SE dipping olivine-
basalt lava lens, the altitude (w350 m) and dip of which allows
the remaining contact trace to be inferred (Fig. 2). As columnar
jointing usually forms orthogonal to the cooling front, which
parallels the intrusion margins (Ryan and Sammis, 1978), the subvertical columnar jointing observed throughout the Ben Hiant
Dolerite is consistent with broadly sub-horizontally oriented the
sheets.
4. Analytical methods
Anisotropy of magnetic susceptibility (AMS) is controlled by the
alignment or distribution of magnetic mineral phases in a rock
sample and can often detect subtle mineral petrofabrics (Khan,
1962; Hargraves et al., 1991; Tarling and Hrouda, 1993).
Numerous studies have demonstrated that the AMS technique
applied to igneous rocks may record primary magma flow (e.g.
Bouchez, 1997; Petronis et al., 2004; Stevenson et al., 2007a,
2007b), in situ crystallisation, post-cumulus modification (e.g.
O’Driscoll, 2007; O’Driscoll et al., 2008) or tectonic strain (e.g.
Borradaile and Henry, 1997; Pressler et al., 2007). The magnetic
susceptibility of each mineral in a sample to magnetisation,
induced by the application of an external applied field, varies
depending primarily on the orientation of the mineral phases
present (shape anisotropy), the crystalline anisotropy of the
mineral, and, in some cases, stress-induced anisotropy (Tarling and
Hrouda, 1993). Therefore, the introduction of a sample in systematically varied orientations into an applied external magnetic field
produces a measurable anisotropic response. In most mafic igneous
rocks, magnetiteeTi-rich magnetite generally dominates the bulk
magnetic susceptibility and the anisotropy is controlled predominantly by the shape and/or distribution of the magnetite crystals
72
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Fig. 5. Intrusive contact between two sheets of Ben Hiant Dolerite defined by the
thinning and curving of columnar jointing. See Fig. 4a for location.
(Tarling and Hrouda, 1993). The magnetic susceptibility tensor can
be visualized as an ellipsoid defined by six independent quantities;
three mutually orthogonal principal susceptibility magnitudes
(K1 K2 K3) and their corresponding set of three directions
(Khan, 1962; Jelínek, 1978). The ellipsoid size (Kmean), shape (m),
strength (H) and the orientations of its axes, referred to as the
magnetic fabric, are often equivalent to the petrofabrics (Knight
and Walker, 1988). The long axis of the ellipsoid (K1) is the
magnetic lineation (L) and the magnetic foliation plane (F) is
defined by the K1eK2 plane, the pole to which is represented by the
K3 axis. For the set of parameters adopted here see Stevenson et al.,
2007a and references therein:
Kmean ¼ ðK1 þ K2 þ K3 Þ=3
(1)
L ¼ ðK1 K2 Þ=Kmean
(2)
F ¼ ðK2 K3 Þ=Kmean
(3)
H ¼ L þ F ¼ ðK1 K3 Þ=Kmean
(4)
m ¼ ðK1 K2 Þ=ðK2 K3 Þ
(5)
m ¼ tan1 m
(6)
In igneous rocks, the magnetic lineation represents the
maximum stretching direction and is typically interpreted to
parallel to the primary magma flow; although several caveats exist
(e.g. Ellwood, 1982; Tauxe, 1998; Geoffroy et al., 2002; Cañón-Tapia,
2004; Cañón-Tapia and Chávez-Álvarez, 2004; Aubourg et al.,
2008). For example, the magnetic response of titanomagnetites is
controlled by the grains’ shape anisotropy and size (Tarling and
Hrouda, 1993). For multidomain (>100 mm grainsize) titanomagnetites, measured K1 axes are parallel to the overall shape long
axis, whereas single-domain titanomagnetites (<1 mm grainsize)
are characterized by a K1 axis orthogonal to the shape long axis
(Tarling and Hrouda, 1993). Titanomagnetites between these two
grainsizes are pseudo-single-domain and display characteristics of
both domain sizes (Dunlop and Özdemir, 1997). From the dependence of principal susceptibility axis orientation on grainsize,
titanomagnetite populations consisting purely of multidomain or
single-domain grainsizes produce ‘normal’ and ‘inverse’ magnetic
fabrics respectively (Rochette et al., 1999; Ferré, 2002). A mixture of
single- and multidomain titanomagnetites may yield intermediate
fabrics, where two of the principal susceptibility axes are switched
(Rochette et al., 1999; Ferré, 2002).
For this AMS study, 41 oriented block samples were collected in
the field (Fig. 6a) and drilled and cut in the laboratory (following
the methods of Owens, 1974), using non-magnetic equipment, to
obtain 4e15 (typically 8) 10 cm3 right cylindrical sub-specimens.
An AGICO KLY-3S Kappabridge (an induction bridge operating at
a field of 300 A/m and a frequency of 875 Hz) at the University of
Birmingham was used to measure the AMS of the sub-specimens,
which were then averaged for each block sample assuming that
the block sample represents a homogeneous multi-normal population (Owens, 2000). Variation within a block is accounted for by
the calculation of 95% confidence ellipses for both directional and
magnitude parameters (Jelínek, 1978).
The magnetic mineralogy of a sample affects the orientation and
intensity of the AMS signal (Tarling and Hrouda, 1993); therefore
identifying the magnetic phases present is key to interpreting the
AMS data and deriving an AMS fabric. Reflected light microscopy
allows the FeeTi oxide components, the likely carriers of AMS in
mafic igneous rocks (Tarling and Hrouda, 1993), to be qualitatively
identified but cannot provide information on the relative contributions of each magnetic mineral phase. High-temperature, lowfield magnetic susceptibility experiments were conducted to
distinguish the magnetic phase(s) contributing to the AMS of six
representative samples (B-4, B-18, B-100, B-103, B-116 and B-117;
Fig. 6a). Experiments on the temperature dependence of low-field
susceptibility for the selected samples were conducted using
a CS3 furnace attachment for the Kappabridge. Stepwise heating/
cooling of the samples from 40 c to 680 c to 40 c in air allows the
magnetic composition to be evaluated from the Curie point
temperature estimates, inferred using the inflection point method
on the heating-cooling curves (Tauxe, 1998). For titanomagnetites,
Curie point estimates may be used to determine the average Ticontent (x) of a sample (Akimoto, 1962; Dunlop and Özdemir,
1997; Lattard et al., 2006). Curie points in high-temperature, lowfield susceptibility experiments are also often associated with
a convex-upwards ‘bump’ in susceptibility, termed a Hopkinson
Peak (Liss et al., 2004). For single-domain grains, the Hopkinson
Peak occurs over a narrower temperature range but results in
a higher susceptibility increase compared to that of multidomain
grains (Dunlop and Özdemir, 1997). High-temperature, low-field
susceptibility curves may therefore provide a proxy for magnetic
domain state (Orlický, 1990; Liss et al., 2004) as well as quantifying
the magnetic mineralogy (Tarling and Hrouda, 1993; Dunlop and
Özdemir, 1997; Tauxe, 1998).
