Metamorphism and Metamorphic Rocks

advertisement
Metamorphism and Metamorphic
Rocks
GLY 2010 - Summer 2013 - Lecture 11
1
Metamorphism
• The mineralogical, chemical, and
structural adjustment of solid rocks to
physical and chemical conditions
2
Metamorphic Rock
• Any rock derived from pre-existing rocks by
mineralogical, chemical, and/or structural
changes, essentially in the solid state
• Derivation is in response to marked changes in
temperature, pressure, shearing stress, and
chemical environment, generally at depth in
the Earth's crust
3
Agents of Metamorphism
• Temperature
• Pressure
• Chemically Active Fluids
4
Temperature
• Minerals in rocks which are heated
may become unstable
5
Pressure
• Minerals in rocks which are under
increased pressure may become
unstable
• Pressure may be directed or
hydrostatic
6
Directed Pressure
• Pressure may be directed (along a single
axis)
7
Metamorphism from Compression
8
Effect of Compression
• Compression has
deformed the
strata and caused
metamorphism
• This represents a
continentcontinent plate
collision
9
Lithostatic Pressure
• Lithostatic pressure is equal in all
directions
10
Confining Pressure
• At depth,
lithosphatic
pressure is due
to confinement
11
Chemically Active Fluids
• Fluids from different sources
• Fluids are capable of dissolving ions and
ion-exchanging with existing minerals, to
produce new minerals
12
Regional Metamorphism
•
•
•
•
Burial
Dynamothermal
Fault
Metasomatism
13
Burial
• When sediments are deposited, they
begin to change
• Often associated with large sedimentary
basins
14
Dynamothermal
• Rocks caught between two colliding plates
• Seen as the roots of ancient mountain
regions, such as in parts of Minnesota,
Wisconsin, and Michigan
15
Fault
• Movement of rock along fault generates
shearing stress in rock (increased
pressure)
• Stress can lead to smearing of rocks
along the fault, and the creation of some
new minerals
16
Metasomatism
• The presence of interstitial, chemically
active pore liquids or gases contained
within the rock body or introduced from
external sources are essential for the
replacement process
• Often, though not necessarily, this occurs at
constant volume with little disturbance of
textural or structural features
17
Contact Metamorphism
aka Thermal
• Proximity to a heat
source may cause
new minerals to
form
• Zone of contact
metamorphism is
known as contact
aureole
18
Contact Metamorphic Diagram
• Development
of contact
metamorphic
aureole
19
Foliation
• A general term for a planar arrangement
of textural or structural features in any
type of rock
• Involves alignment of platy or elongated
minerals grains
20
Foliation exposed in a mine
• Björnevatn Mine, Kirkenes, Norway
21
Foliated Vs. Nonfoliated
• Metamorphic rocks produced by directed
pressure, such as tectonic or shearing
forces, will be foliated
• Rocks produced by load pressure, or not
involving pressure, will not be foliated
22
Development of Foliation
23
Metamorphic Grade
• Grade indicates the severity of conditions
at the time of metamorphism
• Grade is judged on the basis of:
 Foliation
 Grain size
 Mineral assemblage - index minerals
24
Metamorphic Grade Indicators
25
Metamorphic
Mapping
• The first
appearance of
grade minerals
can be used to
produce maps of
metamorphism
26
Foliated Rocks
27
Slate
•Red slate contains
oxidized iron
• A compact, fine-grained metamorphic rock
that possesses slaty cleavage and hence can be
split into slabs and thin plates
• Most slate was formed from shale
28
Slaty Cleavage
Gray slate showing foliation (slaty cleavage) in quarry
29
near Alta, Norway
Uses of Slate
Pool table at
Campobello, F.D.
Roosevelt's home
Slate Roof
30
Phyllite
• A metamorphosed rock,
intermediate in grade
between slate and mica
schist
• (Upper) Cleavage
surfaces have a silky
sheen
• (Lower) Phyllites
commonly exhibit
corrugated cleavage
surfaces
31
Schist
• A strongly
foliated
crystalline rock,
formed by
dynamic
metamorphism
•Garnet schist showing well-developed schistosity
32
Mica Schist
• (Above) Mica
imparts a shiny luster
to rock
• (Left) Garnet mica
schist
33
Gneiss
• High grade regional metamorphic rock
34
Gneiss Continued
• Gneissic foliation often involves felsic
and mafic layers
35
Contortion in Gneiss
36
Migmatite
37
Increasing Metamorphic Grade
38
Non-foliated Rocks
39
Marble
• Metamorphic
equivalent of
limestone
40
Quartzite
• A metamorphic rock consisting mainly of
quartz
• Sioux Quartzite, South Dakota
41
Hornfels
• A fine-grained rock
• Composed of a mosaic of
equidimensional grains
• No preferred orientation
• Typically formed by contact
metamorphism
42
Table Mountain Capetown, South Africa
43
Malmesbury
Hornfels
The Malmesbury Group was made famous by
Charles Darwin during his voyage of scientific
discovery on H.M.S. Beagle in 1844
44
Anthracite
• Anthracite is metamorphosed coal
45
Fault
Metamorphism
• Brittle fracture
under low
confining
pressure
• Ductile flow
under high
confining
pressure
46
Skarn
• Lime-bearing silicates, of any geologic age,
derived from nearly pure limestone and
dolomite with the introduction of large
amounts of silicon, aluminum, iron and
magnesium
47
Skarns As Ores
• Gold-bearing
vesuvianite and
grossular skarn,
near Benambra,
northeast
Victoria,
Australia
48
Download