The document summarizes a study of patchy epidote replacement of plagioclase in metabasic gneisses from Iona, Scotland. The key points are: 1) The gneisses experienced multiple metamorphic events including high-grade metamorphism followed by retrogression. This resulted in albitization of plagioclase and growth of chlorite. 2) Some areas exhibit complete replacement of albite by epidote, forming decimeter-scale patches. Replacement occurred along both diffuse reaction fronts and veins. 3) Pores in the albite provided pathways for fluid infiltration and epidote replacement. The distribution of replacement depends on the interplay between grain-

1. The controls of a patchy plagioclase replacement texture; integrating grain boundary processes with fluid
movement in basement gneisses from Iona, NW Scotland
A. D. Hollinsworth
SchoolofGeographical&EarthSciences,GregoryBuilding,UniversityofGlasgow,Glasgow,G12 8QQ,UK.
ABSTRACT
MetabasicgneissesfromIona, NWScotland,whichhaveundergone multiphasegreenschistfaciesretrogressionexhibitapatchy texturewherealbite
has been completely replacedby segregated decimetre-metre scale patches of epidote. The reactionfront is bothdiffuse across 0.5-2mmand marked
by 1-2mm thick epidote veins. Unconventional methods are employed in order to observe and interpret structures located on grain boundaries.
These are poorly understood yet important sites which provide reactive surfaces for mineral dissolution and precipitation, and act as pathways for
potentially reactive fluids. A series of interdependent processes are shown to be responsible for the distribution of the fluid facilitated epidote
replacement reaction, which occurred following retrogressive chlorite growth and albitisation of plagioclase. Where fluids had access to albite
surfaces, pervasive infiltration and subsequent replacement by epidote occurred, facilitated by intra-crystalline defects in albite. Grain boundary
permeability isshowntobe controlledby mineral dissolutionandprecipitation, and grainsizevariations,withthe interplay betweengrainboundary
flow and focused flow through fractures controlling the reaction distribution. This study has implications for large scale crustal behaviour,
specifically plagioclaserichbasalticoceancrust,whichisknowntoexperiencehydrothermalalterationprocessesobservedintheIonagneisses.
INTRODUCTION
Plagioclase feldspars are a major component of basic and felsic
basement gneisses, and meta-sedimentary and continental and oceanic
meta-igneous rocks. The processes which result in deformation,
alteration and replacement of plagioclase therefore have implications for
regional crustal behaviour. This study aims to determine the interaction
between metamorphic fluids within plagioclase grain interiors and grain
boundaries, and considers the controls that of the development of
metamorphic porosity networks and the equilibration of unstable
minerals with their environments.
Geological Setting
The Iona Gneisses are part of the Lewisian Gneiss Complex
(McAteer et al, 2014) of NW Scotland. The Lewisian Gneisses, initially
interpreted as two distinct groups - basement orthogneisses and
paragneisses, and intrusive metabasic gneisses (Sutton and Watson,
1951) - have recently been reclassified into a series of terranes, defined
by their chronostratigraphic histories and their tectonic relationships
with adjacent terranes (Kinny et al, 2005). The island of Iona is located
within the Tiree-Coll terrane, which is composed of granulites and
retrogressive amphibolites cut by weakly deformed dykes (Park, 2005)
Orthogneisses from Iona yield U-Pb zircon ages of 2.7Ga, with
metamorphic zircon growth occurring at 2.62 Ga; later Pb loss is
associated with pegmatite intrusions at 1.8 and 1.75Ga respectively
(Daly et al, 2006). Six deformation episodes are recorded by the Iona
gneisses, constrained by cross cutting relationships, deformation (or lack
of therein) of basic dykes and recognition of event specific folding and
mineral growth (Fraser, 1977).
This study focuses on extensively retrogressed metabasic gneisses
from western Iona (NM 26200 24500), which exhibit a patchy reaction
texture where decimetre-metre scale patches have hosted the complete
replacement of 0.3-0.5mm albite by fine grained (5-10µm) epidote (fig
1). The reaction boundary is both gradational (across 0.5-1mm) and
marked by epidote veins (fig 1).
The gneisses are composed primarily of 0.5-1mm dark green
amphibole (~35% abundance) and clear albite (~30%) which is
unaltered in areas where epidote replacement has not occurred. Very
fine grained pale green chlorite (<5µm) occurs at amphibole-albite
boundaries, partially replacing amphibole. Chlorite occurs in both
altered and unaltered patches of the gneiss in equal abundance (14%).
