Metamorphic rocks (1)

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 inc

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 existing crystals tend to rotate into this orientation.

pre-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.

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 – 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

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]

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

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]

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

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

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