GEO 101 Lecture Notes - Lecture 14: Quartzite, Hornfels, Metamorphism
23 Jun 2018
School
Department
Course
Professor
Types of Metamorphism
There are two major kinds of metamorphism: regional and contact.
Regional metamorphism. Most metamorphic rocks are the result of regional
metamorphism(also called dynamothermal metamorphism).These rocks were
typically exposed to tectonic forces and associated high pressures and temperatures.
They are usually foliated and deformed and thought to be remnants of ancient mountain
ranges.
Metamorphic grades. The different groups of minerals, or assemblages, that
crystallize and are stable at the different pressure and temperature ranges during
regional metamorphism distinguish distinct metamorphic grades, or faces. The grades
are usually named for the dominant minerals or colors that identify them
In general, proceeding from low grade (lower pressure and temperature) to high
grade (higher pressure and temperature), the following facies are recognized:
Zeolite: low temperature, low pressure
Prehnite pumpellyite‐: low temperature, low medium pressure‐
Greenschist: low medium temperature, low medium pressure‐ ‐
Blueschist: low medium temperature, high pressure‐
Amphibolite: medium high temperature, medium high pressure‐ ‐
Granulite: high temperature, high pressure
Contact metamorphism. Contact metamorphism (also called thermal
metamorphism) is the process by which the country rock that surrounds a hot magma
intrusion is metamorphosed by the high heat flow coming from the intrusion. The zone
of metamorphism that surrounds the intrusion is called the halo (or aureole) and rarely
extends more than 100 meters into the country rock. Geostatic pressure is usually a
minor factor, since contact metamorphism generally takes place less than 10 kilometers
from the surface.
Metamorphic Rock Types
Metamorphic rocks are classified by texture and by mineral composition.
Foliated metamorphic rocks. If a rock is foliated, its name is determined by the type of
foliation present and the dominant minerals—for example, a kyanite schist. If the
minerals are segregated into alternating light colored and dark colored layers, the rock ‐ ‐
is called a gneiss. Slates are generally fine grained, dark colored, metamorphosed ‐ ‐
sedimentary rocks that split easily along slaty foliations and were formed under low‐
grade temperature and pressure conditions. Phyllites are slightly more metamorphosed
than slates and contain mica crystals that impart a glossy sheen. A schist is coarser
grained than phyllite or slate and has aligned minerals that can be identified with the
naked eye. Some varieties of schist are mica, garnet mica, biotite, kyanite, and talc ‐
schist. A schistose rock composed of the mineral serpentine is called a serpentinite.
Migmatites form when temperatures are hot enough to partially melt the rock. The
magma is sweated out, or injected, as layers between foliation planes in the rock.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
There are two major kinds of metamorphism: regional and contact. Most metamorphic rocks are the result of regional metamorphism(also called dynamothermal metamorphism). these rocks were typically exposed to tectonic forces and associated high pressures and temperatures. They are usually foliated and deformed and thought to be remnants of ancient mountain ranges. The different groups of minerals, or assemblages, that crystallize and are stable at the different pressure and temperature ranges during regional metamorphism distinguish distinct metamorphic grades, or faces. The grades are usually named for the dominant minerals or colors that identify them. In general, proceeding from low grade (lower pressure and temperature) to high grade (higher pressure and temperature), the following facies are recognized: Greenschist: low medium temperature, low medium pressure. Amphibolite: medium high temperature, medium high pressure. Contact metamorphism (also called thermal metamorphism) is the process by which the country rock that surrounds a hot magma intrusion is metamorphosed by the high heat flow coming from the intrusion.