Across the reaction fronts, albite is replaced by yellow-green epidote
grains, though amphibole and chlorite remain present. Quartz is present
in low abundances (~10%) throughout the gneisses, which also contain
trace amounts of pyrite, apatite, and calcite. The gneisses do not show
any obvious fabric or mineral lineation, though they are cross cut by
several 0.5-1mm wide veins, composed of calcite, epidote and chlorite.
The presence of hydrous minerals is indicative of significant fluid
availability which facilitated hydrothermal retrograde metamorphism,
which likely occurred at greenschist facies (~200-3000
C).
For extensive, but spatially sporadic replacement of plagioclase
which is observed in the Iona gneisses to be facilitated, a number of
processes are considered in this study to explain the reaction texture
produced, including: the susceptibility of plagioclase to alteration by
reactive fluids, replacement processes such as albitisation and
saussuritization, the role of reactive fluids and their transport pathways
through the gneisses, and the mineralogical controls of fluid
distribution.
Plagioclase is vulnerable to pervasive fluid alteration and subsequent
chemical alteration and replacement (Que and Allen, 1996; Leichmann
et al, 2003; Plumper and Putnis, 2009) due to the presence of intrinsic
primary porosity (Montgomery and Brace, 1975; Que and Allen, 1996).
The development of these structures prior to and during metamorphism
produces internal discontinuities, which may be utilized as intra-grain
fluid pathways, which may facilitate alteration and replacement.
Albitisation and saussuritization are common plagioclase replacement
reactions. Albitisation is the pseudomorphic, isovolumetric replacement
process of Calcic plagioclase by Na rich albite, which requires fluid
presence at the reaction interface (Putnis and Austrheim, 2010).
Saussuritization is a common hydrothermal process in meta-basic and
meta-felsic rock, where plagioclase is replaced by hydrous epidote
(Leichmann et al, 2003; Plumper and Putnis, 2009). This study aims to
establish how albitisation may act as a precursor to facilitate
saussuritization, and considers the potential for fluid infiltration along
existing discontinuities developed during metamorphism.
Despite the establishment of understanding of internal deformation
and replacement of plagioclase, the importance of grain boundary
Figure 1: Field photograph showing a sharp saussuritization
reaction interface marked by epidote veins. Several fractures form
parallel to the reaction front in the saussuritized areas.
5cm

2. 2
interfaces and processes is the least understood factor in plagioclase
behaviour (a statement which is not exclusive to plagioclase), yet these
are dynamic areas which provide reactive surfaces for mineral
dissolution and precipitation, and provide pathways for (potentially
reactive) fluid infiltration.
This study integrates observations and interpretations of grain
boundary structures from the Iona gneisses coupled with petrographic
analyses, with the aims of establishing the controls of a patchy
saussuritization reaction texture in metabasic rocks which have
undergone albitisation. Analyses of the relationship between pervasive
grain boundary fluid flow and focused flow along fractures, as well as
mineralogical and grain size controls of permeability, and the
susceptibility of plagioclase to alteration due to internal defects reveals
that a complex series of processes are responsible in controlling the
spatial distribution of saussuritization in the Iona metabasites.
METHODS
Samples with both vein and diffuse reaction fronts were collected
from Iona and prepared for petrographical analysis and grain boundary
observation under SEM. SEM analyses were undertaken at the Imaging,
Spectroscopy and Analysis Centre (ISAAC) at the University of
Glasgow. The Quanta 200F field-emission environmental SEM allowed
for SEM imaging to be facilitated, whilst x-ray spectroscopy and EBSD
imaging was conducted using the integrated EDAX Genesis and EDAX-
TSL systems. A Zeiss Axio binocular microscope with a Nikon digital
net camera and software attachment were used for optical petrologic
analyses. In order to look at grain boundary surfaces, samples were cut
into thin plates (~5mm) and snapped orthogonally to the reaction front,
following the methods of Dempster et al (2006). Provided there are no
major discontinuities, the rock should fracture along grain boundaries.
The snapped plates where then glued together to form mirror images,
where a mineral surface can – theoretically – be matched with its
opposite hollow. The samples were placed in ultrasonic baths to remove
dust particles, then carbon coated for SEM analysis. Abundance of
chlorite in saussuritized vs. non-saussuritized patches was calculated
using ImageJ software and Microsoft Excel.
