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Transcript
Metamorphic Petrology
Lecture 1: Metamorphic phenomena
and their characterization: An
introduction
by Stephan K Matthäi
MP-SKM, slide 1
Course Objectives
I will try to teach you:
•
To identify common metamorphic rocks in the field and infer their
protoliths (original rock types and composition),
•
Understand how they formed,
•
Get broad estimates of the pressure and temperature conditions under
which the rocks were metamorphosed,
•
How to use overprinting relationships and deformation structures to
determine the geological / metamorphic history of the rocks,
•
Infer the burial depth and thermal history of the metamorphic pile,
•
Make PTt-path diagrams
•
Interpret the plate-tectonic setting of metamorphism,
•
Quantify the chemical changes that the rock underwent during
metamorphism (gains & losses),
Get you ready for independent field work.
MP-SKM, slide 2
ES4.08 Prerequisites
Geology: plate-tectonic settings, basics of sedimentary and igneous rocks,
magmatism and volcanism (Internal Processes, Dynamic Earth)
Mineralogy: ability to determine the main rock-forming minerals in hand
specimen and thin section; ideally, a knowledge of the chemical
composition of minerals (Minerals & Rocks, Optical Mineralogy &
Petrography)
Chemistry: stochiometry (balancing reactions), possible valency states of
cations, law of mass action, equilibrium constants (Geochemistry 1)
Thermodynamics: absolute basics – Gibbs free energy, heat capacity,
entropy, enthalpy, work etc. (Thermodynamics is desirable but not
essential)
Mathematics: basic algebra and elementary calculus (Basic Math,
Introduction to Calculus).
MP-SKM, slide 3
Outcomes
By the end of this course you should be able to:
•
•
•
•
•
•
•
Identify the most important metamorphic minerals and know their
approximate chemical composition
Make inferences on the protolith of metamorphic rocks on the basis of
their mineralogy and chemical composition
Know the key metamorphic reactions and parageneses that define the
boundaries of metamorphic grades
Have a basic knowledge of how you can use crystalline solid solutions
as geo-thermometers and barometers
Distinguish metamorphic fabrics and pre-, syn and post-kinematic
mineral growth as well as pro- and retrograde assemblages
Use petrogenetic grids
Interpret metamorphic history in terms of PTt paths based on field and
laboratory observations
MP-SKM, slide 4
Course Structure & Itinerary
Session 1: Introduction to Metamorphic Petrology
Session 2: Solid solutions and continuous/discontinuous reaction
thermodynamics
Session 3: Contact metamorphism, isograds, geothermometers and
Barometers
Session 4: Metamorphism & deformation, pre-, syn- & post-kinematic
mineral assemblages, brittle-ductile transition
Revision Break
Session 5: Regional metamorphism: facies concept surpassed by
metamorphic grades, petrogenetic grids.
Session 6: Incipient metamorphism and reaction kinetics
Session 7: Medium-, high- grade metamorphism and anatexis
Session 8: Metasomatism and hydrothermal alteration
Extra: Question hour in preparation for examination
MP-SKM, slide 5
Course work
You will have to prepare 2 assignments:
A1: following lecture 4, due for lecture 6:
Exercise on the mineralogy of common metamorphic minerals
A2: following lecture 7, due April 1:
Case study: reconstruction of the metamorphic history of a
Greek island
MP-SKM, slide 6
Recommended Reading
Bucher, K. & Frey, M., 2002, Petrogenesis of metamorphic rocks (7th
ed.), Springer, Berlin, 341 p.
Best, M.G., 2002, Igneous and Metamorphic Petrology (extended version
of Best, M.G. & Christiansen, E.H. Igneous Petrology. Blackwell
Science, ISBN 0-86542-541-8, 458 p.).
Bucher, K., 1997, Petrogenesis of metamorphic rocks. Based on Winkler
(5th ed., Springer, 348 p.).
You may also be interested in…
Pichler, H. & Schmitt-Riegraf, C., 1989, Rock-Forming Minerals in
Thinsection. Chapmann & Hall, 230 p.
Spear, S. & Peacock, S.M. 1989, Metamorphic Pressure-TemperatureTime Paths, AGU Short Course in Geology 7, 102 p.
