METAMORPHIC ROCKS

Metamorphism is the alteration of older parent rocks by high temperatures, high pressures, and fluids.

It can have three effects on the mineral grains in rocks:
it causes them to recrystallize and grow in size;
it deforms and reorients them to create foliation;
it creates new metamorphic minerals.

METAMORPHIC GRADE

Metamorphism occurs over a wide range of temperature and pressure conditions.

Three degrees of intensity of metamorphism are defined:

low grade
medium grade
high grade

METAMORPHIC TEXTURES

There are two textural classes of metamorphic rocks:

foliated rocks: characterized by their layered and banded appearance, due to the parallel orientation of platy and elongate minerals such as micas and hornblende
nonfoliated rocks: characterized by their lack of foliation and a massive, structureless appearance

Foliation results when rocks are subjected to a strong directed pressure (or stress) during metamorphism.

Four foliation textures can be distinguished on the basis of grain size and the thickness of foliation:

slaty: very fine grained, with extremely thin, planar, and parallel foliation bands
phyllitic: fine grained, with thin, wavy, and parallel foliation bands and a silky luster
schistose: medium to coarse grained, with moderately thick, planar foliation bands and coarse, shiny mica flakes
gneissic: coarse to very coarse grained, with thick planar and wavy foliation bands which often alternate in composition and color

METAMORPHIC MINERALS

Metamorphic rocks contain two classes of minerals:

1. minerals that are inherited from their parent rocks, including the common rock-forming minerals quartz, feldspar, amphibole, mica, pyroxene, and calcite; and

2. minerals that recrystallize during metamorphic reactions, including chlorite, muscovite, biotite, staurolite, graphite, garnet, and wollastonite (the "metamorphic minerals")

The metamorphic minerals are index minerals of the metamorphic grade.

METAMORPHIC ROCKS

Foliated rocks are named on the basis of their textures (slate, phyllite, schist, gneiss). The names of schists and gneisses are typically qualified with descriptions of their most abundant or distinctive mineral (such as muscovite schist or garnet gneiss).

Nonfoliated rocks are named primarily on the basis of their compositions (e.g., quartzite, marble, anthracite coal, and amphibolite).

METAMORPHIC PROCESSES

Contact metamorphism is the localized alteration of rocks by the heat and fluids of magma and lava.

It is a high temperature but low pressure process which produces high to low grade nonfoliated rocks.

Regional dynamothermal metamorphism is the alteration of great volumes of rock during tectonic deformation along convergent margins. It is a high temperature-high pressure process which produces high to low-grade foliated rocks.

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