Page 217 - Laboratory Manual in Physical Geology

Contact metamorphism occurs locally, adjacent to Mineralogical composition of a parent rock may change
igneous intrusions. It involves conditions of low to during metamorphism as a result of changing pressure, chang-
moderate pressure and intense heating. The intensity of ing temperature, and/or the chemical action of hydrothermal
contact metamorphism is greatest at the contact between fluids, and processes like neomorphism and metasomatism.
parent rock and intrusive magma. The intensity then In general, as temperature and pressure increase, so does the
decreases rapidly over a short distance from the magma metamorphic grade —the intensity of metamorphism, from
or hydrothermal fluids. Thus, zones of contact metamor- low grade (least intense metamorphism) to high grade (most
phism are usually narrow, on the order of millimeters to intense metamorphism). One group of minerals that was
tens- of-meters thick but some are kilometers wide. stable at a low temperature and/or pressure will eventually
The intruding magma thermally metamorphoses the neomorphose to different minerals at a higher temperature
rock in a narrow zone adjacent to the heat source (magma). and/or pressure. An index mineral is a mineral that is stable
under a specific range of temperature and pressure and thus
Hydrothermal metamorphism occurs along fractures characterizes a grade of metamorphism ( FIGURE 7.6 ).
that are in contact with the watery hot (hydrothermal)
fluids. Like contact metamorphism, there is high heat and
low pressure. Textures of Metamorphic Rocks
Texture of a metamorphic rock is a description of its con-
Dynamic metamorphism occurs along fault zones where stituent parts and their sizes, shapes, and arrangements.
there is local-to-regional shearing and crushing of rocks. If Two main groups of metamorphic rocks are distinguished
the rocks are brittle, then shearing produces fault breccia. on the basis of their characteristic textures, foliated and
But if the rocks are hot and ductile, then a fine-grained nonfoliated .
metamorphic rock called mylonite may result. Mylonite is
a hard, dense, fine-grained rock that lacks cleavage but may
have a banded coloration. Foliated Metamorphic Rocks
Foliated metamorphic rocks (foliated textures) exhibit
foliations — layering and parallel alignment of platy (flat)
Minerals of Metamorphic Rocks
mineral crystals, such as micas. The foliations form when
The mineralogical composition of a metamorphic rock directed pressure causes the platy (flat) mineral crystals to
is a description of the kinds and relative abundances slide parallel to and past one another (shear). This can hap-
of mineral crystals that make up the rock. Information pen as they recrystallize. Crystals of minerals such as tour-
about the relative abundances of the minerals is impor- maline, hornblende, and kyanite can also be foliated because
tant for constructing a complete name for the rock and their crystalline growth occurred during metamorphism and
understanding metamorphic changes that formed the had a preferred orientation in relation to the directed pres-
mineralogy of the rock. sure. Specific kinds of foliated textures are described below:

METAMORPHIC ZONES AND INDEX MINERALS

Parent Low Grade Intermediate Grade High Grade
Slate Schist
Shale
Phyllite Gneiss
Chlorite
Muscovite
Biotite
Garnet
Hornblende
Staurolite
Kyanite
Sillimanite
Quartz / Feldspar

FIGURE 7.6 Index minerals of regionally-metamorphosed clay and mica-rich rocks. Sedimentary rocks rich in clay minerals
neomorphose at low grades to larger foliated crystals of platy (flat) minerals like chlorite, muscovite, and biotite. These minerals neomorphose
to garnet and staurolite at an intermediate grade, and then to sillimanite at a high grade of metamorphism.

192 ■ L ABOR ATORY 7