The document describes flow banding in the Dun Caan Basalt on Raasay, Scotland. It finds that the basalt contains a lower flat domain with sub-horizontal planar banding and an upper contorted domain where the banding is complexly folded. The banding is thought to have formed from intense non-coaxial shear during emplacement of an unusually high-temperature and low-viscosity lava, similarly to a lava-like ignimbrite. Upon cooling and slowing, the upper part underwent slumping which deformed the earlier planar banding into a contorted fabric.

1. 1
Understanding flow banding in basaltic lavas: the Dun Caan Basalt, Raasay.
L.A.H. Gillies
School ofGeographical &EarthSciences,GregoryBuilding,UniversityofGlasgow, Glasgow, G128QQ,UK.
ABSTRACT
Flowbanding is common inevolved lavasandlava-like ignimbritesbutrareinbasic lavas. TheDun CaanBasalt on the Isle of Raasayin north-west
Scotlandis anexample oftherare occurrence offlowbanding within abasiclava. Therearetwofoldingdomains present within DunCaan;alower
‘flat’ and an upper ‘contorted’ domain. This relationship is similar toa lava-like ignimbrite, the Grey’s LandingIgnimbrite, Idaho. However, lavas
and lava-like ignimbrites are emplaced in extremelydifferent ways, which makes this similaritydifficult tocomprehend. Nonetheless the Dun Caan
Basalt is thought to have undergone similar emplacement conditions to lava-like ignimbrites. It is interpreted that the basalt was erupted as an
extremelyhightemperaturelargebodyof inflatinglava of particularlylowviscositywhich facilitatedintense non-coaxial shear producingthe planar
fabric in the lower flat domain. Upon cooling, the lava underwent slumping which is thought to have caused the earlier planar fabric of the lower
domaintodeformandproducethecontortedfabricintheupperdomain.
INTRODUCTION
The Dun Caan Basalt, located on the Isle of Raasay in north-west
Scotland (Figure 1) constitutes part of The Skye Lava Group. This
forms part of The North Atlantic Igneous Province, one of the largest
igneous provinces in the world (Macdougall, 1988) formed due to
rifting of the European and North American plates and impingement of
a mantle hot spot, causing sea floor spreading and the consequent
opening of the North Atlantic Ocean (Saunders et al., 1997). Remnants
of the Paleogene magmatism are found in Greenland, the Faeroe Islands,
Iceland, NE Ireland and NW Scotland. The rocks which are preserved in
NW Scotland are mainly basaltic, (Saunders et al., 1997) erupted from
large central complexes located in the Hebrides, on the Isles of Skye,
Mull, Ardnamurchan and Rum (Bell and Emaleus, 1988; Brown and
Bell, 2013).
The Dun Caan Basalt preserves unusual, complex flow banding
which appears as distinguishable bands defined by colour alternations,
indicative of differences in grain size and mineralogy. The presence of
flow banding in the Dun Caan Basalt is unusual due to the fact that this
is common in more viscous and evolved lavas such as rhyolite and
dacite, and in lava-like ignimbrites. However, it is less prevalent in
higher temperature magmas such as hawaiite and mugearite and
significantly rare in basic lavas, such as basalts (Robins et al. 2010).
The Dun Caan Basalt displays a similar relationship to the Grey’s
Landing Ignimbrite, a lava-like ignimbrite located in Idaho, north-west
America, deposited as the result of a high temperature pyroclastic
density current. Both Dun Caan and Grey’s Landing contain two folding
domains; i) a lower base parallel domain which contains planar sub-
horizontal flow banding; and ii) an upper non-base parallel ‘contorted’
domain where the flow banding fabric displays complex folding
(Andrews and Branney, 2011). These similarities are difficult to
comprehend due to the fact that basaltic lavas and lava-like ignimbrites
are emplaced in very different ways.
It is suggested that the two folding domains within the Grey’s
Landing Ignimbrite were formed due to syn and post emplacement
rheomorphism (Andrews and Branney, 2011). Hot, soft pyroclasts and
rigid crystals agglutinated and coalesced while undergoing welding and
rheomorphism and non-coaxial shear within a rising sub-horizontal
shear zone producing the flow banding. The banding within the lower
flat domain was produced in a flat-lying zone by non-coaxial intense
ductile shear. Within the upper domain, the banding was produced in the
same fashion, however, the fabric was re-folded by larger folds during a
secondary deformation event, which caused its ‘contorted’ nature
(Andrews and Branney, 2011). It is thought that the Dun Caan Basalt
has undergone similar emplacement to the Grey’s Landing Ignimbrite
despite their notably different emplacement modes.
