CHAPTER 10 - METAMORPHISM, METAMORPHIC ROCKS, AND HYDROTHERMAL
ROCKS
Overview
Metamorphism is defined as the changes in texture, new mineral assemblages
or both, occurring in the solid state (no melting), at moderate to great depths
within the earth's interior. Metamorphic rocks are now exposed as a result of
erosion and isostatic adjustment (Canadian Shield and mountain belts).
Texture and composition of the metamorphic rock are different from its parent
rock. Observable relationships indicate that metamorphism involves the high
temperatures, high confining pressure, and movement of fluids that produce
minerals at equilibrium with those conditions. Those high temperature and
pressure conditions cause ductile (plastic) deformation; such conditions exist
at considerable depth within the earth.
The section on factors controlling the characteristics of metamorphic rocks
cites composition of the parent rock, temperature, confining (lithostatic)
pressure and differential stress (compression or shear), foliation, effects of
fluids (usually water), and time as important effects.
The classification of metamorphic rocks is presented in Table 10.1, based on
presence or absence of foliation and minerals present. The metamorphic rock
types are then related to type of metamorphism: contact metamorphism
develops hornfels, marble, and quartzite, all of which are nonfoliated. Regional
metamorphism forms greenschist and amphibolite, while progressive
metamorphism can form a sequence involving slate, phyllite, schist, and
gneiss all produced from shale (parent rock). The concept of metamorphic
facies is well developed in Box 10.4. Migmatites form at higher temperature
and pressure than gneiss and indicate partial melting.
The section on plate tectonics and metamorphism relates the great variety of
metamorphic rocks to variation in temperature and pressure associated with
converging boundaries and subduction zones. Plate tectonics accounts for deep
burial of rocks formed near the earth's surface; it can also create the
pressures necessary for foliation to develop and allows the presence of water
in the deep lithosphere.
The chapter concludes with a discussion of hydrothermal activity at diverging
plate boundaries, metasomatism, and the formation of hydrothermal veins and
minerals summarized in Table 10.3. Water involved in hydrothermal processes
is either ground water or water moved to depth by subduction.
Learning Objectives
1. Metamorphism changes the texture or mineralogy (or both) of its parent
rock usually in response to high temperature and pressure within the Earth's
interior and under conditions that produce ductile (plastic) strain.
2. The chemical composition of the parent rock controls that of the
metamorphic rock, although mineralogy may change. Minerals are stable
within a particular temperature range, but that range varies with pressure and
presence of other substances. The upper limits of metamorphism may overlap
partial melting.
3. New metamorphic minerals crystallize under high confining pressure and
tend to be denser than their low-pressure counterparts. Differential stress
(compression and/or shearing) produces foliated textures described as slaty,
schistose, or gneissic. Water triggers metamorphism and promotes new
mineral formation. Metamorphism is a slow process taking place over millions
of years.
4. The classification of metamorphic rocks is based on texture (foliated versus
nonfoliated) and mineralogy (chemical composition is controlled by parent
rock).
5. Metamorphism is either contact (high temperatures but low confining
pressure) or regional (high-temperatures and high confining pressure).
Metamorphic rocks produced in a contact aureole ("baked zone" of Chapter 6)
include hornfels (shale parent rock), marble (limestone parent rock), and
quartzite (quartz sandstone parent rock). All of these rocks have nonfoliated
textures.
6. Regional metamorphism usually produces foliated rocks such as greenschist
(basalt parent rock) and amphibole schist (basalt parent rock), although
marble and quartzite also form, if their appropriate parent is present.
Progressive metamorphism of shale can produce slate, phyllite, schist or
gneiss as temperature and pressure increase. Partial melting produces
migmatites. Retrogressive metamorphism allows rocks to recrystallize under
lower temperature and pressure conditions than at the peak of metamorphism.
7. Plate tectonics, particularly subduction, create differential stress and
temperature conditions that can cause regional metamorphism.
8. Water plays an important role in metamorphism. Metasomatism involves
hot water transporting ions from outside sources, and can form significant ore
deposits. Hydrothermal rocks are formed by crystallization of minerals from
hot water and most commonly take the form of quartz veins. Disseminated
ore deposits are formed from ore-bearing solutions that percolate throughout
a rock body.
9. The water involved in metamorphism originates as either ground water or
water trapped in descending oceanic crust in a subduction zone.
Boxes
10.1 - ENVIRONMENTAL GEOLOGY – THE OLDEST ROCKS ON EARTH –
Metamorphic rocks are some of the oldest on Earth and are commonly found in
continental cratons such as the Canadian Shield. One of the oldest rocks
known is the Acasta Gneiss, a metamorphosed granite found in the Northwest
Territories, and is thought to have formed around 3.96 billion years ago. The
composition of the gneiss suggests that even in the early stages of Earth
history (around 4 billion years ago) water was present and basaltic crust
formed the floor of ocean basins.
