METAMORPHIC METAMORPHIC ROCKSROCKS

INTRODUCTION – THE ORIGIN OF INTRODUCTION – THE ORIGIN OF METAMORPHIC TEXTURESMETAMORPHIC TEXTURES

In many cases, metamorphism involves at least some In many cases, metamorphism involves at least some increase in pressure compared to that under which increase in pressure compared to that under which the rocks originally formed, and in addition, the the rocks originally formed, and in addition, the pressure is not uniformly distributed or oriented. pressure is not uniformly distributed or oriented. Thus there is one direction [and it could be Thus there is one direction [and it could be anyany direction] along which pressure reaches some direction] along which pressure reaches some maximum value, and this produces what is called maximum value, and this produces what is called directed stress. The effect of this directed stress is to directed stress. The effect of this directed stress is to cause minerals that have at least one longer cause minerals that have at least one longer dimension to orient themselves with the long axis or dimension to orient themselves with the long axis or axes perpendicular to the directed stress. New crystal axes perpendicular to the directed stress. New crystal growth will occur with this orientation, and pre-growth will occur with this orientation, and pre-existing crystals tend to rotate into this orientation.existing crystals tend to rotate into this orientation.

METAMORPHIC FABRIC - 1METAMORPHIC FABRIC - 1

The preferred alignment or orientation of minerals The preferred alignment or orientation of minerals within many metamorphic rocks gives those rocks within many metamorphic rocks gives those rocks fabric. Two principal groups of minerals are involved fabric. Two principal groups of minerals are involved in this. Sheet silicates, especially the micas biotite, in this. Sheet silicates, especially the micas biotite, muscovite, and phlogopite, will align themselves muscovite, and phlogopite, will align themselves with the flakes or flat faces of the crystals parallel to with the flakes or flat faces of the crystals parallel to each other and at right angles to the directed stress. each other and at right angles to the directed stress. This type of fabric is called foliation. As an This type of fabric is called foliation. As an illustration, think of gravity as the stress, and a deck illustration, think of gravity as the stress, and a deck of cards as the micas. If you let go of the deck, the 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 cards will fall to the table top or floor and will all lie flat, their long dimensions (the faces) at right angles flat, their long dimensions (the faces) at right angles to gravity, whose orientation is vertical.to gravity, whose orientation is vertical.

METAMORPHIC FABRIC - 2METAMORPHIC FABRIC - 2

The second group of minerals whose alignment The second group of minerals whose alignment typically produces fabric is the amphibole group. typically produces fabric is the amphibole group. These prismatic or elongate crystals will have their These prismatic or elongate crystals will have their long axes parallel to each other. This fabric is called long axes parallel to each other. This fabric is called lineation. To give an illustration, imagine a handful lineation. To give an illustration, imagine a handful of pencils as representing the amphibole crystals. of pencils as representing the amphibole crystals. Again, the stress is gravity, oriented vertically. If Again, the stress is gravity, oriented vertically. If you let the pencils drop, they will not stand on end, you let the pencils drop, they will not stand on end, but will come to rest in a horizontal position. Unlike but will come to rest in a horizontal position. Unlike the pencils, which will be randomly oriented on the the pencils, which will be randomly oriented on the floor, amphiboles tend to be aligned more like a floor, amphiboles tend to be aligned more like a bundle of pencils in an elastic band. A secondary or bundle of pencils in an elastic band. A secondary or intermediate stress at right angles to the directed intermediate stress at right angles to the directed stress forces this more ordered alignment. stress forces this more ordered alignment.

METAMORPHIC FABRIC - OPTIONALMETAMORPHIC FABRIC - OPTIONAL

We should point not that not all metamorphic rocks have We should point not that not all metamorphic rocks have fabric. Certain minerals tend to be equidimensional fabric. Certain minerals tend to be equidimensional and don’t generally produce fabric. Quartz and calcite and don’t generally produce fabric. Quartz and calcite are two common examples, whose random orientation are two common examples, whose random orientation is seen in such rocks as quartzite and marble, is seen in such rocks as quartzite and marble, respectively [illustrated in the file on metamorphic respectively [illustrated in the file on metamorphic minerals]. As well, in thermal or contact minerals]. As well, in thermal or contact metamorphism next to igneous intrusions, the metamorphism next to igneous intrusions, the absence of directed stress means there is no tendency absence of directed stress means there is no tendency to develop fabric. A common product is a rock called to develop fabric. A common product is a rock called hornfels [named for its common constituents hornfels [named for its common constituents hornblende plus feldspar], which may resemble basalt hornblende plus feldspar], which may resemble basalt or diabase, a shallow intrusive equivalent of basalt. or diabase, a shallow intrusive equivalent of basalt.