5. AMS results
The AMS sample locations, results and magnetic fabric orientations are displayed in Fig. 6 and in the Supplementary data. The
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
a
54
Inferred intrusive sheet contacts
Fabric shape (µ):
Prolate
Triaxial
Oblate
B-104
B-101
B-109
B-107
B-115
B-121
B-6
B-119
B-118
B-126
N
62
B-16
0
0
B-B
B-C
B-D
B-8
B-123
N
1
2
3
4
B-13
500 m
c
Oblate
1
B-125
B-12
B-1
4
2
B-3
B-2
5
3
B-116
B-117
B-100
Triaxial
6
B-120
B-18
B-22
B-4
7
B-103 B-105
B-28
B-24
Prolate
8
B-108
B-27
B-19
B-20
9
L (%)
64
b
B-E
B-106
B-102
73
5
6
F (%)
7
8
9
B-122
54
N = 38
C = 1, 2, 3, 4% contour intervals
Magnetic lineation (smaller length
represents steeper inclination)
d
32
K1 lineation trend
23
Magnetic foliation
Intrusive contacts
14
89
82
16
61
64
27
41
1
36
80
72 1
16
67
77 31
N
2
5
77
86
69
52
56
21
0
79
87
45
23 83 42
Ben Hiant
89
34
61 85
70
34
56
76
73
10
37
N
62
10
16
3
25
500 m
26
500 m
e
magnetic foliation trend
15
6
80
2
62
30
66
81
18 79
87 21
21 1
40
15
N
11
78 45
23
66
61
80
500 m
Fig. 6. (a) Plot of m (ellipsoid shape) distribution and sample positions. There is a distinct absence of prolate fabrics around the summit and a notable NW-SE trend in strong prolate
fabrics to the SW of the summit. (b) L vs F plot showing the range in fabric shapes, from prolate to oblate. (c) Sketch of the Ben Hiant Dolerite incorporating the AMS lineation and
foliation results. Inset: lower hemisphere equal area stereographic projection of K1 lineations with a pronounced NW-SE trend and shallow-moderate plunges. (d) AMS lineations
highlighting a general NW-SE trend with shallow plunges. (e) Magnetic foliation, occasionally defining a convex-outwards strike to the NW of the intrusion. Note that for both (d)
and (e) the interpreted AMS trends can only be made between samples within the same intrusive sheet.
Kmean values for the Ben Hiant Dolerite range from 1.85 to
19.13 102 (SI) and are consistent with the principal AMS carrier
being a ferromagnetic mineral phase (sensu lato), likely titanomagnetite as suggested by reflected light microscopy (Fig. 3c). This
is further corroborated by high-temperature, low-field magnetic
susceptibility experiments. In addition, these experiments also
show that the three major component sheets of the Ben Hiant
Dolerite may be chemically distinguished based on the variable Ti-
74
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
content (x) of titanomagnetites (Fig. 7). In general, on heating the
low-field susceptibility curves are typically characterised by broad
peaks in susceptibility followed by a rapid decrease in susceptibility. Samples B-4 and B-100 were collected from Sheet 1. Sample
B-4 displays a broad peak in susceptibility at 476 C with
a subsequent inflection (Curie point) at 550 C (Fig. 7a), consistent
with a relatively low-Ti magnetite (x ¼ 0.05) population (Akimoto,
1962; Dunlop and Özdemir, 1997). A continued drop in susceptibility to w680 C suggests a minor amount of subsidiary magnetic
mineral phases, likely fine-grained maghaemite or haematite, are
also present. This is supported by the shift in the cooling curve
(Fig. 7a), which reflects the oxidation and breakdown of a less
magnetic phase during heating (Lattard et al., 2006). Sample B-100
displays a minor Hopkinson Peak at 540 C (Fig. 7b), which also
represents a principal contribution from low-Ti magnetite
(x ¼ 0.07), likely of multidomain to pseudo-single-domain grainsize (Akimoto, 1962; Liss et al., 2004). The broad peak, on heating,
at 300 C (Fig. 7b) suggests the presence of pyrrhotite and/or
3000
2500
2000
1500
1000
0
100
200
300
400
500
Temperature (˚C)
9000
600
1500
1000
0
700
d
B-18
100
200
300
400
500
Temperature (˚C)
8000
600
700
B-103
7000
Susceptibility (10-6 SI)
Susceptibility (10-6 SI)
2000
0
0
7000
6000
5000
4000
3000
2000
6000
5000
4000
3000
2000
1000
1000
0
0
0
100
200
300
400
500
Temperature (˚C)
12000
600
0
700
f
B-116
100
200
300
400
500
Temperature (˚C)
600
700
7000
B-117
6000
10000
Susceptibility (10-6 SI)
Susceptibility (10-6 SI)
B-100
Heating
Cooling
8000
e
2500
500
500
c
b
B-4
Susceptibility (10-6 SI)
Susceptibility (10-6 SI)
a
a maghaemite phase (producing the irregularities on the downward slope) also contribute to the AMS. From Sheet 2,samples B18 and B-103 display Curie point temperatures of 424 C (x ¼ 0.25)
and 407 C (x ¼ 0.28) (Fig. 7c and d), respectively, suggestive of
titanomagnetites with moderate Ti-contents (Akimoto, 1962;
Dunlop and Özdemir, 1997). The significant reduction in susceptibility of the cooling curve for both samples suggests the titanomagnetites were oxidised during heating to produce a less
magnetic mineral phase. Susceptibility peaks at 88 C (x ¼ 0.73)
and 194 C (x ¼ 0.59) on the heating curves of B-116 and B-117
(Fig. 7e and f), taken from Sheet 3, are representative of a Ti-rich
magnetite phase (Akimoto, 1962). The continued drop insusceptibility to w680 C in both samples relates to the exsolution of Tipoor magnetite and Ti-rich ilmenite on heating (Dunlop and
Özdemir, 1997). This is reflected in the shift of the cooling curve.