METHODOLOGY
Field work
Field work was undertaken on the Dun Caan Basalt to obtain
measurements of the flow banded fabric and collect samples for further
analysis. To allow for comparison of the two domains, fabric
measurements were taken from both the lower and upper domains.
Thicknesses of the different domains were recorded and a geological
map of Dun Caan was produced.
Optical microscopy and SEM analysis
Two rock samples, one from the lower domain (DCB) and one from
the upper domain (DCT) were selected. An area of specific interest
within the selected samples was chosen and polished thin sections were
produced for further petrographic and SEM analysis. Petrographic and
optical analysis was performed using a Zeiss Axioplan optical
microscope to determine the mineralogy and to define particular areas of
interest for further study using SEM. SEM analysis was undertaken
using a FEI Quanta 200F Field Emission Scanning Electron Microscope
and a Zeiss Sigma VP with Aztec microanalysis in order to focus further
on the mineralogy, textures and grain size in more detail. Quantitative
analyses were performed on darker areas which represent flow bands
and lighter areas where no flow banding is present to obtain a
comparison of their geochemistry. Studying these various factors
facilitated an assessment on the controls of the flow banding.
Figure 1. Location map of Raasay in north-west Scotland. Red
box indicates the location of the Dun Caan Basalt.

2. 2
XRF Bulk Analysis
In order to obtain the bulk chemistry of the Dun Caan Basalt, XRF
(X-Ray Florescence) analysis was used. The same two rock samples
selected for optical microscopy and SEM analysis were crushed using
the Retsch B4 Jaw Crusher. The crushed material was sieved to a very
fine (63µm) powder. This powder was processed into a glass disc to
measure major element concentrations using the Philips PW2404
wavelength-dispersive, sequential X-Ray Florescence Spectrometer
fitted with a rh anode end-window X-Ray tube. The XRF was calibrated
against USGS standards: for major elements analytical error is ~ ±0.01
wt % (Fitton et al. 1998).
RESULTS
Lithology
Within the Dun Caan Basalt, three different lava flows are identified;
B1, B2 and B3. B1 and B2 constitute the lower part of Dun Caan and
represent older lavas which do not display any flow banding. B1 is a
medium grained (0.25-05mm) dark grey/black dolerite with spheroidal
weathering. A red/brown palaeosol defines the boundary between B1
and B2: a fine grained (0.125-0.25mm) dark grey/black basalt. Another
red-ish/brown palaeosol separates B2 and B3: a fine grained (0.125-
0.25mm) amygdaloidal basalt containing stretched vesicles and large (2-
3cm) plagioclase feldspar amygdales. Flow banding is identified within
B3 which constitutes both the upper contorted and lower flat domain.
The flow bands are ubiquitous throughout the basalt and appear darker
and coarser grained.
Structure
i) Lower flat domain
The lower flat domain measures ~27.9m thickness. The first
development of flow banding is seen within the lower part of unit B3.
The flow banding fabric appears as a sub-horizontal planar foliation
(Figure 2) which represents recumbent intrafolial isoclinal folding
(Andrews and Branney, 2011). The fabric trends north-west/south-east
with a general dip of ~16° to the east/south-east.
ii) Upper contorted domain
The gradual transition zone between the lower and upper domain
measures 6m thickness and contains local variations of banding ranging
from sub-horizontal to contorted in nature. The contorted flow banding
fabric begins to develop near the top of unit B3. Large scale
asymmetrical cylindrical inclined folding is present. Smaller-scale
extremely tight recumbent isoclinal folding is also observed where
banding appears almost vertical. The flow banding fabric is variable,
trending W-E and plunging at an average of 27° to the NE in some
areas. In other areas, the banding trends NW-SE and plunges at an
average of 31° to the NE. Within the uppermost part of B3, minor
antiform and synform structures are present within larger fold structures
which are shallow, and plunge in a south-easterly direction (Figure 3a).
Clasts of material from the lower domain are incorporated in the
contorted fabric of the upper domain. The contorted banding displays
definitive colour alterations (Figure 3b).
Figure 2. Recumbent isoclinal folding in lower domain. This folding
produces the sub-horizontal flat nature of the flow banding.
Figure 3. Geological map of Dun Caan. Different flows are indicated along with flow banding fabric. Key displays strati graphical age
and description of flows, and map symbols.

3. 3
Petrography
The mineralogy within the Dun Caan Basalt was determined using
quantitative SEM analysis (Appendix A.1). Olivine (fosterite)
dominates the rock with plagioclase feldspar constituting a slightly
smaller percentage.