METAMORPHISM OF AN ARGILLITE METAMORPHISM OF AN ARGILLITE ––

THE TEXTURAL STORYTHE TEXTURAL STORYJust as progressive metamorphism of an argillite or Just as progressive metamorphism of an argillite or

mudstone parent rock gives rise to a characteristic mudstone parent rock gives rise to a characteristic succession of minerals with increasing grade or succession of minerals with increasing grade or intensity, so too it gives rise to a characteristic intensity, so too it gives rise to a characteristic succession of textures, based on the arrangement succession of textures, based on the arrangement or orientation of the grains. The rock names derive or orientation of the grains. The rock names derive from the names of the textures, and so we see in from the names of the textures, and so we see in order of increasing grade the rocks slate, phyllite, order of increasing grade the rocks slate, phyllite, schist, and gneiss, which will be illustrated in turn. schist, and gneiss, which will be illustrated in turn. The corresponding textural terms are slaty The corresponding textural terms are slaty cleavage, phyllitic cleavage, schistosity, and cleavage, phyllitic cleavage, schistosity, and gneissosity, all of which are admittedly awkward gneissosity, all of which are admittedly awkward terms.terms.

Slate.Slate.This This represents represents the lowest the lowest grade of grade of metamorphismetamorphism. There is m. There is grain grain reorientation reorientation without without appreciable appreciable grain growth.grain growth.

The reorientation of fine sheet silicates in response to the directed The reorientation of fine sheet silicates in response to the directed stress leads to slaty cleavage, wherein the rock splits very cleanly stress leads to slaty cleavage, wherein the rock splits very cleanly along essentially planar surfaces. A red shale would be less compact along essentially planar surfaces. A red shale would be less compact and lack the sheen of the cleavage surfaces of this red slate.and lack the sheen of the cleavage surfaces of this red slate.

Slate – Exploiting its PropertiesSlate – Exploiting its Properties

These discards from the roofing material used (by one of These discards from the roofing material used (by one of Edmonton’s more prosperous citizens) in preference to shakes Edmonton’s more prosperous citizens) in preference to shakes or asphalt shingles could be cleaved more finely [green or asphalt shingles could be cleaved more finely [green arrows]. Alignment of the platy grains makes the material arrows]. Alignment of the platy grains makes the material impermeable, and the fact it splits into thin sheets makes the impermeable, and the fact it splits into thin sheets makes the weight manageable. Note the low sheen on the cleavage weight manageable. Note the low sheen on the cleavage planes, which pass between grains.planes, which pass between grains.

Slate – Original Identity PreservedSlate – Original Identity Preserved

In this low grade metamorphic rock, the original identity of the In this low grade metamorphic rock, the original identity of the parent rock can be seen. We see graded beds [purple arrows] parent rock can be seen. We see graded beds [purple arrows] with light, coarse grains at the base grading to dark, fine grains with light, coarse grains at the base grading to dark, fine grains at the top. As well, we notice that the foliation and cleavage, at the top. As well, we notice that the foliation and cleavage, parallel to the broad faces of the sample, are unrelated to the parallel to the broad faces of the sample, are unrelated to the original bedding, whose planes are parallel to the light blue original bedding, whose planes are parallel to the light blue arrows.arrows.

Phyllite.Phyllite.This is also a This is also a low grade low grade metamorphic metamorphic rock, but is of rock, but is of a higher a higher grade than grade than slate. slate. Cleavage Cleavage planes are planes are highlighted highlighted by yellow by yellow stars.stars.With higher metamorphic grade comes increased crystal size. The With higher metamorphic grade comes increased crystal size. The faces of the larger grains are more reflective than smaller grains, and 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 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].in slate. Even irregular fracture surfaces have a sheen [blue star].

Phyllite.Phyllite.As with slate, As with slate, cleavage is a cleavage is a bulk property bulk property of the rock of the rock and passes and passes between between grains, unlike grains, unlike mineral mineral cleavage cleavage which passes which passes through through crystals.crystals.This phyllite has less regular cleavage planes and a higher sheen This phyllite has less regular cleavage planes and a higher sheen due to the coarser grains. The intermediate stress has influenced the due to the coarser grains. The intermediate stress has influenced the growth of these crystals such that they form ridges and swales growth of these crystals such that they form ridges and swales aligned parallel to the green arrows and perpendicular to this stress. aligned parallel to the green arrows and perpendicular to this stress.

