Ig Met Pet Lecture 18: Metamorphic Toolbox One

How do we interpret the geologic processes of metamorphism?

Mineralogy and texture of rocks, and how they change across field exposures

What are the agents of metamorphism

Temperature, pressure, fluid activity, stress

Texture

Mineral distribution, relations, and reactions that show equilibrium growth and replacement during progressive/retrogressive metamorphic processes

Structure

Large scale features that occur in the hand specimen, outcrop, or regional scale of metamorphic rocks

What does an increase in lithostatic pressure result in for a metamorphic rock?

Grain size reduction

Stress

A force acting on a point

Strain

The response to an applied stress

Deviatoric stress

When stress is not the same in all directions, resulting in strain and deformation

What does an increase in deviatoric stress result in for a metamorphic rock?

Deformation of grains

What causes textural changes in metamorphic rocks?

Temperature and fluids

What does increasing temperature do to metamorphic rocks?

Promotes recrystallization, drives endothermic reactions, overcomes kinetic barriers

What does recrystallization do to grain size?

It increases grain size

How do fluid affect metamorphic rocks?

They can change chemistry and help with the transport/diffusion of chemical components

What does metamorphic mineral growth involve?

Detachment of ions from the surface of reacting minerals, nucleation of new minerals, diffusion of material to new growth, growth of new mineral incorporating components from carried to the surface, and transportation of excess waste away

Why do fluids affect metamorphic mineral growth?

They make processes go much quicker

How does metamorphic grade affect fluid content?

Higher grade metamorphic rocks are hotter and deeper, resulting in fluid being driven off

What does texture give clues about in metamorphic rocks?

P-T conditions, fluid, and the protolith

What does blast mean as a prefix?

Relict, from the protolith

What does blast mean as a suffix?

Of metamorphic origin, not from the protolith

What are the five metamorphic texture types?

Palimpsest, crystalloblastic, exsolution, replacement, anatectic

Palimpsest (relict)

Textures inherited from the parent rock that have been retained in the metamorphosed rock, occurring in low-grade rocks

Crystalloblastic

Textures newly developed in a rock during the process of metamorphism

Exsolution

Mineral intergrowth formed by solid exsolution, common in both igneous and metamorphic rocks

Replacement (reaction)

Mineral reactions under varying metamorphic conditions, caused by P, T, fluid, or stress

Anatexis

Partial melting of crustal rocks that occur between the solidus and liquidus, producing migmatites

What are examples of palimpsest textures?

Relict sedimentary bedding and clastic/igneous primary textures

What facies do palimpsest textures occur in?

Sub-greenschist to greenschist

Where can crystalloblastic textures occur?

All grades

When does exsolution texture occur?

When temp cools or pressure drops

Where in the PTt path do replacement textures occur?

Prograde or retrograde

What does the presence of replacement suggest?

That metamorphic reactions are not fully completed

What are the two types of reaction textures?

Complete pseudomorphs and incomplete rims

What grades do anatexis textures occur in?

Very high grade rocks, as they require high temperature

What is diagnostic of an anatexis texture?

Interaction between melts and solid phases, with transportation and segregation of melts along grain boundaries

Undeformed metamorphic rocks

Nonfoliated rocks composed of hard, equidimensional minerals weakly affected by deformation

What texture do nonfoliated rocks exibit?

Granoblastic, as mineral grains are nearly random

Where are textures which record increasing T or strain rate most commonly found?

Regional metamorphism

What are the five ways through which rocks deform (become foliated)?

Cataclastic flow, pressure solution, intercrystalline deformation, recovery, recrystallization

What external factors affect foliation?

grain size, mineralogy, presence of fluids, temperature, pressure, deviatoric stress, fluid pressure, strain rate

Do deformational processes work alone?

Not always, they typically work in unison depending on internal and external factors

What is the difference between brittle and ductile deformation?

Brittle involves the fracturing of mineral grains, while ductile does not

What are the pressure temperature conditions which result in brittle deformation, and where are these found?

High strain, low temperature, associated with earthquakes

Pseudotachylyte

Small melt veins formed during short bursts of very high strain

Do grains have to be bound in brittle deformation?

No, they can be cohesive or non cohesive

Under what conditions does ductile deformation occur?

Lower stress and higher P-T conditions, relevant to middle-lower crust, and upper mantle environments

What are the ductile deformation structures, in order of increasing metamorphic grade?

Slate, phyllite, schist, gneiss

What does ductile deformation result in?

Pressure solution, intracrystalline deformation, recovery, recrystallization

Pressure solution

Dissolution of material at contacts between grains. Requires an intergranular fluid, to dissolve and transport material.

Intracrystalline deformation (plastic)

No loss of cohesion in rock, but significant changes in position of atoms/ions and chemical bonds, reorienting crystal lattice

Recovery

Highly-strained grain forming two low-strain subgrains

Recrystallization

Reducing stored energy through movement of grain boundaries and development of new boundaries

What do grain boundaries want to do?

minimize energy, through triple point angles

Ductile deformation microstructures

Shape change of a material through bending or flowing, where chemical bonds break but are reformed

Types of ductile deformation microstructures

Folds, foliation, lineation, mechanical twins and kink bands, bulging, subgrain rotation, grain boundary migration, rigid body rotation, crystallographic preferred orientation