Clinopyroxene is also present along with amphibole and iron-oxides,
mainly magnetite. There is also a small percentage of chlorite found
whereby olivines have undergone serpentinization. Distinctive dark and
light areas are evident, with dark areas representing the presence of flow
bands. In darker areas, there are a higher percentage of opaque, iron-
oxide minerals, along with a greater abundance of olivine and chlorite
(Figure 4a, Appendix A.2). Grain sizes differs between dark and light
areas, where dark areas are coarser (0.1mm) than lighter areas
(0.08mm). The olivine phenocrysts display heavy fracturing, more so
within the flow bands (Figure 4c, Appendix A.3).
The basalt exhibits a directional fabric whereby plagioclase feldspars
‘wrap’ around the larger crystals of olivine (Figure 4b). These features
appear both in the lower and upper domain, with the only difference
being that within the upper domain flow banding is more easily
identified and olivines are slightly more fractured.
Geochemistry
Both samples plot within the basalt field of a total alkali versus silica
(TAS) diagram (Figure 5, Appendix B). The red dot indicates the lower
domain (DCB) and the green, the upper domain (DCT). DCT plots
slightly higher than the DCB but the difference does not appear to be
largely significant.
Within dark areas which represent flow bands, there is a higher
percentage of MgO and Fe2O3 whereas in lighter areas where flow
banding is not present, CaO and SiO2 are more abundant (Figure 6,
Appendix A.4). This corresponds with mineralogy of the bands
(dominant olivine and Fe-oxides).
a
Figure 3. Field images of the upper contorted domain. (a) Antiform
and synform structures. (b) Flow banding shows colour
alternations. Bands appear darker brown and coarser grained.
b
Figure 4. Optical microscope and SEM images. (a) Plagioclase
feldspar crystals displaying a directional fabric ‘wrapping’ around
phenocryst of heavily fractured olivine which has serpentinized to
chlorite. (b) Image of flow banding, bands appear greener in colour
due to the high abundance of olivine present. Minerals within bands
are also coarser.
b
Figure 5. Total alkali versus silica (TAS) diagram. Red dot
represents DCB, green dot represents DCT.
a

4. 4
DISCUSSION
Controls on flow banding
Microfabric structures within flow bands display colour alternations
which are indicative of textural and compositional variations (Seaman,
et al. 1995). The darker colour of the bands in the Dun Caan Basalt
represents the high abundance of coarse olivine and iron oxide minerals
present. This indicates that mineralogy and grain size both act as a
control on the banding. The presence of heavily fractured olivine within
flow bands suggests that the olivine has undergone hydrothermal
alteration to chlorite along fractures, most likely through the process of
serpentinization. The presence of chlorite and amphibole are suggestive
of the hydrothermal metamorphic reaction occurring at greenshist facies
conditions (Chen, et al. 2014). The formation of pores and fractures
would have allowed for pathways to be created facilitating fluid flow
through the rock (Jamtveit et al., 2009).
The presence of a higher abundance of olivine and iron-oxide
minerals within flow bands is due to their high density compared to
plagioclase and pyroxene. The higher density olivine is getting ‘clogged
up’ within bands as the less dense plagioclase feldspars are sheared
more easily.
Geochemistry
Geochemical analysis indicates that chemistry is not a major
controlling factor on flow bands. While there is some variation between
the two domains it is not significant and is essentially “within error”,
confirming they are part of the same lava flow. The flow bands contain
more MgO and Fe2 O3 as they are more abundant in olivine and iron-
oxide while areas with no flow banding contain a higher percentage of
silica indicating the presence of less dense minerals such as plagioclase
feldspar.
Textures
The directional fabric observed is an unusual feature to occur within
basaltic magmas, this type of texture is most common in more evolved
and viscous lavas and lava-like ignimbrites which have undergone
shear. Directional fabrics in “normal” ignimbrites and lava-like
ignimbrites are formed due to (i) sedimentary alignment of elongate
particles during deposition, (ii) shearing casing deformation within the
non-particulate flow and (iii) post-emplacement slumping or loading
(Branney and Kokelaar, 1992). Crystals with long axes oriented parallel
to the direction of inferred shear record fluid escape dominated
deposition from a flow boundary zone with shear close to the flow
boundary zone and a component of granular flow. (Branney and
Kokelaar, 2002).
An example of a similar fabric is recorded in the basalt lavas of the
southern Larnington Volcanics, eastern Australia where the alignment
of crystals is interpreted as shear zones formed during solidification of
magma (Smith, 1998). The directional fabric observed within the Dun
Caan Basalt indicates this magma has undergone intense non-coaxial
shear, whereby the plagioclase crystals which wrap around larger
olivine phenocrysts have sheared past the denser olivine producing the
observed directional fabric.