Schist.Schist.This texture This texture also appears also appears first at low first at low grades, but grades, but higher than higher than both slate both slate and phyllite. and phyllite. The The significance significance of the stars is of the stars is explained on explained on the next the next slide.slide.This schist has a reasonably coarse grain size, wherein individual grains This schist has a reasonably coarse grain size, wherein individual grains of biotite in particular are readily distinguished with the unaided eye. of biotite in particular are readily distinguished with the unaided eye. This low grade schist is This low grade schist is friablefriable, that is, can easily be crumbled with , that is, can easily be crumbled with normal finger pressure. This is not generally seen in slate or phyllite.normal finger pressure. This is not generally seen in slate or phyllite.

Schistosity and Weathering - 1Schistosity and Weathering - 1

The foliation produced by alignment of the biotite grains is the The foliation produced by alignment of the biotite grains is the schistosity, and accounts for the easy way in which this rock schistosity, and accounts for the easy way in which this rock splits [green arrows]. This exposes the fresh surface [yellow splits [green arrows]. This exposes the fresh surface [yellow stars], which is nearly black as expected for biotite, whereas the stars], which is nearly black as expected for biotite, whereas the weathered surface [blue stars] is brown because of oxidation weathered surface [blue stars] is brown because of oxidation [rusting] of the iron content of the mafic sheet silicate biotite.[rusting] of the iron content of the mafic sheet silicate biotite.

Schistosity and Weathering - 2Schistosity and Weathering - 2

Again, the iron content of biotite causes the weathered surface Again, the iron content of biotite causes the weathered surface [blue stars] to turn brown, as distinct from the dark fresh surface [blue stars] to turn brown, as distinct from the dark fresh surface [yellow stars]. This sample is of higher grade than the previous [yellow stars]. This sample is of higher grade than the previous one, as indicated by the coarser crystal size. The coarser grains one, as indicated by the coarser crystal size. The coarser grains result in a loss of well developed cleavage planes; the rock splits result in a loss of well developed cleavage planes; the rock splits very irregularly, but still parallel to the foliation [green arrows].very irregularly, but still parallel to the foliation [green arrows].

Schist.Schist.At a higher At a higher grade, in this grade, in this case medium case medium grade, new grade, new minerals minerals such as such as garnet garnet [yellow [yellow arrows] may arrows] may appear, appear, within a within a matrix of matrix of muscovite in muscovite in this case.this case.The garnet crystals have a strong force of crystallization, and will The garnet crystals have a strong force of crystallization, and will usually be larger in size than surrounding micas. Such larger crystals usually be larger in size than surrounding micas. Such larger crystals are called are called porphyroblastsporphyroblasts, and generally their appearance marks the , and generally their appearance marks the onset of medium grade metamorphism, still within schists.onset of medium grade metamorphism, still within schists.

Schist.Schist.Generally at Generally at a higher a higher grade than grade than for garnet, for garnet, staurolite staurolite [blue arrows] [blue arrows] may appear may appear as as porphyroblasporphyroblasts.ts.

In this case, the disparity in grain size between staurolite In this case, the disparity in grain size between staurolite porphyroblasts and the biotite of the matrix is extreme. Even though porphyroblasts and the biotite of the matrix is extreme. Even though the sample is not bounded by large planar surfaces, the biotite grains the sample is not bounded by large planar surfaces, the biotite grains at its surface are parallel to each other and define the foliation.at its surface are parallel to each other and define the foliation.

Schist Schist with a with a Twist.Twist.In this In this sample, the sample, the grains of grains of various various minerals are minerals are clearly not clearly not uniformly uniformly distributed.distributed.

Alignment of micas produces a strong foliation [green arrow]. Parallel Alignment of micas produces a strong foliation [green arrow]. Parallel to the foliation is a thin band of quartzofeldspathic [mix of quartz plus 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 feldspars] material. If this was introduced to the sample later, it could be a schist, but the banding is suggestive of another rock, gneiss.be a schist, but the banding is suggestive of another rock, gneiss.

