METAMORPHIC ROCKS Prepared by Dr F Clark Department

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 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.