Metamorphic Rocks
Gneiss
Marble-Limestone
Metamorphic Rocks
• Metamorphism occurs when any
previously existing rock, the parent rock,
is buried in the earth under layers of other
rock. The deeper the rock is buried the
hotter it gets, and the higher the pressure
becomes. Eventually, rock must adjust to
the new conditions, whether it is baked, or
squeezed, or both, and in the process
becomes a metamorphic rock.
Metamorphic Rock Texture
•
The new metamorphic textures are of two types, granular (also
non-foliated) where the grains are equidimensional, and foliated
where the minerals are layered .
Granular rocks are usually uniform in composition; they are all
one mineral (e.g. quartz = quartzite, calcite = marble; exception =
hornfels) so the minerals do not segregate into layers –
Foliated textures result when the new metamorphic minerals
line up producing a distinct layering in the rock. The layering
produces three distinctly different looking rocks, those with slaty
cleavage (e.g. slate), schistosity (e.g. schist), and mineral
banding (or gneiss texture). Metamorphic rocks are identified on
these textures
Metamorphic Textures
• Foliation is a broad term referring to the
alignment of sheet-like minerals. Types of
foliation:
– Schistosity - alignment of large mica flakes, as in a
mica schist derived from the metamorphism of shale.
– Slaty cleavage - alignment of very fine-grained
micas, as in a slate derived from the metamorphism
of shale.
– Phyllitic structure - alignment of fine-grained micas,
as in a phyllite.
– Gneissic banding - segregation of light and dark
minerals into distinct layers in the rock, as in a gneiss.
Lineation
• Lineation refers to the alignment of
elongated, rod-like minerals such as
amphibole, pyroxene, tourmaline, kyanite,
etc. Lineation is a texture commonly seen
in the metamorphic rock amphibolite
derived from the metamorphism of basalt.
Non-foliated or granular
• Non-foliated or granular metamorphic
rocks are those which are composed of
equidimensional grains such as quartz or
calcite. There is no preferred orientation.
The grains form a mosaic
Metamorphic Rock Texture
Continued
Kinds of Metamorphism or rocks
• most introductions to metamorphism focus
on just two: Barrovian (" regional" ) and
Contact.
Regional Metamoprhism
•
Regional Metamorphism
Regional metamorphism takes place during mountain building events when very
large areas of sedimentary rocks are buried, squeezed, and heated. Sometimes the
metamorphism occurs just because the burial is deep enough, but often it is also
associated with major igneous intrusions that supply most of the heat. Regional
metamorphism produces both textural changes and compositional changes in the
rock. The compositional changes take place because sedimentary minerals are
stable only at the earth's surface. As they become buried the sedimentary minerals
become unstable and transform into new metamorphic minerals. For example, clay
goes to chlorite, and chlorite eventually goes to quartz, feldspar, and mica, as well as
other minerals. At the upper end of metamorphism the rock melts and becomes
igneous.
But the texture of the rock also changes during metamorphism. The rocks become
foliated, that is the texture sequence of slaty cleavage to schsitosity, to banded, or the
rock sequence slate to phyllite to schist to gneiss.
Thus, metamorphic rocks not only tell us the kind of metamorphism, they are also
a measure of the intensity of metamorphism. The closer we get to the source of heat
and pressure the more altered the rock becomes.
Diagram o f Regional
Metamorphism
Contact Metamorphism
• Contact metamorphism occurs in the " country rock" (the rock
intruded by and surrounding an igneous intrusion). Rocks are "
baked" into a ceramic from heat escaping from intrusives, often
enhanced by hot fluids. However, unlike Regional metamorphism,
pressure is not a significant factor in the contact process, especially
directed pressure. Without directed pressure foliation does not
develop. So, the clay (shale) which in regional metamorphism
develops a strong foliation (slate, phyllite, schist, gneiss) in contact
metamorphism develops a granular texture (the rock hornfels). The
analogy is putting a clay pot in a kiln; the clay is simply baked.
Contact metamorphism can occur next to any igneous intrusion,
although it is most easily seen next to smaller intrusions. The
intensity of metamorphism decreases with distance from the
intrusion, until at some distance away the rock is unaltered country
rock. However, large intrusions such as batholiths usually alter the
country rock so much that other styles of metamorphism override
the contact metamorphism.
Contact Metamorphic Rocks
Key to Identifying Metamorphic Rocks
Classification of Metamorphic
Rocks
• Metamorphic Rocks are divided into two
basic divisions, summarized in the table
below:
1. Foliated/Banded
2. Non-Foliated (also, granular or
equidimensional).
Chart of Foliated and Non-Foliated
Metamorphic Rocks
Foliated Rocks
• Most Metamorphic rocks form in the influence of a directed stress
field. Because of this they develop conspicuous directional textures.
For example, the top illustration to the right shows the stress field
before application (arrows), with the mineral grains randomly
oriented. As metamorphism proceeds, the sheet structure silicates
(flat minerals with basal cleavage) such as mica (biotite and
muscovite) and chlorite start to grow. The sheets orient themselves
perpendicular to the direction of maximum stress. The new parallel
mineral flakes produce a planar texture called foliation. (from the
Latin folium - leaf). Foliation can be subtle or pronounced depending
on the degree of metamorphism.
Types of Foliated Rocks
Slaty• cleavage
- a pervasive, parallel foliation
.
(layering)
of fine-grained platy minerals (chlorite) in a
direction
perpendicular to the direction of maximum stress.
Produces the rocks slate and phyllite
Schistosity - the layering in a coarse grained, crystalline
rock due to the parallel arrangement of platy mineral grains
such as muscovite and biotite. Other minerals present are
typically quartz and feldspar, plus a variety of other minerals
such as garnet, staurolite, kyanite, sillimanite
Mineral Banding (Gneiss) - The layering in a rock in which
bands or lenses of granular minerals (quartz and feldspar)
alternate with bands or lenses in which platy (mica) or
elongate (amphibole) minerals predominate.
Chart to Classify Metamorphic
Rocks