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METAMORPHIC ROCKS
Prepared by Dr. F. Clark,
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
INTRODUCTION – THE ORIGIN OF
METAMORPHIC TEXTURES
In many cases, metamorphism involves at least some
increase in pressure compared to that under which the
rocks originally formed, and in addition, the pressure is
not uniformly distributed or oriented. Thus there is one
direction [and it could be any direction] along which
pressure reaches some maximum value, and this
produces what is called directed stress. The effect of this
directed stress is to cause minerals that have at least
one longer dimension to orient themselves with the long
axis or axes perpendicular to the directed stress. New
crystal growth will occur with this orientation, and preexisting crystals tend to rotate into this orientation.
METAMORPHIC FABRIC - 1
The preferred alignment or orientation of minerals within
many metamorphic rocks gives those rocks fabric. Two
principal groups of minerals are involved in this. Sheet
silicates, especially the micas biotite, muscovite, and
phlogopite, will align themselves with the flakes or flat
faces of the crystals parallel to each other and at right
angles to the directed stress. This type of fabric is called
foliation. As an illustration, think of gravity as the stress,
and a deck of cards as the micas. If you let go of the
deck, the cards will fall to the table top or floor and will
all lie flat, their long dimensions (the faces) at right
angles to gravity, whose orientation is vertical.
METAMORPHIC FABRIC - 2
The second group of minerals whose alignment typically
produces fabric is the amphibole group. These prismatic
or elongate crystals will have their long axes parallel to
each other. This fabric is called lineation. To give an
illustration, imagine a handful of pencils as representing
the amphibole crystals. Again, the stress is gravity,
oriented vertically. If you let the pencils drop, they will
not stand on end, but will come to rest in a horizontal
position. Unlike the pencils, which will be randomly
oriented on the floor, amphiboles tend to be aligned
more like a bundle of pencils in an elastic band. A
secondary or intermediate stress at right angles to the
directed stress forces this more ordered alignment.
METAMORPHIC FABRIC - OPTIONAL
We should point not that not all metamorphic rocks have
fabric. Certain minerals tend to be equidimensional and
don’t generally produce fabric. Quartz and calcite are
two common examples, whose random orientation is
seen in such rocks as quartzite and marble, respectively
[illustrated in the file on metamorphic minerals]. As well,
in thermal or contact metamorphism next to igneous
intrusions, the absence of directed stress means there is
no tendency to develop fabric. A common product is a
rock called hornfels [named for its common constituents
hornblende plus feldspar], which may resemble basalt or
diabase, a shallow intrusive equivalent of basalt.
METAMORPHISM OF AN ARGILLITE –
THE TEXTURAL STORY
Just as progressive metamorphism of an argillite or
mudstone parent rock gives rise to a characteristic
succession of minerals with increasing grade or intensity,
so too it gives rise to a characteristic succession of
textures, based on the arrangement or orientation of the
grains. The rock names derive from the names of the
textures, and so we see in order of increasing grade the
rocks slate, phyllite, schist, and gneiss, which will be
illustrated in turn. The corresponding textural terms are
slaty cleavage, phyllitic cleavage, schistosity, and
gneissosity, all of which are admittedly awkward terms.
Slate.
This represents
the lowest
grade of
metamorphism.
There is grain
reorientation
without
appreciable
grain growth.
The reorientation of fine sheet silicates in response to the directed
stress leads to slaty cleavage, wherein the rock splits very cleanly along
essentially planar surfaces. A red shale would be less compact and lack
the sheen of the cleavage surfaces of this red slate.
Slate – Exploiting its Properties
These discards from the roofing material used (by one of Edmonton’s
more prosperous citizens) in preference to shakes or asphalt
shingles could be cleaved more finely [green arrows]. Alignment of
the platy grains makes the material impermeable, and the fact it
splits into thin sheets makes the weight manageable. Note the low
sheen on the cleavage planes, which pass between grains.
Slate – Original Identity Preserved
In this low grade metamorphic rock, the original identity of the parent
rock can be seen. We see graded beds [purple arrows] with light,
coarse grains at the base grading to dark, fine grains at the top. As
well, we notice that the foliation and cleavage, parallel to the broad
faces of the sample, are unrelated to the original bedding, whose
planes are parallel to the light blue arrows.
Phyllite.
This is also a
low grade
metamorphic
rock, but is of
a higher grade
than slate.
Cleavage
planes are
highlighted by
yellow stars.
With higher metamorphic grade comes increased crystal size. The faces
of the larger grains are more reflective than smaller grains, and so the
cleavage planes of the rock have a greater sheen than they do in slate.
Even irregular fracture surfaces have a sheen [blue star].
Phyllite.
As with slate,
cleavage is a
bulk property
of the rock and
passes
between
grains, unlike
mineral
cleavage which
passes through
crystals.
