Metamorphic Rocks

Metamorphic rocks form when a pre-existing rock, called the parent rock or protolith, is subjected to some combination of three new “agents” of metamorphism: 1) Elevated Temperature – Different minerals are stable at different temperatures, as we see

from Bowen’s Reaction Series in igneous rocks. The same idea applies in metamorphic rocks though the minerals that form are much more diverse and complex. The complexity results because heat is not the only agent involved, usually.

2) High pressure, either confining (equally high in all directions) or directed (compressed from two opposite directions). Pressure also controls which minerals are stable. You can get carbon as hot as you want and it won’t turn into diamond. You can squeeze it as hard as you want and it won’t turn to diamond. Both things must happen of there’s no diamond. Pressure also aligns flat or elongate minerals in a metamorphic rock, giving it a texture called foliation.

3) Water or “fluids”. Water’s primary role in the process is to move ions around. If there’s plenty of water this is easy, if the rocks are very dry it is not. Ion mobility makes it easier to change one mineral into another, particularly if it involves a chemical change. Water also introduces ions from other rocks making minerals possible that weren’t possible with the elements at hand. Finally, water can move ions out of a metamorphic region where they can concentrate those elements. Much of the world’s store of metals is derived from such “hydrothermal deposits”. They are therefore important, but we will not consider them any further.

TEMPERATURE

Rocks that result from alterations caused by elevated temperature alone are called contact metamorphic rocks because the heat source for the change is usually a nearby magma, with which the protolith was in contact. Such rocks will have minerals stable at various temperatures but will not be foliated, even if minerals are present that could foliate. Mica, for example, is very common. In a foliated rock all the flat mica “books” are aligned parallel to each other, giving the rock an obvious layered appearance. Mica-bearing rocks resulting from contact metamorphism do not have the alignment, nor the layering. The heart source is normally surrounded by concentric zones or aureoles of different temperature minerals, as the diagram shows.

Igneous Rock (hot magma at the time

of metamorphism)

*Unmetamorphosed protolith *Low temp. minerals *Medium temp. minerals *Highest temp. minerals

PRESSURE

A directed compressional pressure on the protolith will cause platy or elongate minerals to align themselves perpendicular to the direction of the stress. This can happen in three ways.

This rock is slate. Its protolith was shale, and this is obvious because it is made of clay minerals, just like shale. Remember that clays are tiny sheet silicates like mica. The original layering in the shale is evident crossing the rock, as indicated by the dashed red line. This bedding probably resulted from slightly different grain sizes in the original mud. Otherwise it would not have been preserved. Instead it would have been destroyed when all the clays were realigned by the directed pressure that created the foliation obvious on the side of the rock and indicated by the yellow dotted line. This type of foliation as called slaty cleavage after the rock or simply rock cleavage. From which directions was the compressional stress that foliated this rock operating?

Way #1

Perpendicular to the foliation, of course.

This is a rock called schist. You should be able to recognize the muscovite that makes up the bulk of the rock because lots of cleavage faces – virtually all the crystals – are reflecting light toward your face. This can only happen if the crystals are all oriented the same way

Viewed from the side the layering that results from the alignment of all the micas is obvious. This type of foliation is called schistosity after the rock.

How was the stress oriented that foliated this rock?

Way #2

Perpendicular to the foliation, of course.

This is a rock called gneiss and the type of foliation is called gneissic foliation. In this case there are feldspars (white – probably plagioclase) and ferromagnesian minerals (black – probably amphibole) and they are fairly well separated into different layers – not perfectly, but almost.

How was the stress oriented that foliated this rock?

Way #3

Perpendicular to the foliation, of course.

The pressures along faults are directed, but oriented differently than in the previous slides. The laterally directed stress pulverizes and grinds the rock along the fault zone, grinding it to a powder called “mylonite”. Because the grains are so small they cannot be foliated, though a different type of vague layering is sometimes apparent in them. Mylonite is a metamorphic rock that results from this sort of pressure. We call this type of metamorphism dynamic metamorphism.