A lack of strong Hopkinson Peaks in the high-temperature, lowfield susceptibility experiments implies that the titanomagnetite
grains are of a multidomain grainsize (Liss et al., 2004), suggesting
8000
6000
4000
2000
5000
4000
3000
2000
1000
0
0
0
100
200
300
400
500
Temperature (˚C)
600
700
0
100
200
300
400
500
Temperature (˚C)
Fig. 7. High-temperature, low-field susceptibility experiment results. See text for explanation.
600
700
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
the AMS fabrics are ‘normal’ (Rochette et al., 1999; Ferré, 2002).
Combined with the petrographical observation of anhedral intercumulus magnetite occupying the interstices of a plagioclase and
clinopyroxene primocryst ‘framework’, the high-temperature, lowfield susceptibility experiments suggest the AMS fabrics can be
interpreted as the equivalent of the titanomagnetite petrofabric,
itself controlled by the fabric of the primary silicate mineralogy.
Several studies, combining rock magnetic techniques and quantitative thin section image analysis have similarly demonstrated that
late-stage crystallising magnetite crystals use primary silicates (e.g.
plagioclase) as a “template” during grain growth; suggesting the
magnetic fabric likely reflects the petrofabric of the silicate phases
(Cruden and Launeau, 1994; Archanjo et al., 1995; Launeau and
Cruden, 1998). These observations suggest the measured AMS
fabrics may be interpreted as primary magmatic flow fabrics.
The strength of the anisotropy (H) ranges from a very weak
(0.35%) to a strong (11.35%) fabric. Fig. 6b, a plot of lineation (L)
versus foliation (F), indicates a wide range of fabric shapes from
prolate to strongly oblate with a predominance of oblate fabrics.
Some of the strongest oblate fabrics are located along the coast
where mineral layering (040/16 NW) is developed in the Ben Hiant
Dolerite, to which the magnetic foliation (037/26 NW) is subparallel (Figs. 2 and 6). Predominantly, lineation trend and foliation strike are often sub-parallel, particularly if the fabric is strongly
prolate (Fig. 6a and c). Throughout the inferred sheets there is
a preponderance of K1 magnetic lineations with sub-horizontal to
moderate plunges (68% are <45 ) and a NW-SE trend (Fig. 6c).
However, to the north of the Ben Hiant Dolerite, the magnetic
lineations are oriented more towards the N-W (Fig. 6d). There
appears to be little correlation between the AMS fabrics and the
exposed Ben Hiant Dolerite boundaries, particularly adjacent to the
steeply inclined Sheet 1 margins, suggesting that the crystallising
magma did not interact strongly with or flatten against the country
rock margins (Fig. 6c).
The identification of separate intrusive sheets of Ben Hiant
Dolerite in the field requires that the magnetic fabrics be interpreted with caution, such that correlation of AMS fabrics at individual locations (i.e. Fig. 6d and e) should only be made if the
samples are from the same intrusive sheet. The magnetic foliations,
and to a lesser extent the magnetic lineations, in Sheet 1 are
moderately inclined inwards adjacent to the NW and SE margin and
sub-horizontal in the centre of the coastal outcrop; describing
a concave-up geometry (Fig. 8a). In some areas of Sheet 2,
convergent lineation trends are observed (Fig. 6d). Towards the NW
margin of the Ben Hiant Dolerite (Sheet 2), the magnetic foliations
define a convex-outwards (towards the NW) strike (Fig. 6e). Along
the SE margin of the main Ben Hiant Dolerite body, within Sheet 2,
NW
SE
B-4 B-100
B-3
B-2
B-1
Sheet 1 - Ben
Hiant Dolerite
Magnetic foliation
dip
Host rock
Inferred magnetic
foliation trace
Inferred
normal faults
Fig. 8. (a) Schematic cross-section through the vertical cliff exposure to the SW of the
Ben Hiant summit. Magnetic foliations from the coastal AMS samples are transposed
onto the section and an inferred trace extrapolated to highlight fabric deflection
associated with the margins.
75
a similar rotation in magnetic foliation strikes is present (Fig. 6e).
The fabrics of Sròn Mhor and Stallachan Dubha are also slightly
variable but define a sub-horizontal to moderately plunging NW-SE
trend to the lineations (Fig. 6c). Sheet 3 contains magnetic fabrics
that are more variable (Fig. 6c).
6. Discussion
6.1. Construction of the Ben Hiant dolerite
6.1.1. Incrementally emplaced sub-horizontal sheets
Previous emplacement models of the Ben Hiant Dolerite are
based on the interpretation of Ben Hiant as a volcanic vent complex
and suggest that magma originated from the Ardnamurchan
Central Complex. However, the volcanic vent interpretation has
been brought into question by the work of Brown and Bell (2006,
2007) who reclassified the volcaniclastic breccias as high-energy
debris flow deposits formed on the flanks of a central volcano. In
the present study, several dolerite sheets of variable thicknesses
(20 cm to >70 m) have been defined structurally and magnetically
(i.e. titanomagnetite Ti-content variations and variations in overall
magnetic lineation trends). The main body of the intrusion has
been divided into Sheets 1, 2 and 3. Minor intrusive sheets are also
defined, particularly at Stallachan Dubha and Sròn Mhor where
they are thinner (w10e20 m) and more laterally restricted
compared to the major sheets observed within the main body of the
Ben Hiant Dolerite. The absence of palaeosol horizons between the
observed dolerite sheets and the non-radial NW-SE sub-horizontal
to moderately plunging magnetic lineation pattern suggest that the
Ben Hiant Dolerite does not consist of a series of radiating lava
flows as proposed by Judd (1874), but rather that it is an intrusive
mass. Identification of discrete sheets (Fig. 2) also implies that the
Ben Hiant Dolerite is not a single homogeneous intrusion (cf. Richey
and Thomas, 1930). Furthermore, the geometrical form of the
stacked sheets, the gently plunging (2e25 ; Fig. 6c) magnetic
lineations, prevalent sub-vertical columnar jointing and the presence of underlying Proterozoic Moine metasedimentary host rock
along the coast are not consistent with Sheet 1 representing
a vertical feeder zone as suggested by Richey and Thomas (1930).
