EARTH AND PLANETARY SCIENCES 101 Fall, 2002

THE WAY THE EARTH WORKS

METAMORPHIC ROCKS AND METAMORPHISM, A WINDOW INTO THE EARTH'S CRUST

I. Introduction, why important?

A. Metamorphism requires pre-existing rock.

B. Transformation of pre-existing material into texturally and/or mineralogically new material

C. Factors include temperature, pressure, chemistry, and TIME

D. parent rock (protolith)

E. Chemical reactions take place, with chemical equilibrium maintained

F. hydrothermal fluids--associated with metamorphic (and igneous!) processes

II. Metamorphic rocks--characteristics, factors

A. Composition of the Parent Rock

1. assume that metamorphic reactions involve little element transfer (element mobility)

2. example--start with a basalt. metamorphic minerals formed are those relatively low in silica

B. Temperature--range of “stability” existence of a particular phase

1. minerals stable at high temperatures are generally those that are less dense than others

2. higher temperatures speed chemical reactions.

3. lower temperature limit is about 200C, and the upper is melting of the composition rock!

a) digress on the effect of water--recall melting relations and the importance of fluids

b) origin of water? or other components to a fluid?

C. Pressure

1. static or confining pressure is due to the rock column

a) increase in pressure favors the formation of a denser phase

b) solid pressure is easily determined

2. directed pressure (dynamic) Applied unequally [squeezing or shearing]

a) results in foliation--constituents (minerals, rock fragments, etc.)

b) parallel alignment of these constituents (e.g., sheet silicates like micas)

c) textural variations : slaty cleavage, schistocity, gneissic fabric (solid state diffusion)

D. Fluids

1. why infer presence? Many metamorphic minerals are hydrous!

2. Importance--fluid flux for chemical transport over relatively short distances

E. Time

1. rates of mineral growth must be measured over millions of years.

III. Classification

A. Texture--foliated vs. non-foliated (which are classified on the basis of composition)

1. if foliated, determine the type of foliation, then the mineralogy

B. Nature of Protolith--maximum range of temperature and pressure.

IV. Types of Metamorphism

A. Contact (Thermal) metamorphism; related to intrusive activity

1. high temperatures, not directed stresses, nonfoliated rocks, usually

2. examples: shale to hornfels, limestone to marble, sandstone to quartzite

B. Regional metamorphism; relatively high temperature AND pressure

1. dynamothermal in character (directed stresses); function of depth of burial, temperature

2. progressive--same material affected to greater or lesser degree.

3. Intensity of metamorphism

a) progressive changes in pressure, temperature (Prograde pathway, vs. retrograde pathway)

b) mudstone/claystone----goes to a slate (clays aligned), then to a phyllite (some micas formed and you can actually see “prograde” metamorphic minerals); then to a schist (relatively coarse grained metamorphic minerals); then to a gneiss (solid state diffusion of elements, leads to mineralogic banding; generally near the stage of melting); finally a migmatite (mixed rocks).

c) DEHYDRATION REACTIONS; water released to favor higher temperatures and pressures.

V. Metamorphic Facies

A. Different Temperature and Pressure fields of mineral “stability”; within which a characteristic assemblage of minerals will form assuming a particular bulk composition.

B. Purpose-use describe general concepts of pressure/temperature fields for the formation of metamorphic rocks, at a stage above burial/diagenesis and ultimately ending in melting.

C. Understand position, relative to Pressure-Temperature space, of the different met. facies!

D. Importance of the geothermal gradient--how does a rock’s path differ???

E. Relation to plate tectonics?

1. subduction zone environments and depression of isotherms (lines [actually planes] of constant temperature) due to subduction of cold, dense oceanic lithosphere.

2. Relatively low geothermal gradients near the trench and within the subduction zone, and high pressure, low temperature metamorphic facies result!

3. Relatively high temperature and low pressure facies metamorphism within the magmatic arc environment, where the geothermal gradient is considerably greater.

4. THIS IS BASICALLY EXACTLY WHAT WE OBSERVE IN THE FIELD, LEADING TO THE INFERENCES ON THE ACTUAL “POLARITY” OF ANCIENT SUBDUCTION ZONES.

VI. hydrothermal fluids

A. Hot fluids, water and dissolved ions, associated with a number of important processes

B. Metasomatism--introduction of one or more ions from a distant source

1. due to magmatic activity?

2. also the possibility of metamorphic fluids being generated in dehydration reactions; conversion of a metamorphic rock to feldspars and quartz (potassium metasomatism)

C. Hydrothermal processes

1. hot solutions emanating from crystallizing intrusion, a terrane undergoing metamorphism, etc. pass into cooler, surrounding rocks, largely along cracks and deposit different minerals

2. vein formation--minerals (quartz, calcite, sulfides of different elements, native elements)

D. Origin of water?

1. ground/meteoric

2. juvenile--magmatic

3. ocean water in subduction zones--role of subducted sedimentary material

VI. Metallic Ore deposits (covered to some degree in discussions of igneous rocks/magmas)

A. Ore--material of economic significance containing an unusually high concentration of one or more elements. definition somewhat dependent on world economy

B. Ores formed by igneous processes

1. magmatic fractionation (e.g., Stillwater Complex, Montana)

2. hydrothermal fluids (veins, Butte, Questa, etc.)

3. contact metamorphism (Bingham, Utah)

4. hot springs (mid-ocean ridges)

5. pegmatites

C. Ores formed by surface processes

1. chemical precipitaiton in layers

2. place deposits

3. concentration by weathering and soil formation (laterites)

D. Relatations to plate tectonics -----plenty!

1. mid-ocean ridges (layered igneous deposits)

2. ophiolites (obducted oceanic lithosphere)

3. subduction processes--arc magmas and prograde metamorphism; dehydration reactions.