Wood, B. J. & Fraser, D. G. 1977, Elementary Thermodynamics for
Geologists. Oxford University Press, 303 p.
Yardley, B., 2001, Introduction to metamorphic petrology (2nd ed.).
Blackwell Scientific Communications.
MP-SKM, slide 7
Lecture 1: Topics
A overview of Metamorphic Petrology:
1. The main driver: heat
2. Exercise: common metamorphic rocks
3. Metamorphic reactions
4. Mineral paragenesis
5. An overview of metamorphic settings
6. Classification of metamorphism: index minerals,
metamorphic facies and metamorphic grades
MP-SKM, slide 8
Definition of Metamorphism
The change of the mineral assemblage (and composition of a
rock) in response to changes in temperature, pressure, or volatile
content.
Mineralogical and usually structural transformation of a rock in the solid
state, as a consequence of physical/chemical conditions which differ
from those under which the protolith was formed (Schreiner, Mehnert &
Winkler).
Distinctions: Gradual transition to metasomatism where changes are not
isochemical
Gradual transition to hydrothermal alteration which, in some cases,
may be referred to as metamorphism in response to changes in
temperature and volatile content.
Gradual transition to diagenesis, poorly defined as “the transformation
of a rock between sedimentation and metamorphism” (Correns).
NB: The transition between diagenesis and metamorphism can be defined
as the boundary above which mineralogical changes can be clearly
related to elevated temperature and-or pressure.
MP-SKM, slide 9
Where does metamorphism occur?
MP-SKM, slide 10
1. The main driver: Heating in the Crust & Mantle
MP-SKM, slide 11
Geotherms &
gradients
Put values on the
axis,
Mark the boundary
of the continental
and the oceanic
crust?
Estimate some
typical geothermal
gradients.
MP-SKM, slide 12
2. Metamorphic rocks and their protoliths
Increasing temperature and pressure
Granite
Basalt
Dunite
Shale
Sandstone
Limestone
Write the names of corresponding metamorphic rocks into the empty
fields in the columns; circle those rocks which we had a chance to look
at in handspecimen.
MP-SKM, slide 13
Assigning names to metamorphic rocks
Names often contain the word schist (puff-pastry like), fels
(massive, fine-crystalline), marble (predominantly carbonate), or
gneiss
Determine relative volumetric proportions of minerals, for example:
garnet–biotite schist with 30 vol.% gnt & 25 vol.% biotite
Start with a generic name and become more specific:
metapelite -> quartz phyllite
MP-SKM, slide 14
3. Metamorphic Reactions
Solid-solid (a) and solid-fluid (b = dissolution precipitation)
reactions, e.g. A + B = C + D; classification:
Continuous reactions (over PT range)
chlorite(Fe-rich) + phengite ↔ biotite + chlorite(Fe-poor) + quartz + H2O
Discontinuous reactions (at fixed PT conditions)
andalusite ↔ sillimanite
Recrystallization versus. grainsize reduction during deformation.
b) devolatilization:
Dehydration
loss of H2O
Decarbonation
loss of CO2
Pyrolysis
liquification of C
MP-SKM, slide 15
Discontinuous reactions: Al2SiO5 pPolymorphs
Pressure
[GPa]
MP-SKM, slide 16
Continuous reactions are exchange
reactions
Garnet (Fe,Mg)3Al2Si3O12
and
Biotite K(Fe,Mg)3AlSi3O10(OH)2
FeMg(garnet) = FeMg(biotite)
(Fe,Mg)3Al2Si3O12 + KAlSi3O8 + H2O =
Al2SiO5 + K(Fe,Mg)3AlSi3O10(OH)2 + 2 SiO2
With increasing temperature andradite garnet becomes more Mg-rich
and Fe-poor while biotite does the opposite.
MP-SKM, slide 17
4. Mineral paragenesis
Paragenesis
= A group of
minerals which
formed
contemporaneousl
y and in contact
with one-another,
at the same PT
conditions in the
rock, implying that
these minerals
were in chemical
equilibrium when
they formed.
crossed polarizers, 6mm
MP-SKM, slide 18
Types of events recorded by mineral parageneses
One classifies metamorphic paragenetic mineral assemblages as
prograde, peak-metamorphic and retrograde. This interpretation is
based on the PT conditions defined by petrogenetic stability fields and
mutual overprinting relationships.