Our findings favour the stacked intrusive sheet model proposed by
Geikie (1897), although Geikie (1897) suggested the intrusive
sheets dipped N to NW whereas our field observations indicate an
overall gentle eastward dip.
The spatial and temporal variation in titanomagnetite composition (i.e. Ti-content increases with topographic height in younger
sheets) potentially reflects replenishment of the source reservoir or
progressively increasing oxygen fugacity conditions in successive
magma pulses associated with an evolving magma chamber (cf.
Lindline et al., 2011). Given the overall silicate mineralogical similarities observed throughout the Ben Hiant Dolerite, it is unlikely
the observed pattern of titanomagnetite compositional variation is
an indicator of magma differentiation; a similar conclusion reached
for early Miocene Cieneguilla basanite in the northern Rio Grande
Rift, New Mexico, USA (Lindline et al., 2011).
To strengthen the interpretation that the Ben Hiant Dolerite is
constructed of vertically stacked, gently SE dipping intrusive sheets,
a geologically valid explanation for the steeply inclined margins of
Sheet 1 (Fig. 2) is required. Within Sheet 1 the Ben Hiant Dolerite is
relatively coarse-grained, compared to Sheets 2 and 3, and contains
mineral layering as well as cross-cutting felsic veins adjacent to the
SE lateral margin. From these observations, Richey and Thomas
(1930) suggested that the lateral contacts are primary and that
the coastal Ben Hiant Dolerite (here reclassified as Sheet 1) represents a cumulate rock that cooled relatively slowly. However, the
Ben Hiant Dolerite exposed adjacent to the SE lateral margin of
76
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Sheet 1 is unchilled, contrasting with all other exposed primary
contacts throughout the intrusion where chilled margins are
prevalent. In addition, magnetic lineations (interpreted to parallel
the primary magma flow axis) are predominantly oriented NW-SE,
the magnetic foliations are discordant at a moderate angle to the
steeply inclined contacts (Fig. 8) and sub-vertical columnar joints
are consistently displayed. These observations suggest that the
contacts are not primary and that the Ben Hiant Dolerite has been
juxtaposed against the volcaniclastic country rock. For example,
columnar jointing should form orthogonally to a primary contact
(Ryan and Sammis, 1978).
On closer inspection, the moderately dipping, inwardly inclined
magnetic foliations of the marginal AMS samples (i.e. B-1, B-4 and
B-100) appear to be deflected from the sub-horizontal magnetic
foliation (Samples B-2 and B-3) and magmatic layering dips
measured within the centre of the coastal outcrop (Fig. 8). This
concave-upwards geometrical relationship is reminiscent of
normal fault drag folds (see Twiss and Moores, 2007, their
Fig. 3.11b) and suggests that both lateral contacts may be normal
faults. Potential development of the two faults whilst Sheet 1 was
still at a high temperature, immediately after solidification, may
also explain why the dolerite is unchilled (i.e. the chilled margin
sheared off during faulting) yet the adjacent country rock is thermally altered. Although the cross-cutting felsic veining in the Ben
Hiant Dolerite adjacent to the SE margin may be interpreted as
evidence of country rock partial melting associated with a primary
contact (Richey and Thomas, 1930), the age relationship between
the veining and potential faulting is unknown.
Field observations and AMS data appear to suggest that the
steep NW and SE lateral margins of Sheet 1 may be normal faults.
The orientation of the two inferred NE-SW striking normal faults is
orthogonal to the expected orientation of normal faults formed in
an extensional regional stress field where s3 is oriented NE-SW (i.e.
the BIPIP; England, 1988). Instead, these two faults and some other
normal faults observed on Ardnamurchan strike tangentially to the
Ardnamurchan Central Complex, suggesting fault formation or
reactivation was related to extensional volcano-tectonic processes
(cf. Day, 1989).
As the topographically higher intrusive sheets of Ben Hiant
Dolerite (i.e. Sheet 2 and 3) are more laterally extensive than Sheet
1, apparently cross-cutting the proposed north-western normal
fault, we argue that emplacement of the Ben Hiant Dolerite may be
divided into two principal intrusive phases (i.e. phase 1 corresponds to Sheet 1 intrusion and phase 2 represents the period of
Sheet 2 and 3 emplacement) by an intervening period of extensional faulting (Figs. 2 and 9). These distinct events may reflect
concomitant caldera forming processes active within the Ardnamurchan Central Complex. Volcanic edifice evolution is often
cyclical, with inflation driven by the intrusion of igneous bodies
(e.g. Sheet 1) and consequent eruptions resulting in the deflation
(Gudmundsson, 1998a; Walter and Troll, 2001). In some volcanotectonic settings, instabilities in the edifice caused by deflation
result in extensional caldera collapse and peripheral normal fault
development (e.g. Walter and Troll, 2001). Continuation of the
magma supply to the volcanic edifice essentially re-inflates the
edifice.
