By: Alan V. Morgan
Continuing our story of the respective rock families we reach the third great group, the metamorphic rocks. Igneous rocks were covered in What On Earth 2002 Fall issue and the Sedimentary rocks in the Spring issue of 2003. All three categories are now covered in a new teaching poster that can be obtained at a minor cost from What On Earth.
Metamorphic rocks are complex because they have been derived from either pre existing igneous, sedimentary or metamorphic rocks. These "parent" rocks are modified during metamorphism and the attempt to re establish equilibrium with the changed conditions creates another "daughter" rock type. After metamorphism, pre-existing parent rocks that had quite different origins might end up looking essentially the same. Alternatively, just one original parent rock type might end up with several different daughter stages - and different metamorphic names - depending upon the degree of change that it has been through.
There are a number of processes that cause metamorphic change, and these take place from about 200° C to ~850°C, depending on different factors. The first is heat and this can be from local temperature changes (for example from nearby magmatic bodies that are being emplaced) or as the result of deep burial and a geothermal gradient. The latter is seen as a gradual increase of temperature as you go deeper into the Earth. However, even in the case of the deepest mines or oil wells, temperatures are usually insufficient for metamorphism. Very deep burial in places with a higher than average geothermal gradient are necessary for regional metamorphism.
The chemical changes that take place in rocks are considerably speeded if water is present and circulating through the rock unit. Hot dry rocks will change less than hot wet rocks, and the grain size and arrangement of grains are also important. Pressure by the weight of the overlying sediments promotes movement of water within the rock. This pressure may be directed by tectonism or other large-scale Earth movements and rocks that are stressed or sheared, for example along faults, often show localised (cataclastic) metamorphism.
The literature on metamorphic rocks is complex and very detailed and this article only outlines metamorphic rocks in a very general way. Let's take hypothetical eroded landscape and a hike across the surface. On the way we will encounter outcrops of rocks. In the distance, a number of kilometres away, is a low line of hills. According to our geological map they are made up of granites that were intruded into the rocks we are going to walk across. At the start of our walk we examine the first outcrops that consist of mudstone and shales. As you will recognise from the sedimentary rocks outlined in the last What On Earth, these were formed from very fine-grained clastic sediments made up largely of clay-sized particles. Such sediments are very susceptible to heat, pressure and chemically active fluids.
Foliated metamorphic rocks
Before we travel far we see a change in the rocks. They gradually become greenish or silvery-coloured due to the development of the minerals chlorite and muscovite within them. The clay minerals will likely also acquire a new orientation that is perpendicular to the direction of regional pressure. In this area we might see slate developed, with its excellent parallel cleavage. This chlorite (and slate) stage is considered as low-grade metamorphism where temperatures are in the range of ~200+°C.
Moving on towards the mountains the slate gradually changes. The white mica, muscovite, often accompanied by black mica known as biotite, becomes more obvious. The excellent fissility in the slate breaks down. It no longer cleaves into thin flat slabs but breaks into shorter crenulated fragments. The rock type is now a phyllite formed at temperatures of about 300°C. As you move still closer to the granite the phyllite changes to a foliated rock dominated by layers of muscovite and/or biotite micas in 1-5 mm crystals known as a mica schist. This formed at temperatures close to ~400°C.
Within the schist a deep maroon-red mineral might start to appear, and if the rock is still relatively fine grained, these minerals stand out as raised "spots". The spots are red garnets, and garnets together with other bladed minerals such as andalusite, staurolite and cordierite indicate that you are getting well into the medium rank of the metamorphic grade. The rock type now changes to gneiss. Gneisses are typified by bands of feldspar, quartz and micaceous minerals, created under high temperatures (400 - 500°C) and under moderately high pressures. These rocks were once deeply buried in the footings of former mountain ranges.
Still more minerals start to appear closer to the granite outcrop. Staurolite is an intriguing mineral that forms curious "crosses", sometimes known as "fairy crosses" as a result of twinning during its formation. Continuing our treck we encounter the mineral kyanite. Some years ago when I was traversing a particularly muddy, slumped, roadway in Tibet, I noticed that the engineers had used a crushed fill that contained a spectacular bright blue mineral - Kyanite. Kyanite, and sillimanite, seen as needle-like, rather glassy crystals appear closest to the granite outcrop and mark the highest metamorphic zone.
My students often have problems remembering these zones from lowest to highest metamorphic grades so I suggest a simple gnomonic: "Charlie Brown's groundhog stomps killer slugs" where the initial letter of each word defines a metamorphic zone. C, B, G, S, K, and S, mark the Chlorite, Biotite, Garnet, Staurolite, Kyanite, and Sillimanite zones. These diagnostic zonal minerals are illustrated on the second line of the centre-fold in this issue. The rock types that are typically found associated with them are shown on the top line below the James Bay image.
A few concluding words about these "foliated" rocks. Because of the platy nature of many of the minerals, particularly in the lower metamorphic grades, the rocks have fissility due to grain re-orientation. I have already mentioned this in slate (excellent), phyllite (poor), and mica schists (even more poor). By the time the gneiss category is reached there is a foliation but many of the minerals have grown "fat", and fissility starts to be lost. Small veinlets of quartz and feldspar have replaced the original clay minerals although the rock does retain some linearity because of zones of oriented micas. The gneiss example illustrated in the centrefold shows pink feldspars, grey quartz and some crumpled bands that are delineated by biotite micas. Finally, as I mentioned in "The Rock Cycle" (Wat On Earth, Fall 1999) metamorphic rocks that are heated substantially will melt. A migmatite (illustrated at the far right of the top rock line in the centrefold) illustrates the highest grade of metamorphism and one that is in the process of being converted back to an igneous rock. This occurs in the temperature range of ~600 to ~850°C.