NB: This interpretation of sequential equilibrium assemblages is in
conflict with genuine equilibration which is not obtained as is indicated
by presence of remnant minerals from the previous assemblage.
The mitigating factor is the reaction rate. Reaction kinetics influenced
by factors such as temperature, grain-to-grain contact, deformation
rate, presence of fluids, grain-surface area, mineral zonations etc.
control whether a mineral is preserved or replaced.
Thus - it is not always easy to determine which part of a PT path a
mineral assemblage is related to. Sometimes prograde assemblages
have vanished, often there is no retrograde assemblage.
MP-SKM, slide 19
5. Metamorphic Settings
from geoscience flyer (1998)
Univ. Minnesota, St Paul
MP-SKM, slide 20
Island Arcs & Active Continental Margins
MP-SKM, slide 21
Metamorphism
of a
Subducting
Slab
MP-SKM, slide 22
Deep Intrusions (>10 km)
MP-SKM, slide 23
Metamorphism of the Oceanic Crust
MP-SKM, slide 24
Metamorphism & Rifting
MP-SKM, slide 25
Extension: metamorphism & detachment faults
MP-SKM, slide 26
6. Classification of Metamorphism
Historical evolution:
1. Index minerals (=Barrowian zones, Barrow, 1893)
2. Phenomenological classification based on common
transformations of specific rock types:
Metamorphic Facies, Eskola (1915)
3. Classification in terms of the peak pressure (P) and
temperature (T) which the rock experienced, grouped
into 4 Metamorphic Grades, Winkler (1976)
4. Petrogenetic grids
MP-SKM, slide 27
6.1: Index minerals
•
Developed for medium pressure rocks in Scottish Highlands by
Barrow (1893).
•
Simple: The occurrence of index minerals is mapped, e.g.
•
Chlorite – biotite – almandine – staurolite –kyanite – sillimanite
•
Temperature conditions are inferred
Problems
•
Specific to rock type
•
Ignores pressure dependence of mineral stability (different
sequences are possible)
•
Imprecise
MP-SKM, slide 28
6.2: Eskola’s (1920) facies classification
for basaltic rocks
MP-SKM, slide 29
Facies definition & its shortcomings
• Each facies comprises a set of minerals which
formed over a poorly defined range of PT conditions
from a specific protolith.
• The common facies names refer to mineral
assemblages common in basaltic rocks. Applying this
strictly one would classify a staurolite micaschist as
part of the amphibolite facies
• The introduction of sub-facies (e.g., Turner 1960) led
to a proliferation of facies names
• It follows that metamorphic facies is not indicative of
specific PT conditions – It is purely descriptive.
MP-SKM, slide 30
6.3: Metamorphic grade (Winkler 1976)
Winkler replaced the facies classification by 4 metamorphic
grades as defined by largely protolith-independent univariant
mineral reactions:
Winkler
Eskola and later.
Metamorphic Grade
Facies Equivalent
very-low
zeolite facies
low
greenschist facies
epidote-amphibolite facies
medium
amphibolite facies
pyroxene fels facies
high
- anatexis
Example: Reaction boundary between medium & high grade:
muscovite + quartz = K-feldspar + sillimanite + H2O
MP-SKM, slide 31
The four metamorphic grades
Pressure
[MPa]
Depth
[km]
diagenesis
nonexistent
conditions
Temperature [oC]
MP-SKM, slide 32
Solution – exercise 1 (slide 12)
We had a look at hand specimens of the following rocks:
1. slate - phyllite - mica-schist gneiss, granulite - hornfels
2. basalt - greenschist / blueschist - amphibolite / eclogite
3. quartzite
4. limestone - marble - calc-silicate rock (marble with olivine
or diopside, tremolite, wollastonite etc.)
5. dunite - serpentinite - soapstone
MP-SKM, slide 33
Revision questions for Session 1
1. Try to memorize the new terminology
2. Make a list of metamorphic minerals noting which are
hydrous and which are not
3. Calculate by how much the temperature of the granite
from slide 11 (radiogenic heat production) should rise
by radioactive decay of unstable elements within a
years time, assuming that the sample is thermally
insulated from its surroundings. Hint: Revise what the
heat capacity is.
MP-SKM, slide 34