6.1.2. The internal structure of Sheet 2
To the north of the Ben Hiant summit, several linear (N-S),
vertically bounded screens of host rock occur within Sheet 2. For
example, one outcrop of Mesozoic metasedimentary rock extends
from the base to the top of a stream valley immediately to the east
of Beinn na h-Urchrach (Fig. 2). The screen is cut at its base by a subhorizontal sheet of Ben Hiant Dolerite but at its top is attached to
the stratigraphically younger olivine-basalt lavas, possibly through
a primary sub-horizontal unconformity (Fig. 2 and Fig. 9c). The
isolated outcrops of host rock to the north of the Ben Hiant summit
display a similar linear arrangement and may represent another
host rock screen; albeit one that has been cross-cut multiple times
by the Ben Hiant Dolerite. We suggest that the host rock screens
developed between two separate segments of Ben Hiant Dolerite
and originally formed an intact bridge between the host rock below
and above the intrusion plane (cf. Hutton, 2009; Schofield et al.,
2010). This is supported by the unconformity observed between
the Mesozoic metasedimentary rocks and the olivine-basalt lavas at
the top of the stream gully. The coalescence of the Ben Hiant
Dolerite segments that occurs in several places (e.g. the base of the
stream gully) likely reflects the exploitation of fractures generated
during deformation of the bridge as the individual segments
inflated (cf. Hutton, 2009; Schofield et al., 2010). This process
results in the formation of broken bridges and bridge xenoliths (e.g.
Fig. 9a).
Magnetic fabrics within the Ben Hiant Dolerite adjacent to the
host rock bridge remnants are typically moderately to strongly
prolate and probably relate to the higher finite strains formed due
to frictional drag of flowing magma adjacent to the magma-bridge
contact (Horsman et al., 2005). Subsequently the maximum magma
stretching direction, or flow direction, is parallel to the contact
(Morgan et al., 2008). Several studies have also shown that the long
axes of broken bridges and bridge xenoliths can be used to roughly
infer the primary magma flow axes (Rickwood, 1990; Hutton, 2009;
Schofield et al., 2010). Here, the bridges trend approximately N-S,
sub-parallel to the local K1 lineations and magnetic foliation strikes,
further supporting interpretation of the magnetic fabric as
a magma flow fabric. Based on this relationship between magnetic
fabric styles and contact zones, we suggest that a conspicuous zone
of strongly prolate (i.e. samples B-24, B-28, B-29, B-101 and B-123;
Fig. 6a), sub-horizontal NW-SE trending magnetic lineations and
parallel magnetic foliation strikes, observed to the west of the Ben
Hiant summit, represents an internal boundary within the Ben
Hiant Dolerite (Fig. 9a).
With distance from the interpreted internal contacts within
Sheet 2, the magnetic foliation strikes rotate from contact parallel
to an orientation orthogonal (i.e. NE-SW) to the contact (Fig. 6e).
Although data coverage is relatively sparse, there is a suggestion
that this rotation is progressive and to the NW margin of the Ben
Hiant Dolerite describes a convex north-westwards geometry
(Fig. 6e and Fig. 9a). Similarly, magnetic foliation strikes rotate away
from an orientation parallel to the lobeelobe contact towards the
SE margin of the main Ben Hiant Dolerite body, although no strikes
orthogonal to the inferred NW-SE magma flow trend were
observed. This rotation of foliation strikes to an orientation
orthogonal to the magma flow axis is consistent with divergent
magma flow (i.e. not restricted by proximal contacts) and similar
geometries have been observed near to parallel bulbous magma
lobe terminations in field examples (e.g. Cruden et al., 1999;
Stevenson et al., 2007b; Morgan et al., 2008), as well as in analogue
models (Kratinová et al., 2006). The inward dip of the magnetic
foliations is consistent with sampling in close proximity to the base
of Sheet 2 (cf. Cruden et al., 1999; Horsman et al., 2005).
The interpretation of several sub-vertical contacts, trending
approximately NNW-SSE, and variations in AMS fabrics suggests
that Sheet 2 consists of at least four incrementally emplaced
magma lobes. Examples of magmatic lobes have been identified in
tabular intermediate-felsic intrusions (e.g. Dinkey Creek pluton,
USA, Cruden et al., 1999; Maiden Creek Sill, USA, Horsman et al.,
2005; Trawenagh Bay Granite, Ireland, Stevenson et al., 2007b)
and in seismically imaged sills (e.g. Rockall Trough, Thomson and
Hutton, 2004; Solsikke lobate sill, Hansen and Cartwright, 2006).
Our results suggest that AMS fabrics within each individual lobe
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
77
Fig. 9. (a) Division of the most extensive sheet, Sheet 2, into magmatic lobes that propagated in a north-westward direction. (b) Section through the possible primary magma feeder
tubes. Note their convex-upwards geometry and tilting (white arrow) of phase 1 magmatic sheets by phase 2 intrusion. (c) Cross-section through the lobes detailing their inflated
form and the geometry of the broken bridge and bridge xenoliths.
reflect primary magmatic flow fabrics. Preservation of primary
magmatic flow fabrics within individual lobes of Ben Hiant Dolerite
implies that internal boundaries were maintained after coalescence
and no magma mixing or crystallisation processes occurred
between lobes after emplacement.
6.2. Magma flow direction and source
The supposition that the Ben Hiant Dolerite magmas are derived
from the Ardnamurchan Central Complex is central to the models of
Geikie (1897) and Gribble (1974). The central assumption of these
models arises from the proximity of the Ben Hiant Dolerite to the
Ardnamurchan Central Complex and is consistent with NW-SE
oriented magnetic lineations reported in the present study.
Furthermore, the sub-horizontal to moderately plunging magnetic
lineations and gently SE dipping sheeted form are consistent with
a lateral magma flow pattern. Gribble (1974) suggested that the Ben
Hiant Dolerite represents an amalgamation of Centre 1 cone sheets
that dip NW. Whilst the gently-dipping sheets of the Ben Hiant
Dolerite do not conform to this model, the cone sheets are potential
feeder conduits. There are two possible candidates within the Ben
Hiant Dolerite that may represent cone sheet feeders and should
therefore contain magma flow indicators oriented NW-SE; the Ben
Hiant Dolerite observed along the coast adjacent to the NW margin
(i.e. sample B-4 and B-100; Fig. 6), which dips parallel to the local
cone sheets (Fig. 2 and Fig. 3a), and the shallowly dipping protrusion of Ben Hiant Dolerite to the NE (i.e. sample B-E; Fig. 6).
