DEFINITION OF KINDS OF METAMORPHIC ROCKS

The names of metamorphic rocks are derived from their original

rock type, their texture, and mineral assemblages.

METAMORPHISM OF SHALE AND MUDSTONE Shales and

mudstones have an initial mineral assemblage of quartz,

clays, calcite, and feldspar. Slate is the low-grade metamorphic

equivalent of shale and, with recrystallization, is made

of quartz and micas. At intermediate grades of metamorphism,

the mica grains grow larger so that individual grains

are now visible to the naked eye and the rock is called a phyllite.

At high grades of metamorphism, the rock (ex-shale)

now becomes a schist, which is coarse grained and the foliation

becomes a bit irregular. Still higher grades of metamorphism

separate the quartz and the mica into different layers;

this rock is called a gneiss. For both schists and gneisses, a

prefix is commonly added to the names to denote some of the

minerals present in the rock. For instance, if garnet grows in

a biotite schist, it could be named a garnet-biotite schist.

METAMORPHISM OF BASALT Fresh basalts contain olivine,

pyroxene, and plagioclase, none of which contains abundant

water. When metamorphosed, however, water typically enters

the rock from outside the system. At low grades of metamorphism,

the basalt is turned into a greenstone or greenschist,

which has a distinctive color because of its mineral assemblage

of chlorite (green)+albite (clear)+epidote (green)+calcite (clear).

At higher metamorphic grades, the greenschist mineral

assemblage is replaced by one stable at higher temperature

and pressure, typically plagioclase and amphibole, and the

rock is known as amphibolite. Amphiboles have a chain

structure, which gives them an elongated shape. When they

crystallize in a different stress field like that found in a metamorphic

rock, the new minerals tend to align themselves so

that their long axes are parallel to the least compressive

stress, forming a lineation. At even higher metamorphic

grades the amphiboles are replaced by pyroxenes and the

rock is called a granulite.

METAMORPHISM OF LIMESTONE When limestone is metamorphosed,

it is converted to marble, which consists of a network

of coarsely crystalline interlocking calcite grains. Most

primary features, such as bedding, are destroyed during metamorphism

and a new sugary texture appears.

METAMORPHISM OF SANDSTONE When sandstone is metamorphosed,

the silica is remobilized and fills in the pore

spaces between the grains, making a very hard rock called a

quartzite. Primary sedimentary structures may still be seen

through the new mineral grains.

Kinds of Metamorphism

Metamorphism is a combination of chemical reactions

induced by changing pressure and temperature conditions

and mechanical deformation caused by differential stresses.

The relative importance of physical and chemical processes

changes with metamorphism in different tectonic settings.

THERMAL OR CONTACT METAMORPHISM Near large plutons

or hot igneous intrusions, rocks are heated to high temperatures

without extensive mechanical deformation.

Therefore, rocks next to plutons typically show growth of

new minerals but lack strong foliations formed during metamorphism.

Rocks adjacent to these large plutons develop a

contact metamorphic aureole of rocks, altered by heat from

the intrusion. Large intrusions carry a lot of heat and typically

have large contact aureoles, several kilometers wide.

The contact metamorphic aureole is made of several concentric

zones each with different mineral groups related to

higher temperatures closer to the pluton. A hornfels is a hard,

fine-grained rock composed of uniform interlocking grains,

typically from metamorphosed and suddenly heated shale.

BURIAL METAMORPHISM When rocks are buried by the

weight of overlying sedimentary rocks, they undergo small

changes called diagenesis, until they reach 390°F (200°C). At

about 570°F (300°C), some recrystallization may begin, particularly

the formation of a group of water-rich minerals

known as zeolites.

REGIONAL METAMORPHISM The most common types of

metamorphic rocks are the regional metamorphic rocks.

Regional metamorphism involves a combination of chemical

and mechanical effects and so these rocks tend to have a pronounced

foliation (slate, schist). Most regional metamorphic

rocks are found in mountain belts or old eroded mountain

belts, which were formed by the collision of two tectonic

plates. In regional metamorphic conditions, the rocks are

compressed horizontally, resulting in large folds and faults,

which place some rocks on top of other ones, burying them

quickly and elevating their pressure and temperature conditions.

In this type of environment there is a wide range of

pressure/temperature conditions over which the rocks were

metamorphosed, and geologists have defined a series of different

metamorphic zones which reflect these conditions.

These metamorphic zones are each defined by the appearance

of a new metamorphic index mineral, which include, in

progressively higher grade order (for shale), chlorite-biotitegarnet-

staurolite-kyanite-sillimanite. In the field, the geologist

examines the rocks and looks for the first appearance of these

different minerals and plots them on a map. By mapping out

the distribution of the first appearance of these minerals on a

regional scale, the geologist then defines isograds, which are

lines on a map marking the first appearance of a given index

mineral on the map. The regions between isograds are known

as metamorphic zones.

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