10.2 - ASTROGEOLOGY - IMPACT CRATERS AND SHOCK METAMORPHISM
- Asteroids and comets occasionally collide with the earth and other planets
producing impact craters that display shock metamorphism. Shocked quartz
and the rare silicate coesite, tektites (local melt rock), and atmospheric dust
are produced by these impacts. The Sudbury Structure in Ontario is a large
impact crater that formed around 1.85 billion years ago. Broken, shocked and
partially melted rocks created by this impact are host to extremely rich ore
deposits in the Sudbury region. Large meteorite impacts have had devastating
effects on the Earth - past impacts may have changed global climate and
caused mass extinction of many varieties of creatures including the dinosaurs.
10.3 - IN GREATER DEPTH - INDEX MINERALS - Certain minerals, called
index minerals, form within limited ranges of temperature and pressure during
metamorphism. Three of the best known are: andalusite, kyanite and
sillimanite, which represent increasing temperature respectively, although
pressure also influences mineral formation. Regional metamorphic patterns
across the Grenville Province of the Canadian Shield can be identified from the
distribution of andalusite, kyanite and sillimanite minerals and allow ancient
pressure and temperature conditions to be reconstructed.
10.4 - IN GREATER DEPTH - METAMORPHIC FACIES AND THE
RELATIONSHIP TO PLATE TECTONICS - In the early part of this century, it
was recognized that different mineral associations in metamorphic rocks
reflected the temperature and pressure at the time of their metamorphism.
Rocks with the same mineral assemblage belong to the same metamorphic
facies, named after the dominant mineral component. Different metamorphic
facies form at different temperature and pressure conditions. Early geologists
recognized metamorphic facies but could not explain their environmental
significance until the plate tectonic theory was introduced. Converging plate
boundaries exhibit a variety of metamorphic facies depending upon proximity
to the descending oceanic crust. Temperature and pressure increase with
depth, but at the same depth, temperatures are lower along the cool
subducting plate compared to lithosphere under continental crust or volcanic
plutonic complexes. The resulting metamorphic facies reflect those differences
in temperature.
10.5 - ENVIRONMENTAL GEOLOGY - THE WORLD'S LARGEST HUMANMADE HOLE - THE BINGHAM CANYON COPPER MINE - Bingham Canyon
open-pit copper mine near Salt Lake, Utah is the largest hole in the world (800
meters deep; 4 km wide) made by humans in the search for copper, gold,
silver and other metals. Initial underground shaft mining in the last century
followed silver- and lead-bearing veins. It was discovered early in this century
that copper sulfides were disseminated throughout the granite stock, and since
then, 12 million tons of metallic copper have been produced from Bingham
Canyon. This production comes with an environmental price. The open pit is
viewed as an eyesore, processed waste rock can be blown into dust storms,
and smelting produces toxic smoke containing sulfuric acid.
Short Discussion/Essay
1. Why does metamorphism preclude melting?
2. Why is foliation only associated with regional metamorphism?
3. What role does water play in metamorphism?
4. Why does shale produce so many different types of metamorphic rocks,
while other parent rocks have limited metamorphic equivalents?
5. Why are so many semiprecious gemstones associated with metamorphism?
Longer Discussion/Essay
1. What are metamorphic facies? Do all major rock groups exhibit these
facies? What do these facies represent?
2. What is actually happening to the minerals as foliation increases?
3. Why are metamorphic rocks so limited in their distribution at the earth's
surface?
4. Explain the affects of temperature and pressure on the process of
metamorphism.
5. Why are quartz and other metallic minerals, such as gold, commonly
associated with hydrothermal rocks?
Selected Readings
There are numerous textbooks dealing with metamorphic rocks. Most treat
petrology and petrography. A few of the more recent editions are given here.
Blatt, Harvey and Tracy, R.J. 1996. Petrology. W.H. Freeman and Co.
Fichter, L.S. 1996. "Tectonic Rock Cycles," Journal of Geoscience Education
44(2): 134148
Philpotts, A.R. 1989. Petrography of Igneous and Metamorphic Rocks.
Englewood
Cliffs, NJ: Prentice-Hall.
Raymond, L.A. 1995. Petrology. Dubuque IA: Wm. C. Brown Publishers.
An interesting article on classification.
Brown, V.M. and Harrell, J.A. 1991. "Megascopic classification of Rocks,"
Journal of
Geological Education 39: 379-387