However, the resolvable magnetic lineations at these locations
plunge gently to either the NE (i.e. B-100) or ESE (i.e. B-E), implying
these areas are not likely to represent feeder zones (Fig. 6).
Although the NW-SE trending magnetic lineations are consistent with magma possibly being fed laterally from the Ardnamurchan Central Complex to the northwest, this observation does
not preclude a source to the southeast. Magnetic lineation trends
and sub-parallel broken bridge axes in the north of the Ben Hiant
Dolerite are oriented more N-S compared to the general NW-SE
trend. This might suggest that the lobes and magma flow
branches from a point in the southeast (Fig. 9a; cf. Thomson and
Hutton, 2004). These observations suggest that the Ben Hiant
Dolerite magma propagated in a north-westward direction from
a source area located to the southeast (Fig. 6e and Fig. 9). This may
be supported by the convex north-westward foliation trends
within the lobes, which would mimic the frontal lobe geometry
expected from magma flow in a north-westward direction (cf.
Cruden et al., 1999; Thomson and Hutton, 2004; Horsman et al.,
2005; Stevenson et al., 2007b; Hansen and Cartwright, 2006).
The possibility that magma flow was directed north-westward
suggests that the laterally restricted sheets observed at Stallachan
Dubha and Sròn Mhor may represent primary magma feeder tubes.
Primary magma transport tubes feeding magma lobes are
commonly observed to feed magma lobes and are comparatively
thinner, elongated in the magma flow direction and develop
a convex-upwards roof (e.g. Pollard et al., 1975; Thomson and
Hutton, 2004; Miles and Cartwright, 2010). Alternatively, if
magma flowed towards the southeast, these sheets may instead
denote terminal offshoots originating from the main Ben Hiant
78
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Dolerite body. Both models are consistent with the gently plunging,
NW-SE trending magnetic lineations measured in both the Stallachan Dubha and Sròn Mhor sheets and in the main body of the Ben
Hiant Dolerite to the northwest, which suggest that the two were
once linked by a subsequently eroded section of Ben Hiant Dolerite.
However, the age relationships and geometry of the Stallachan
Dubha and Sròn Mhor sheets are difficult to determine. Secondary
to the overall gentle SE dip of the Stallachan Dubha and Sròn Mhor
Ben Hiant Dolerite masses, the stepped topography of the gullies on
Sròn Mhor (Fig. 4a) suggests three component sheets have
a moderate SW dip (Fig. 9b). These sheets are cross-cut by a later
intrusive sheet on Sròn Mhor inclined at 064/12 SE (strike and
dip); similar to the orientation of the Stallachan Dubha mass (Fig. 2
and Fig. 9b). It is suggested here that the moderately SW dipping
sheets belong to the older phase 1 and were tilted, from an original
gentle SE dip, to their current position by the intrusion of the later
phase 2 Stallachan Dubha Ben Hiant Dolerite body. The crosscutting sheet, oriented 064/12 SE (strike and dip), on Sròn Mhor
is also attributed to phase 2 (Fig. 9b). Alternatively, the sheets
moderately dipping to the SW may be dyke-like intrusions associated with the Ben Hiant Dolerite.
Two potential sources for the Ben Hiant Dolerite therefore exist.
Firstly, a source within the Ardnamurchan Central Complex may be
postulated if some of the transgressive Centre 1 cone sheets
extending to the southeast of Ben Hiant (Emeleus, 2009) fed the
Ben Hiant Dolerite; assuming the cone sheet themselves were
centrally sourced (Richey and Thomas, 1930). This implies that the
radiating upward-and-outward cone sheets (cf. Palmer et al., 2007)
were reoriented into north-westwardly propagating sills or magma
tubes (Fig. 10). Richey and Thomas (1930) highlight geochemical
similarities between the Ben Hiant Dolerite and the cone sheets,
which may support this hypothesis, although they also note that
there are distinct mineralogical differences. Similar examples of
inclined sheets feeding sills are observed in the field (e.g. Burchardt,
2008) and in seismic data (Smallwood, 2008). However, the relatively thin (typically <1 m) Ardnamurchan cone sheets display
well-developed chilled margins (Richey and Thomas, 1930), suggesting that they did not form magma conduits capable of
supplying the major Ben Hiant Dolerite intrusion.
The second possibility, which we tentatively prefer, is that the
AMS data and field observations presented here are consistent with
the Ben Hiant Dolerite being fed by NW-SE striking regional dykes
originating from the Mull Central Complex to the SE (Fig. 11). In this
scenario, the Ben Hiant Dolerite magma flowed laterally through
the regional dykes and into primary magma transport tubes
preserved at Sròn Mhor and Stallachan Dubha, from which
Fig. 10. Possible reconstruction of the emplacement of Sheet 2 from a cone sheet
assumed to be fed from a central source at depth beneath the Centre 1 foci. Extrapolated traces of Sheets 1 and 3 have not been included for clarity.
individual lobe buds diverged, and propagated for approximately
1.5 km before subsequently inflating (Figs. 9 and 11). The regional
dykes located in southern Scotland and northeast England (e.g. the
Cleveland dyke) extend over 600 km from the Mull Central
Complex, with which they share a geochemical affinity and are
interpreted to have laterally propagated from (Jolly and Sanderson,
1995; Macdonald et al., 2009, 2010). Speight et al. (1982; their
Fig. 33.3) mapped an apparent reduction in the percentage dilation
of regional dykes between Ardnamurchan and Mull. Although this
potentially suggests laterally propagating regional dykes extending
to the northwest of the Mull Central Complex were deflected
around Ardnamurchan through Morvern and Coll, the observed
percentage dilation distribution may alternatively reflect the distal
effects of a stress field interference pattern about Ardnamurchan
and an inhibition to significant crustal extension imparted by the
Ardnamurchan volcanic edifice (cf. Walker, 1992). This latter effect
may also explain the preponderance of coherent cone sheet
intrusions on Ardnamurchan compared to dyke intrusions (cf.
Walker, 1992). Importantly, Speight et al. (1982; their Fig. 33.1)
identified over 300 outcrops of NW-SE trending regional dykes that
connect the Ardnamurchan and Mull central complexes. It therefore seems plausible that regional dykes propagating laterally to
the northwest from the Mull Central Complex interacted with the
Ardnamurchan Central Complex.
