MetamorphicMetamorphic PetrologyPetrology LectureLecture 1:1: MetamorphicMetamorphic phenomenaphenomena andand theirtheir characterization:characterization: AnAn introductionintroduction byby StephanStephan KK MatthäiMatthäi

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-Temperature- Time 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 andpressure 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 Depth diagenesis [MPa] [km]

non- existent 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