Structure
The lower flat domain of the Grey’s Landing Ignimbrite was formed
in a lower, flat-lying zone of intense non-coaxial ductile shear
producing the primary flow sub-horizontal planar foliation (Andrews
and Branney, 2011). Flow bands in rheomorphic lava-like ignimbrites
are formed by the movement of particulate flow to non-particulate flow.
This transition occurs within the agglutination zone, where particles
agglutinate and undergo welding and rheomorphism to form
macroscopic flow banded structures (Kobberger and Schmincke, 1999).
The Dun Caan Basalt is thought to have undergone similar emplacement
conditions whereby intense non-coaxial shear played a major role in
forming the recumbent isoclinal planar foliation. For this fabric to be
produced in a basic magma, it would require an anomalously high
temperature which would have caused extremely low viscosity and
therefore exceptionally fast flow. The speed of the flow, rather than an
over-riding pyroclastic density current (the case for lava-like
ignimbrites) would have caused intense non co-axial shearing to occur
which is evident from the directional fabric. The upper contorted
domain of the Dun Caan Basalt is interpreted to have formed as a result
of the lava cooling and becoming more viscous, consequently slowing
its speed. This process of cooling would have caused slumping and
relaxation of the magma causing it to buckle up and deform the earlier
isoclinal fabric produced in the lower flat domain.
Emplacement model
It is interpreted that the Dun Caan Basalt has undergone similar
emplacement conditions to lava-like ignimbrites. Within the lower
domain, the fabric seen is not typical flow banding, suggesting that a
different mechanism must have been in place to create such structures.
The controls on flow banding are the same regardless of domain,
meaning the pattern must be related to emplacement conditions rather
than mineralogy or grain size differences between domains.
It is interpreted that the Dun Caan Basalt was erupted as an extremely
high temperature lava at ~1200°C which is similar to the temperature
proposed for lava-like ignimbrites (Andrews and Branney, 2011). The
high temperature would have allowed the lava to have a remarkably low
viscosity, consequently providing a mechanism for fast flow. This rapid
flow would have led to the occurrence of syn-emplacement
rheomorphism and intense non-coaxial shearing resulting in the
formation of recumbent isoclinal folding preserved within the lower
domain. As Dun Caan is a large exposure of basalt, there would have
been very high fusion occurring in a large body of extremely hot lava,
which accounts for the high rates of shearing being produced. Coarser
layers where flow bands occur are due to flow segregation of denser
minerals, such as olivine. The less dense minerals, pyroxene and
plagioclase would have sheared past these denser minerals, producing
the directional fabric.
The large body of lava would have started to cool, whereby it would
have begun to behave more like an evolved lava, resulting in an increase
in viscosity and consequently lower speed. This would have caused
downslope slumping and relaxation of the lava. It is this cooling which
is thought to have produced the contorted flow banding, due to the
maximum amount of shearing occurring at the top and edges of the
flow, deforming the isoclinal folds previously formed within the lower
domain. Some fold structures within the upper contorted domain appear
almost upright and represent clasts of the same material found within
the lower domain which suggests that this is due to the crust of the flow
flaking off during cooling and solidification and being incorporated
Figure 6. Graph comparing dark areas (presence of flow bands)
to light areas (absence of flow bands). Darker areas indicate more
Fe2O3 and MgO. Lighter areas indicate more SiO2 and CaO.

5. 5
within fold structures in the upper domain. Present day erosion has left a
landscape with the older lavas partly eroded at the surface and the
preservation of both domains (Figure 7).
CONCLUSIONS
The Dun Caan Basalt was emplaced as a large, inflating body of
extremely hot lava with low viscosity allowing intense non-coaxial
shearing to occur producing the planar fabric recorded within the lower
flat domain. This fabric was then folded and deformed when the body of
lava started to cool and behave more like an evolved lava, this
mechanism produced the contorted fabric in the upper domain. The Dun
Caan Basalt is the first recorded example of such banding in a basaltic
lava, and has important implications for our understanding of the
emplacement of lavas.
AKNOWLEDGMENTS
I would like to thank my supervisor Dr. David Brown for his extremely useful
guidance and support throughout the project. I would also like to thank Robert
Macdonald, John Gilleece and Peter Chung at The University of Glasgow for
help with sample preparation and Nic Odling at The University of Edinburgh for
assistance with XRF analysis.
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Figure 7. Emplacement model for Dun Caan Basalt.
Occurrence of syn and post emplacement rheomorphism along
with downslope slumping and relaxation and later erosion.