The area of metamorphism represented by the "baked" rocks around any type of large igneous intrusion is known as the "metamorphic aureole". This will range in size depending on the nature and characteristics of the country rock and the emplacement depth, temperature and chemical composition of the intrusion. It might occupy a belt many kilometres wide around the igneous body, or it might be just a few cm or mm wide adjacent to small intrusions, such as dykes or sills. Alteration immediately adjacent to a hot igneous body is called "contact" metamorphism.
We have seen what might happen under ideal circumstances to a sequence of fine-grained sediments under regional metamorphism. Generally they become "foliated" but many other rocks fall into a category called "non-foliated" and these are illustrated by seven images in the bottom line of the centrefold.
Non-foliated metamorphic rocks
Under metamorphism, coarse-grained rocks will often become "stretched" because of stress. In former sedimentary rocks such as conglomerate, the clasts can become elongated in a metaconglomerate. Metaquartzite is a former sedimentary quartzite that has been altered by heat and pressure. Individual silica grains and the silica cement has been recrystallised into a granular, interlocking mosaic of silica. This is an extremely hard rock type and difficult to distinguish between its sedimentary counterpart. Limestones will also recrystallise to form marble, which, because of its soft massive nature can be carved into almost any shape. Gasses and fluids rich in iron, magnesium, silicon and aluminum often invade carbonate-rich rocks close to igneous intrusions. Such metamorphosed areas are termed "skarns" and they contain distinctive suites of minerals, many of which have economic importance.
Former igneous rocks that were in the basic or mafic category (such as basalts) will become greenish coloured because of the development of chlorite and epidote within them; they acquire the generalised name of "greenstones". Greenstone belts are often of considerable economic importance since they contain many sulphide minerals as well as gold and platinum group elements, formerly contained in igneous bodies or in their metamorphic aureoles that have been remobilised during tectonism.
As I mentioned under sedimentary rocks, coals are also "changed" rocks. They have lost water and volatiles and have acquired increasing carbon content due to deep burial and heating. For this reason I have included anthracite coal in the metamorphic rock category as well as under sedimentary rocks. Temperatures involved in the formation of anthracite fall into the metamorphic range of ~200 to ~270°C.
The last two images include Hornfels, a rather glassy metamorphic rock, that formed in a high-temperature regionally-baked environment adjacent to a "wet" magma mass that cooled over a long period of time. Finally Serpentinite is a metamorphosed rock that originally was an ultrabasic igneous rock (such as dunite or peridotite) reheated to somewhat less than 500°C. It is made up of serpentine, a mass of commonly white, grey-green, red and yellow hydrous magnesium-rich minerals, with an appearance rather like a discarded snake skin.
Metamorphic Facies
As we have seen there are a variety of metamorphic rocks that have been formed under different temperature and pressure regimes. These can be converted into a temperature-depth diagram as seen in Figure **.
Zeolite Facies: This is developed in regionally metamorphosed rocks under low temperature and pressure conditions. The minerals that are created (complex hydrous aluminosilicates) are developed from sedimentary rocks and also from volcanic rocks. The facies represents the transition from sedimentary diagenetic processes and those seen in the Greenschist facies.
Greenschist Facies: So-called because many of the minerals created in regional metamorphism under these conditions are green in colour (chlorite, epidote and serpentine). The pre-existing rock types are commonly basic volcanics. High shear stresses may be common and this factor together with the platy nature of many of the green minerals commonly forms schist.
Amphibolite Facies: The rocks in this facies have formed under relatively high temperatures and pressures and are characterised by minerals that include amphibole, garnets (almandine and grossularite) and woolastonite (a calcareous silicate). These rocks are fairly common in Precambrian Shield areas and are believed to represent the deeper parts of ancient, exhumed, folded mountain ranges.
Granulite Facies: Rocks in this category represent the highest grade of regional metamorphism. At the top end of the temperature range the rocks are transitional with migmatites (see above). Minerals formed include sillimanite, pyroxenes and even calcium plagioclase.
Hornfels Facies: Progressing through an elevated temperature gradient, at low pressures, the rocks of the Hornfels Facies are encountered. This major metamorphic category represents rocks formed by contact metamorphism, with temperatures as high as 800°C to 850°C. Mineral compositions vary according to the parent rock. (see Hornfels above).
Blueschist Facies: Typically formed where oceanic rocks have been subducted under high pressure but low temperature conditions, the facies is rich in the blue-coloured amphibole mineral, glaucophane.
Eclogite Facies: Created under high pressures and high temperatures the rocks are rich in garnets and pyroxenes. Eclogite is a spectacular red and green coloured rock.
Slates from the Llechwydd quarries, Nantlle region of North Wales (left) have long been renowned for their properties of thin fissility (about 2mm) and durability. More intense metamorphism allows feldspars to grow.
The image below illustrates a gneiss along the shore of Lake Superior. The foliation directions can be clearly seen, but this rock type is extremely hard and is often used as polished ornamental stone.
“Baked” rocks that retain original bedding are often seen as Hornfels (above). This example is from the wide metamorphic aureole on the north side of the Dartmoor granite in southwestern England. Re-crystallisation can allow silica cements to re-organise, changing a former sedimentary quartzite to a metamorphic rock (below). Here the original silica grains (dark blue) in a silica cement (light blue) have acquired an interlocking texture.