RESULTS
Non-saussuritized areas
X-ray spectroscopy revealed that plagioclase in non-saussuritized
patches have been completely albitised. Most albite grain surfaces
albites are mantled by fine grained (1-5µm) chlorite (fig 2A), oriented at
high angles (80-90˚) to the host (fig 2A), and occasionally forming sub-
parallel to albite cleavages (fig 2B).
Fracturing of albite during snapping enabled the observation of
intra-crystalline structures. Albite exhibits both circular and tabulate
pores (fig 2B). Tabulate pores are ca. 5µm across, and have partially
coalesced with adjacent pores (fig 2B). Circular pores are typically
smaller (<1µm) and more abundant than tabulate pores (fig 2B). Pores
abundances are higher at twin/ cleavage planes than in the general body
of the grain (fig 2B). Epidote is observed to occur sporadically within
albite in un-saussuritized areas (fig 3C), becoming more abundant
approaching the saussuritization reaction front (fig 4B).
Several veins cross-cut albites and amphiboles in non-saussuritized
areas, filled by lineated and gently folded chlorite (fig 2C; fig 3A) and
epidote (fig 4A). Chlorite is often present at the margins of veins, whilst
epidote is concentrated centrally (fig 4A). Albite adjacent to veins
shows minimal alteration, though amphibole has undergone partial
retrogression adjacent to chlorite veins (fig 4A).
Amphibole and quartz form the other major constituents of the non-
saussuritized sections. Both minerals exhibit heavily pitted surfaces (an
example of a quartz surface is shown in fig 3D). Amphibole and albite
occasionally exhibit granoblastic textures, evidenced by triple junctions
at grain boundaries. However, amphibole-albite grain boundaries are
more often characterised by chlorite (fig 2D) due to the replacement of
amphibole by chlorite. Chlorite is equally abundant (~14%) in both
saussuritized and non-saussuritized areas of the gneisses. Less common
mineralogical components of the non-saussuritized regions include rare
orthopyroxenes with heavily pitted surfaces, pyrite, and calcite, which
occurs internally within albite and in veins.
Saussuritization reaction front
At diffuse saussuritization interfaces, albite grains with cleavages
oriented sub-orthogonal to the reaction front exhibit “stepped” grain
surface geometries in contact with the fine grained epidote matrix (fig
3A). Reaction interfaces with no relationship to structure exhibit
undulating geometries (fig 3B). Reaction fronts marked by veins exhibit
more defined planar geometries (fig 4B). Complete saussuritization
occurs abruptly across veins, with only minor epidote growth observed
in the non-saussuritized area adjacent to the vein (fig 4B). In contrast,
diffuse interfaces have gradational fronts across ca. 5mm, characterised
by increasing epidote concentrations within albite grains (fig 3C)
towards the reaction front.
Saussuritized areas
Saussuritized areas are defined by complete replacement of albite by
5-10µm grains of epidote. The epidote matrix is heavily fractured (fig
4C). Chlorite and amphibole occur in equal proportions in saussuritized
sections as with non-saussuritized. Quartz surfaces are characterised by
5µm concentric etch-pits (fig 3D).
GEOLOGICAL INTERPRETATION
Rare orthopyroxene occurrences and relict granoblastic textures at
amphibole-albite boundaries provides evidence for high grade
amphibolites-granulite facies metamorphism of a basic igneous
protolith. Considering the anorthite component of plagioclase increases
with metamorphic grade (Goldsmith, 1982), significant leaching of Ca
must have occurred during albitisation following peak metamorphism,
which would require significant fluid input. This occurred prior to the
retrogression of amphibole to chlorite, which mantles albite grain
boundaries. The growth of chlorite on albite grain boundaries typically
shows alignment with the ambient stress conditions, signaling a regional
metamorphic event during or shortly following chlorite growth; Vein
chlorite has later experienced gentle folding in a secondary
Figure 2: Scanning electron (SE) and backscatter electron images
(BSE) from non-saussuritized sections A) albite grain boundary
surface mantled by chlorite (SE) B) Tabulate and circular pores
within albite (BSE + SE) C) Chlorite vein proximal to
saussuritization reaction front (SE) D) Typical chlorite mantled
amphibole-albit grain boundary (BSE + SE). Scale bar 20µm

3. 3
metamorphic event (fig. 2C). Structures within albite also potentially
control the orientation of chlorite, which is observed to form sub-
parallel to cleavage planes (fig 2B).