This phyllite has less regular cleavage planes and a higher sheen due
to the coarser grains. The intermediate stress has influenced the
growth of these crystals such that they form ridges and swales aligned
parallel to the green arrows and perpendicular to this stress.
Schist.
This texture
also appears
first at low
grades, but
higher than
both slate and
phyllite. The
significance of
the stars is
explained on
the next slide.
This schist has a reasonably coarse grain size, wherein individual grains
of biotite in particular are readily distinguished with the unaided eye.
This low grade schist is friable, that is, can easily be crumbled with
normal finger pressure. This is not generally seen in slate or phyllite.
Schistosity and Weathering - 1
The foliation produced by alignment of the biotite grains is the
schistosity, and accounts for the easy way in which this rock splits
[green arrows]. This exposes the fresh surface [yellow stars], which
is nearly black as expected for biotite, whereas the weathered
surface [blue stars] is brown because of oxidation [rusting] of the
iron content of the mafic sheet silicate biotite.
Schistosity and Weathering - 2
Again, the iron content of biotite causes the weathered surface [blue
stars] to turn brown, as distinct from the dark fresh surface [yellow
stars]. This sample is of higher grade than the previous one, as
indicated by the coarser crystal size. The coarser grains result in a
loss of well developed cleavage planes; the rock splits very
irregularly, but still parallel to the foliation [green arrows].
Schist.
At a higher
grade, in this
case medium
grade, new
minerals such
as garnet
[yellow arrows]
may appear,
within a matrix
of muscovite in
this case.
The garnet crystals have a strong force of crystallization, and will
usually be larger in size than surrounding micas. Such larger crystals
are called porphyroblasts, and generally their appearance marks the
onset of medium grade metamorphism, still within schists.
Schist.
Generally at a
higher grade
than for
garnet,
staurolite [blue
arrows] may
appear as
porphyroblasts.
In this case, the disparity in grain size between staurolite
porphyroblasts and the biotite of the matrix is extreme. Even though
the sample is not bounded by large planar surfaces, the biotite grains
at its surface are parallel to each other and define the foliation.
Schist
with a
Twist.
In this sample,
the grains of
various
minerals are
clearly not
uniformly
distributed.
Alignment of micas produces a strong foliation [green arrow]. Parallel
to the foliation is a thin band of quartzofeldspathic [mix of quartz plus
feldspars] material. If this was introduced to the sample later, it could
be a schist, but the banding is suggestive of another rock, gneiss.
Schist or Gneiss? The Debate Continues.
These views of the same sample show dominant biotite grains
producing foliation, which is seen in high reflectance from biotite grains
at the surfaces of the sample highlighted by yellow stars. Some would
suggest that a schist has more than 50% orientable grains (e.g. micas,
as seen in this sample, amphiboles, and kyanite), and a gneiss less
than 50%, regardless of the development of mineral banding. Here we
see two discrete but minor quartzofeldspathic layers [blue and purple
arrows] that are not quite parallel to the foliation (note they do not
bend, whereas the foliation does), suggesting they may have been
introduced to the rock afterward as fracture fillings, or veins.
Amphibolite –Equivalent Grade to Schist
Where elongate or prismatic amphibole crystals grow instead of micas,
the rock’s fabric is lineation, parallel to the long axes of the grains
[green arrow]. Garnet crystals are highlighted by yellow arrows. In
the view on the right, you are looking down the length of the
amphiboles, which in this perspective have no long dimension and
therefore no lineation is apparent.
Gneiss – A High Grade Metamorphic Rock
At the highest grades, minerals segregate into distinct compositional
bands. Generally there will be dark bands dominated by mafic
minerals such as biotite and amphibole, and white/grey/pink bands
[arrows] dominated by quartz plus feldspars. Feldspars may occur
as discrete pockets called augen, from the German for “eyes”
[stars]. Left view is cut surface, right is weathered.
Gneiss.
This is called a
granitic gneiss
because its
mineralogy
matches that
of the igneous
intrusive rock
granite. Its
texture marks
it as
metamorphic.
The orientation of the sample in this view means that we are not
looking parallel to the fabric. Nevertheless, the distinct segregation into
compositional bands is apparent. Turning a sample around will often
show things not apparent in your first view.
Twice as Gneiss (Sorry, couldn’t resist it!)
Two more examples of gneiss illustrate differing degrees of segregation
between mafic and quartzofeldspathic minerals. The left sample
exhibits a discrete layer of dark, mafic minerals parallel to what is
otherwise a barely developed foliation [green arrow], whereas the
right sample shows alignment of distinct elongate pods of the
lighter minerals that have nevertheless not coalesced into bands.
Gneiss – Banding Without Segregation
The foliation [blue arrow] is defined by banding, with alternating
biotite-rich dark layers [yellow arrows and stars] representing
metamorphism of mudstones, and quartz-plus-feldspar-rich light
layers representing metamorphism of sandstones. In this case
banding does not reflect high grade metamorphism, but rather low
to medium grade metamorphism of layered sedimentary rocks.