BOTH TEMPERATURE & PRESSURE

The foliated rocks that we saw while talking about foliation do not result from pressure alone. Our evidence for this is twofold. First, increasing pressure always increases temperature as well. In the second case, except for slate (whose minerals are clay) none of the minerals in foliated rocks can form without elevated temperature as well. Even the clay in slate is often partly recrystallized to higher temperature minerals. Foliated rocks occur over large refions of the Earth. The Appalachian region – both the Piedmont and Blue Ridge – are regionally metamorphosed rocks, and the Piedmont is partially buried beneath young sediments of the Coastal Plain. The range extends from Alabama to Canada and has metamorphic rocks throughout. The foliation in the Appalachian rocks, as is true in all metamorphic belts in all mountain chains, suggest that thee directed pressure that made the foliation was directed across the long axis of the mountains – roughly SE/NW. Other indicators of stress direction indicate the same thing. In the next few slides we’ll see one way to refine our interpretation and decide on only one direction of stress. So what is big enough to squeeze the entire side of North America hard enough to metamorphose the rocks all along the eastern ¾ (or more) of the mountains?

Metamorphic Grade

We looked have at three foliated metamorphic rocks and I introduced them in the order I did for a specific reason. The most common protolith for all three is shale (or mudstone), so why do you get different rocks by metamorphosing the same thing?

Slate is formed at T&P that can foliate the rock, but not change the minerals (much). Schist requires higher T&P in order for the clay to be altered to mica. As you’ll recall from Bowen’s Reaction Series amphibole and plagioclase are even higher temperature minerals than muscovite, so the T&P must have been even higher. Metamorphosing a shale to higher and higher T&P would change it into each of these rocks in series, starting with slate and ending with gneiss. Even higher T would melt the rock! (Unless the P increased even faster and kept it from melting.)

We have created the idea of metamorphic grade to make it easier to suggest that both T and P are changing, without having to say, “both T & P” every time. Low grade rocks form at low T & P, mid-grade rocks at moderate T & P, and high grade rocks at the highest T & P. Like a lot of things in the natural world the rocks we know don’t divide the possible range of T&P conditions into equal parts. It would be nice if they did, but they don’t. There is only a small range of T&P’s that can form both slate and gneiss, and a big fat middle range that can form schist. So we have discovered a series of minerals that are formed within the schist range, but at increasing grade within that range. These minerals are called metamorphic index minerals.

There are seven of these index minerals shown here arranged by grade from the bottom left. There is considerable overlap in the stability fields of these minerals so one rock might have more than one of them in it. In that case it is the highest grade mineral that tells the grade of the rock.

Chlorite – a Green Mica

Muscovite you already know

Biotite you already know

Garnet occurs as roundish, soccer-ball shaped crystals

Staurolite has a distinctive shape in cross-section

Kyanite is usually a very pretty blue

Sillimanite is often white and fibrous, like asbestos.

Staurolite also sometimes has two different crystals intergrown to form a “twin crystal” known colloquially as a “fairy cross”

You should see two different minerals in this rock. Both are index minerals. Before you go on, see if you can identify them from the previous picture and indicate which is the higher grade mineral.

Garnet is the higher grade mineral of the two, therefore this rock reached a T&P adequate to form garnet.

A geologic map of an area in Scotland by George Barrow in 1893. This was the first time that anyone had recognized that various minerals in metamorphic zones Were not distributed randomly. The “isograds” were picked at the boundary between rocks that had and rocks that did not have a particular mineral in them. For example, the rocks north of the staurolite isograd have staurolite, those to the south do not. (“Isograd” just means “same grade”). This version of the map was modified slightly for the textbook: Long, L., 1974, Geology, McGraw-Hill, NY, 526 p.

Rocks 1,2,3, and 4 are younger than the metamorphic rocks we are interested in because they disrupt the continuity of those older rocks and faults. In other words, the faults that broke the older rocks did not break the rocks labeled 1-4. Notice that all of Barrow’s isograds are between two faults, which I’ve emphasized in red. The foliation in the rocks would be roughly parallel to the isogads, indicating that the rocks were squeezed by a compression coming from the NNW and SSE as the arrows indicate. (The rocks were later deformed in a different direction, bending them). Which way do you think the main push was actually coming from? Why?

This way. The highest grade rocks, indicating, if you will, the worst T&P regime for preserving the original rocks unaffected is on this side of the belt. (Compare the mineral photos with the index minerals a few slides back.)

High Grade

Low Grade

Staurolite Isograd

Kyanite + staurolite

Only kyanite

Blue Ridge Blairsville

Ellijay

Dahlonega

We see something similar in the mountains of north Georgia. Which way to the zone of maximum T & P?

Garnet isograd

Kyanite isograd

And in the Piedmont near Carrollton, GA. Which way to the zone of maximum T & P?

Increasing grade indicates this way to the zone of highest T&P. Remember the earlier question: What can do this kind of damage to the edge of a continent?

Any ideas?