Although analytical comparison between the Ben Hiant Dolerite
and the Palaeogene regional dyke swarm is beyond the scope of this
study, a potential approach to test the hypotheses presented above
is through geochemical and isotopic study. The limited existing
geochemical and isotopic data for the Ben Hiant Dolerite is insufficient to correlate the intrusion with either the Ardnamurchan or
Mull Central Complexes. However, several inferences can be made.
Whole rock major and trace element analyses of rock glass within
the Ben Hiant Dolerite, conducted to determine the composition of
the primary magma, highlight comparable similarities in major
element composition to the ‘Non-porphyritic Magma-type’ of the
Mull central complex (Gribble, 1974), now renamed the Central
Mull Tholeiite Magma-type (Emeleus and Bell, 2005). In the current
absence of a more detailed geochemical and isotopic study of the
Ben Hiant Dolerite and the Mull regional dyke swarm, this correlation tentatively suggests that a model involving laterally propagating regional dykes from the Mull magmatic system sourcing the
Ben Hiant Dolerite (w35 km NW of the Mull Central Complex) is
plausible.
6.3. Controls on emplacement position
A multitude of numerical studies have shown that intrusive
sheets are preferentially emplaced orthogonal to the least
compressive principal stress axis (i.e. s3; Anderson, 1951) when the
internal magma pressure exceeds the magnitude of s3 (Rubin,
1995). Traditionally, sheet intrusion is considered to be controlled
by either a local or regional stress field (Anderson, 1936, 1951;
Gautneb and Gudmundsson, 1992; Gudmundsson, 1998b, 2006;
Geshi, 2005; Acocella and Neri, 2009). Analysis of sheet geometry
may inherently allow the principal stress axes active during
emplacement to be reconstructed (Jolly and Sanderson, 1997).
The approximate sub-horizontal orientation of the Ben Hiant
Dolerite sheets implies that s3 was oriented sub-vertically during
intrusion. Throughout the evolution of the BIPIP the regional
stress field s3 axis was sub-horizontal and oriented NE-SW
(England, 1988). The distribution and nature of the Centre 1
intrusions (e.g. the major intrusions and the cone sheets) are
consistent with a syn-intrusive local compressional stress regime
generated by an inflating central major magma reservoir, 3e5 km
below the present surface (Richey and Thomas, 1930; Anderson,
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
a
Possible magma
feeder tubes
SE
NW
79
E
Sills dipping gently SE
Individual lobe position controlled
by position of previous lobes
Regional dykes
b
NW
Normal faults
SE
E
Normal faults - heave
and throw unknown
c
NW
SE
c. i
Phase 2 Ben Hiant Dolerite
sills cut faults
Basalt lens
E
Present erosion level
Key
Ben Hiant Dolerite
(Phase 2)
Ben Hiant Dolerite
(Phase 1)
Broken bridge of
Mesozoic metasedimentary rock
Magmatic lobes coalesce
to form a sheet
c. ii
E
Basalt lens
Olivine basalt
lavas
Mesozoic limestones, shales
and sandstones
Normal
faults
Lateral magma flow regime propagating from the SE
Phase 1 magma tubes rotated by
inflating phase 2 magma tube
Fig. 11. Simplified potential emplacement model of the Ben Hiant Dolerite involving laterally propagating regional dykes (a) that feed sills through an intermediary magma tube.
Initially sill growth is lateral. (b) Normal faulting downthrows the central portion of the sills. (c) The second phase involves intrusion and inflation of sills, sourced from regional
dykes and divided into discrete magmatic lobes. Later reactivation of the SE fault displaces the second phase sheets.
1936). Gudmundsson (1998a, 1998b, 2006) numerically modelled
the local principal stress axes generated by overpressured magma
chambers, with spherical or sill-like geometries, within a regional
extensional setting similar to the BIPIP. From these models it is
apparent that sub-vertical s3 trajectories may be developed
superjacent to the upper margins of the magma chamber
(Gudmundsson, 1998b, 2006). Given that the Ben Hiant Dolerite is
a satellite intrusion located w2 km laterally to the SE of Ardnamurchan and was emplaced at a higher stratigraphic level
compared to the Centre 1 magma reservoir (Richey and Thomas,
1930), it is unlikely the local compressive s3 principal stress axes
during its intrusion was oriented sub-vertically. Both the local and
regional stress field s3 axes, active during the emplacement of the
Ben Hiant Dolerite, are therefore inconsistent with the subhorizontal sheet orientations observed. Although this disparity
may result from post-emplacement deformation (i.e. the Ben
Hiant Dolerite sheets were rotated into a sub-horizontal orientation), the horizontal, older olivine-basalt lavas adjacent to the Ben
Hiant Dolerite (Fig. 2) suggest that little or no tilting occurred
locally.
Instead, we suggest that significant rigidity contrasts within
the host rock in the Ben Hiant region controlled magma
emplacement. Numerous studies, based on field observations
(Horsman et al., 2005; Morgan et al., 2008; Burchardt, 2008;
Thomson and Schofield, 2008; Schofield et al., 2010) and analogue
experiments (Kavanagh et al., 2006; Menand, 2008), have highlighted the importance of such rigidity contrasts in controlling
sheet intrusion emplacement. In the Ben Hiant area, these rigidity
contrasts are thought to relate to the juxtaposition of mechanically different lithologies via the development of unconformities
between the Proterozoic Moine metasedimentary rocks and the
overlying Mesozoic limestones and shales as well as Early
80
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
Palaeogene poorly consolidated volcaniclastics and weathered
olivine-basalt lavas (Fig. 2; Richey and Thomas, 1930; Emeleus and
Bell, 2005; Brown and Bell, 2006). As the Ben Hiant Dolerite
magma flowed into the area (regardless of its source position or
flow direction) and interacted with the unconformities it is likely
the sheet intrusion reoriented to exploit the host rock planes of
weakness (Fig. 2). This reorientation may have been facilitated by
local perturbations in the active stress field associated with either
sheet emplacement (e.g. Chadwick and Dieterich, 1995) or interference between the local and regional stress field (e.g. Geshi,
2005). Important mechanical anisotropies may also have been
generated between the country rock and previous sheet intrusion
of Ben Hiant Dolerite. The vertical stacking of the Ben Hiant
Dolerite sheets is consistent with intrusion of successive magma
pulses along lithological planes of weakness developed between
the host rock and previously intruded sheets of Ben Hiant
Dolerite.