Pore development in albite is likely a result of both albitisation
(Hovelmann et al, 2010) or exploitation of intrinsic plagioclase pores by
hydrothermal fluids (Que et al, 1996). Coalescence of pores is indicative
of dissolution by reactive fluids. The nucleation of pores along
cleavage/ twin planes provides evidence for pervasive fluid flow
through albite, allowing for epidote replacement to occur internally (fig
3C).
Veins in non-saussuritized sections show reactive fluids had access to
these areas of the gneisses (fig 4A), though there is minimal
saussuritization associated with the generation of these veins. The
channelization of fluids within the vein may have prevented epidote
bearing fluids from seeping into grain boundaries or through albite
defect due to the focused nature of the flow. Initial formation of chlorite
at vein walls (as observed in fig 2C; 4A) may also have inhibited
epidote nucleation on surfaces of albite when the vein reopened at a
later stage carrying epidote bearing fluids.
Chlorite nucleation on albite surfaces is considered to play a similarly
important role in limiting reactive fluids at grain boundaries from
initiating saussuritization. Extensive formation of chlorite at grain
boundaries must be facilitated by the retrogression of amphibole (and
potentially pyroxene). Significant dissolution of amphibole is evidenced
by “peeling” textures along cleavage planes and partial replacement by
chlorite, which precipitates at albite-amphibole grain boundaries (fig
2D). However, chlorite formation at grain boundaries cannot be the
primary control of the saussuritization reaction distribution, as
abundances are equal throughout the gneisses.
At the saussuritization reaction front, the geometry and nature of the
reaction interface is key to determining the controls of fluid movement.
Diffuse reaction fronts marked by undulating interfaces (fig 2B) with
gradationally increasing epidote concentrations towards the
saussuritized patches suggest that reactive fluids infiltrated along grain
boundaries, having access to the interiors of albite along internal defects
and pores. The stepped geometry of the grain boundary reaction front in
fig. 2A indicates a structural control to the reaction propagation, where
epidote may have initially nucleated along cleavage planes, before
consuming the remainder of the grain. Reaction interfaces marked by
veins tend to be planar and more abrupt (fig 4B). Veins are well known
fluid conduits which provide evidence for the introduction of reactive
fluids to rocks that are out of equilibrium with their environments
(Jamtveit et al, 2000; Putnis and Austrheim, 2010), however in this
instance epidote veins signify the removal of fluid from the reaction
front, as the introduction of fluids via fracture networks should generate
equilibration reactions on both sides of the fracture, which is not
observed in the Iona gneisses. The continued build up of fluid pressure
at the reaction front may generate fracturing, providing a mechanism to
remove fluids from the front and limit the progression of the reaction at
that point. The continued input of fluids to the reaction front therefore
must be facilitated by grain boundary from the saussuritized patches.
There is significant evidence for pervasive fluid infiltration along
grain boundaries within the saussuritized patches. Heavily pitted quartz
surfaces indicate pressure-solution where quartz is in contact with
epidote (fig 3D); the pits have similar size and morphology to the
epidote matrix. Fracturing within the epidote matrix is also attributed to
hydrostatic pressure increase. The finer grained epidote matrix relative
to the coarser grained albite rich areas may encourage channelization of
fluids within the saussuritized areas; there are more grain boundaries
(fluid pathways) which may be utilized by fluids, thus allowing
continual fluid supply to the reaction front.
DISCUSSION
Albitisation and Saussuritization
Albitisation resulted from low grade hydrothermal alteration of an
initially anorthitic plagioclase; Na rich plagioclase becomes more stable
with lower temperatures (Goldsmith, 1982). The porous nature of the
albite in this study is a product of albitisation, during which the Al-Si
framework of the plagioclase is completely reworked in a dissolution-
precipitation reaction (Hovelmann et al, 2010), with growth and
coalescence of pores suggesting dissolution during fluid infiltration
(Que and Allen, 1996). Evidence of fluid infiltration and replacement
reactions occurring on cleavages and twin planes are well documented
in plagioclase (Leichmann et al, 2003; Plumper and Putnis, 2009).
Coupled with the occurrence of pores (fig 2B), there are several
potentially interlinking pathways within the albite in this study, which
have facilitated extensive saussuritization.