Schist or Gneiss? The Debate Schist or Gneiss? The Debate Continues.Continues.These views of the same sample show dominant biotite grains These views of the same sample show dominant biotite grains producing foliation, which is seen in high reflectance from producing foliation, which is seen in high reflectance from biotite grains at the surfaces of the sample highlighted by biotite grains at the surfaces of the sample highlighted by yellow stars. Some would suggest that a schist has more than yellow stars. Some would suggest that a schist has more than 50% orientable grains (e.g. micas, as seen in this sample, 50% orientable grains (e.g. micas, as seen in this sample, amphiboles, and kyanite), and a gneiss less than 50%, amphiboles, and kyanite), and a gneiss less than 50%, regardless of the development of mineral banding. Here we regardless of the development of mineral banding. Here we see two discrete but minor quartzofeldspathic layers [blue and see two discrete but minor quartzofeldspathic layers [blue and purple arrows] that are not quite parallel to the foliation (note purple arrows] that are not quite parallel to the foliation (note they do not bend, whereas the foliation does), suggesting they they do not bend, whereas the foliation does), suggesting they may have been introduced to the rock afterward as fracture may have been introduced to the rock afterward as fracture fillings, or veins. fillings, or veins.

Amphibolite –Equivalent Grade to Amphibolite –Equivalent Grade to SchistSchist

Where elongate or prismatic amphibole crystals grow instead of Where elongate or prismatic amphibole crystals grow instead of micas, the rock’s fabric is lineation, parallel to the long axes of micas, the rock’s fabric is lineation, parallel to the long axes of the grains [green arrow]. Garnet crystals are highlighted by the grains [green arrow]. Garnet crystals are highlighted by yellow arrows. In the view on the right, you are looking down yellow arrows. In the view on the right, you are looking down the length of the amphiboles, which in this perspective have the length of the amphiboles, which in this perspective have no long dimension and therefore no lineation is apparent.no long dimension and therefore no lineation is apparent.

Gneiss – A High Grade Metamorphic Gneiss – A High Grade Metamorphic RockRock

At the highest grades, minerals segregate into distinct At the highest grades, minerals segregate into distinct compositional bands. Generally there will be dark bands compositional bands. Generally there will be dark bands dominated by mafic minerals such as biotite and amphibole, and dominated by mafic minerals such as biotite and amphibole, and white/grey/pink bands [arrows] dominated by quartz plus white/grey/pink bands [arrows] dominated by quartz plus feldspars. Feldspars may occur as discrete pockets called feldspars. Feldspars may occur as discrete pockets called augenaugen, , from the German for “eyes” [stars]. Left view is cut surface, from the German for “eyes” [stars]. Left view is cut surface, right is weathered.right is weathered.

Gneiss.Gneiss.This is called This is called a granitic a granitic gneiss gneiss because its because its mineralogy mineralogy matches that matches that of the of the igneous igneous intrusive rock intrusive rock granite. Its granite. Its texture texture marks it as marks it as metamorphicmetamorphic..The orientation of the sample in this view means that we are not The orientation of the sample in this view means that we are not looking parallel to the fabric. Nevertheless, the distinct segregation looking parallel to the fabric. Nevertheless, the distinct segregation into compositional bands is apparent. Turning a sample around will into compositional bands is apparent. Turning a sample around will often show things not apparent in your first view.often show things not apparent in your first view.

Twice as Gneiss (Sorry, couldn’t resist Twice as Gneiss (Sorry, couldn’t resist it!)it!)

Two more examples of gneiss illustrate differing degrees of Two more examples of gneiss illustrate differing degrees of segregation between mafic and quartzofeldspathic minerals. The segregation between mafic and quartzofeldspathic minerals. The left sample exhibits a discrete layer of dark, mafic minerals left sample exhibits a discrete layer of dark, mafic minerals parallel to what is otherwise a barely developed foliation [green parallel to what is otherwise a barely developed foliation [green arrow], whereas the right sample shows alignment of distinct arrow], whereas the right sample shows alignment of distinct elongate pods of the lighter minerals that have nevertheless not elongate pods of the lighter minerals that have nevertheless not coalesced into bands.coalesced into bands.

Gneiss – Banding Without SegregationGneiss – Banding Without Segregation

The foliation [blue arrow] is defined by banding, with alternating The foliation [blue arrow] is defined by banding, with alternating biotite-rich dark layers [yellow arrows and stars] representing biotite-rich dark layers [yellow arrows and stars] representing metamorphism of mudstones, and quartz-plus-feldspar-rich light metamorphism of mudstones, and quartz-plus-feldspar-rich light layers representing metamorphism of sandstones. In this case layers representing metamorphism of sandstones. In this case banding does not reflect high grade metamorphism, but rather banding does not reflect high grade metamorphism, but rather low to medium grade metamorphism of layered sedimentary low to medium grade metamorphism of layered sedimentary rocks.rocks.