6.4. Correlations and implications for other areas
The observations and data presented suggest the possibility that
the Ben Hiant Dolerite may have been fed by regional dykes, likely
originating from the Mull Central Complex w35 km SE of Ardnamurchan, that propagated laterally to the NW and were reoriented
into sub-horizontal sheets as they exploited local rigidity contrasts.
Local host rock (e.g. rigidity contrasts between unconformities) and
stress field (e.g. instigating the normal faulting) conditions probably favoured and accommodated construction of a vertically
stacked sheet intrusion rather than continued propagation. Several
studies have identified other laccolithic and sill-like intrusions that
are fed by laterally propagating, extensive regional dykes (e.g. Ernst
et al., 1995). Hyndman and Alt (1987) describe eight satellite
laccoliths fed by radial dykes originating from the Adel Mountains
(Montana) volcanic pile located 8e23 km to the south. The individual laccoliths have length (0.5e5 km parallel to the flow direction axis) and thickness ranges (80e200 m) (Hyndman and Alt,
1987) similar to the sheets of the Ben Hiant Dolerite (<1.5 km
length and w70 m thick). In contrast to the Ben Hiant Dolerite,
which may have been fed through intermediary primary magma
tubes, Hyndman and Alt (1987) observed the Adel Mountain
laccoliths formed by ‘roll-over’ of the radial dykes. Two other
examples include: laterally propagating dykes of the MacKenzie
dyke swarm (Canada) feeding sill-complexes 800e1500 km away
from the plume head source (Ernst et al., 1995) and; the Paraná sills
in South America, which White (1992) similarly suggested were fed
by extensive regional dykes. These examples suggest satellite
intrusions and whole intrusive networks (e.g. sill-complexes) may
have been emplaced at significant lateral distances from their
magma source.
The observed controls on emplacement of the Ben Hiant
Dolerite, the host rock structure and local stress field, are consistent with similar conclusions derived from field studies (Geshi,
2005; Burchardt, 2008; Schofield et al., 2010) and experimental
modelling (Gudmundsson, 1998b, 2006; Kavanagh et al., 2006;
Menand, 2008). Attributing the Ben Hiant Dolerite to the Mull
Central Complex, rather than the Ardnamurchan Central Complex,
implies that close spatial relationships may be insufficient in
determining magma sources for satellite intrusions and that the
importance of lateral magma transport between adjacent volcanic
systems may be greater than previously considered. Several
studies have identified geochemically distinctive lavas around
volcanoes on Hawaii (i.e. Kilauea; Rhodes et al., 1989) and in the
Galapagos (i.e. Ecuador and Wolf volcanoes; Geist et al., 1999) that
were laterally sourced from adjacent volcanoes <20 km away.
Geist et al. (1999) noted that the laterally sourced lavas were
erupted from dykes at the base of the volcano flank facing the
adjacent source area. Similarly, Kervyn et al. (2009) highlight that
peripheral vents to active volcanoes are often located at the base of
volcano flanks. This study suggests that the position of satellite
intrusions, possible feeders of peripheral eruptive vents, may be
strongly controlled by pre-existing host rock structure regardless
of their source position.
7. Conclusion
Field observations reveal that the Ben Hiant Dolerite is constructed from incrementally emplaced intrusive sheets stacked
vertically, in agreement with the emplacement models proposed by
Geikie (1897) and Gribble (1974). Implicit in previous models is the
assumption that the Ben Hiant Dolerite was fed from the Ardnamurchan Central Complex located w2 km to the northwest. We
have used AMS analysis to constrain the internal structure of the
Ben Hiant Dolerite and identify a NW-SE trending, subhorizontalemoderately plunging primary magma flow pattern.
Several magmatic lobes, the primary component of the intrusive
sheets, are defined by broken bridges, zones of high finite strain
associated with frictional drag of magma and convex striking
magnetic foliations. In contrast to previous models our results
suggest a potential magma source to the southeast. This is supported by (1) the branching nature of the lobes and magnetic
lineations and (2) the convex magnetic foliation strikes within the
magmatic lobes. Although, the north-westerly, shallowly plunging
primary magma flow regime identified from these data may be
linked to a centrally sourced cone sheet feeding a sill-like intrusion,
it is also plausible that laterally propagating regional dykes originating from the Mull Central Complex located w35 km to the
southeast fed the Ben Hiant Dolerite. Regardless of source position,
we suggest that the host rock lithologies and pre-existing structure
controlled both the position and geometry of the Ben Hiant
Dolerite.
The emplacement models presented here for the Ben Hiant
Dolerite emphasises the importance of incremental magma pulses
to the construction of major intrusions. The suggestion that intrusive sheets can be further sub-divided into smaller magmatic lobe
components implies our current understanding of emplacement,
host rock deformation and crystallisation processes may be too
simplistic. The effects that adjacent volcanic systems may have on
each other in terms of magma supply and input is also highlighted
here. This bears particular importance for our understanding of
edifice construction in terms of determining eruption dynamics
and pre-cursors.
Acknowledgements
Craig Magee was funded by a Natural Environment Research
Council Ph.D. studentship (NER/S/A/2008/85478). Thanks to Eric
Horsman for useful discussions on satellite intrusion emplacement.
Paul Hands is thanked for aid in thin section preparation and Trevor
Potts for his generous hospitality in the field. Sven Morgan and
Eoghan Holohan are thanked for their thorough reviews that helped to significantly improve the manuscript. We also thank Henry
Emeleus and two anonymous reviewers for their constructive
insights and helpful comments on a previous version of this
manuscript.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jsg.2012.03.005.
C. Magee et al. / Journal of Structural Geology 39 (2012) 66e82
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