Saussuritization occurs internally within albite and at grain
boundaries. Internal epidote nucleations have two possible origins: Ca
leaching during albitisation resulting in epidote formation on cleavage
surfaces or in micropores (Leichmann et al, 2003; Plumper and Putnis,
2009), or Ca bearing fluids infiltrating porous albite at a later stage
causing saussuritization. An influx of Ca-fluid is the more likely source
in this case, due to the extensive occurrence of albite-replacing epidote
in segregated patches. Direct epidote precipitation resulting from
albitisation is discounted, as epidote would show a more uniform
distribution throughout the rock.
Concentric dissolution pits on quartz grain surfaces (fig. 3D) are also
attributed to the precipitation of epidote in extensively saussuritized
areas. Similar grain surface morphologies are proposed to occur during
serpentinisation of olivine, where dissolution of the host grain is focused
at the centre of an etch pit. These become the points of highest
hydrostatic pressure, promoting hydraulic fracturing and exposure of
fresh reaction surfaces (Plumper et al, 2012). Minimal exposure of albite
grain surfaces due to replacement by epidote and mantling by chlorite
meant no direct observations of this texture were made. However, cross
sectional views of the undulating saussuritization reaction fronts at grain
margins (fig. 3B) are morphologically comparable between quartz and
albite, suggesting that albite surfaces were characterised with similar
dissolution structures as quartz.
Figure 3: Secondary electron (SE) and backscatter electron (BSE)
images from saussuritized sections A) structurally controlled epidote
replacement of albite with “stepping” geometry (BSE) B) Undulating
albite-epidote replacement reaction front (SE) C) Epidote nucleation
within albite (SE) D) Quartz grain surface characterized by spherical
dissolution pits (SE). Scale bar 20µm.

4. 4
The saussuritization reaction interface morphology in the Iona
gneisses is observed in several dissolution-precipitation reactions
(Jamtveit et al, 2000; Malthe-Sorenssen et al, 2006; Jamtveit et al, 2009;
Jonas et al 2014), it is therefore proposed here that saussuritization of
albite is facilitated by the same mechanism, where albite is dissolved at
a fluid-mineral interface, and replaced by precipitating epidote. The
development of a patchy reaction texture is the result of: fracturing of
the reactant (albite) at the reaction interface exposing non-reacted
surfaces (Plumper et al, 2012), volume changes resulting from
precipitation of the product (epidote) generating fractures (Jamtveit et
al, 2009), and the development of a porous product allowing fluids to
continually reach the reaction front (Putnis and Putnis, 2007). There is
little evidence for fracturing of albite surfaces in contact with epidote,
though these may have subsequently been filled by precipitating
epidote. Both volume expansion and contraction have been proposed for
saussuritization reactions based on the Fe content of epidote (Arghe et
al, 2011), and both volume expansion (Jamtveit et al, 2009) and
contraction (Malthe-Sorenssen et al, 2006) may generate fracturing with
volume changing reactions, imparting less significance on the nature of
the volume changes associated with saussuritization. Fracturing occurs
within the epidote matrix (fig. 4C; 4B), creating permeable channels
which facilitate fluid flow to the reaction front, allowing continued
interaction with fresh albite surfaces. The dendritic, fractured texture
observed within the epidote matrix reflects that of modelled fracture
propagation networks associated with volume changes (Jamtveit et al,
2000; Malthe-Sorenssen et al, 2006; Jamtveit et al, 2009). Permeability
within saussuritized patches resulting from fracturing therefore plays a
major role in controlling the extent to which albite is replaced in the
gneisses, however, mineralogical controls within saussuritized and non-
saussuritized areas also exert controls of the reaction distribution.
Mineralogical Controls of Permeability
Chlorite and epidote are shown to have important roles in
determining permeability. Chlorite is a common authigenic grain
coating mineral in petroleum reservoirs, often enabling the preservation
of primary porosity by preventing the growth of pore-filling cements
(Sun et al, 2011 and references therein). Chlorite coating albite surfaces
at grain boundaries and in veins indicates chlorite formation may a)
reduce grain boundary permeability and b) whether reactive fluids come
into contact with albite, drawing parallels with authigenic chlorite
inhibiting cement formation in petroleum reservoirs.
