Metamorphic rocks

      How do metamorphic rocks form?

 

Metamorphism is a solid state transformation

      Stable vs Metastable

            At certain temperatures and pressures certain minerals are stable

            As pressure and temperature increases these minerals become unstable. 

            New minerals will form that are stable at the new pressures and temperatures

 

      Polymorphic transitions

            No chemical change in rock

            Crystal structure changes (atomic rearrangement)

                  Low P&T:     Kyanite     or    Graphite

                  High P&T:  Sillimanite   or    Diamond

      Recombination

            Creation of new minerals from existing minerals

                  Calcite + Quartz à  Wollastonite + CO2

                   CaCO3 + SiO2    à  CaSiO3 + CO2

 

Agents of Metamorphism

      Heat:

            Provides energy

            Ions begin to vibrate and move around easier

            Where Does heat come from?

 

      Fluids

            Helps dissolve existing minerals (metastable)

            Increases mobility of ions in system and through rocks

            Re-deposits ions to create stable minerals

            Fluids flow through system and away from high pressure to lower pressure

            Minerals form in pressurized system and are aligned perpendicular to pressure

            Where do the fluids come from?

      Pressure:

            Important in developing textures of metamorphic rocks

                  Foliation

                  Banding/layering

                  Tensional stresses

                        Strain Ellipsoids

                        Boudins

 

Metamorphic Rock Types

      Slate

      Phyllite

      Schist

      Gneiss

      Marble

      Quartzite

      Hornfels

      Migmatites

      Tekties

 

What Types of Environments Might Metamorphism Occur?

 

Types of Metamorphism

      Contact Metamorphism

            Magma comes in contact with rock

            Heat (no real pressure)

            Creates hornfels (baked rock)

      Hydrothermal Metamorphism

            Often associated with magma crystallization

            Magma crystallizes and leaves fluid behind

            Fluids rich in ions

            Chemically alters host rock and often deposits important ores

            Often found at mid-ocean ridges

      Burial Metamorphism

            Only pressure

            Subduction zone at plate contact

            Creates blue schist

                  Glaucophane:  type of amphibole

      Dynamic metamorphism

            Low temperature high pressure

            Associated with deformation of crust

            Found along fault zones

            Creates Mylonites and Fault Breccia

            Gives sense of fault motion

      Regional Metamorphism

            Large region of metamorphism

            Heat, pressure and fluids  involved

            Associated with convergent boundaries (lab manual pg 78)

      Impact metamorphism

            Created by extreme pressure

            Meteorite impacts

            Creates tektites

 

Grades of Metamorphism

      Grade of metamorphism is based on how metamorphosed a rock becomes

      High pressure and temperature leads to high grade metamorphism

      Low pressure and temperature often leads to low grade metamorphism

 

What is a Migmatite

      When pressure is great small scale melting of rock can occur.

      Cross over to igneous activity

 

Index Minerals

      A group of minerals used to establish the temperature/pressure certain metamorphic rocks      form at.