Abundance of chlorite is equal in the saussuritized and non-
saussuritized patches, however, suggesting that chlorite formation at
grain boundaries is not the primary control over permeability. Epidote
also influences the permeability of the gneisses. The fine grain sizes and
higher grain boundary concentrations of epidote relative to the coarser
grained granoblastic albitic areas promote channelization of fluid within
saussuritized sections, developing permeable metamorphic aquifers
(Wark & Watson, 2000). Grain size therefore plays an important role in
metamorphic permeability, and is considered to be a significant
component for channelizing reactive fluids (Wark & Watson, 2000)
within fine grained epidote zones (Arghe et al, 2011). Grain boundary
chlorite contributes to this channelization process by deflecting fluids at
the margins of epidote aquifers back into the saussuritized patches,
limiting the reaction propagation away from the reaction front.
Fluid pathways: focused flow vs. pervasive infiltration
Evidence for fluid flow both along grain boundaries and in fractures
in the Iona gneisses provides an opportunity to establish a hierarchical
fluid flow order. In several cases observed in the Iona gneisses, epidote
bearing veins mark the termination of the saussuritization reaction
progression (the reaction front), suggesting reactive fluids were flowing
Austrheim, 2010) suggest. Modeling has shown that fractures may
develop sub-parallel to dissolution-precipitation reaction fronts resulting
from volume changes (Jamtveit et al, 2009). Fractures may also generate
Figure 4: A) plane polarized image of chlorite and epidote veins in
The non-saussuritized area. Retrogressive chlorite replaces
amphibole along fractures, with only minimal replacement of
albite by epidote B) plane polarized image showing a reaction
front marked by an epidote vein. Only partial replacement occurs
on the left of the vein, whilst near complete replacement is shown
on the right. Amphibole has not seen retrogression in this case,
despite the vein cross cutting several grains C) cross polarized
image of amphibole and epidote rich saussuritized area; some
adjacent epidote grains have the same optical properties
suggesting intra-grain fracturing
(A)
(B)
(C)

5. 5
as a result of continued influx of fluids along epidote grain boundaries,
enhanced by mineralogically controlled fluid channelization (as
discussed above), alleviating fluid pressures at the reaction front to the
point where the tensile strength of the rock is overcome, and fracturing
occurs (Ferry, 1994). The generation of fractures resulting from these
processes may drain fluids from adjacent grain boundaries (Vernon,
2004). Fluid flow is consequently inhibited across some fractures,
leaving a relatively unaltered patch across the vein (fig 4B), providing a
potential inlet for pervasive grain boundary flow and hydrostatic
pressure accumulation elsewhere. Grain boundary flow and fracture
generation (and subsequent fluid “vacuuming”) are therefore considered
to be interlinking processes rather than hierarchical in this case.
CONCLUSION
Metabasic gneisses from Iona have a complex metamorphic and
metasomatic history, during which plagioclase has potentially
undergone several physical and chemical changes over six recognized
metamorphic events (Fraser, 1977). The most recent changes considered
in this study are albitisation, which was followed by complete
saussuritization of segregated patches. This is suggested here to be an
interface coupled dissolution-precipitation replacement process.
Petrographic analyses, and observations of grain boundary structures
show that the development of crystalline defects within the host
plagioclase, reaction-driven fracture propagation, mineralogical controls
of permeability, and the interplay between grain boundary and focused
fluid flow through conduits interdependently control the distribution of
reactive fluids, and by extension, the resulting patchy alteration texture.
Albite boundaries provide reactive sites for saussuritization to occur,
which augments and focuses permeability. Albite also provides
nucleation sites for grain coating chlorite, which inhibits reactive fluid
infiltration. Major fracture development is shown to halt reaction
propagation at specific sites by removing fluids from the reaction front
and redistributing them to other areas of the rock. Fluids pervasively
infiltrate albite, along cleavage/ twin planes, and in pores, which
developed during albitisation (Hovelmann et al, 2010). Fluids may also
have utilized deformation induced permeable pathways developed
throughout polyphase metamorphism.
Due to its high abundance in the crust, understanding plagioclase
alteration and replacement processes has implications for large scale
crustal behaviour. Greenschist facies metabasic assemblages similar to
those studied here are common in meta-volcanic and meta-intrusive
rocks in Canada (Harrigan and Maclean, 1976; Gelinas et al, 1982), the
Scottish Highlands (Arghe et al, 2011) and in hydrothermally altered
ocean crust (Humphris and Thompson, 1978). This study highlights the
need to better constraint fluid flow dynamics, and the role of grain
boundary processes with respect to plagioclase stability in the crust.
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