Welcome to GLG101E Introduction to Geology Fall 2001 Professor James Tyburczy
Department of Geological Sciences
	Chapter 8

Chapter 8. METAMORPHIC ROCKS

Changes in solid rocks due to effects of heat and pressure. Metamorphic processes occur deep in the crust or at plate boundaries

Factors controlling character of metamorphism and of metamorphic rocks - Temperature, Pressure, Fluids

1) Heat, Temperature

• Geothermal gradient - earth is hotter as you go deeper, about 25 Deg C per km of depth. Rocks can be moved down by burial, subduction, or continental collision
• Heat from intrusion of igneous bodies - contact metamorphism

  Effects of heat

  • Changes minerals to high-temperature stable forms
  • Releases water from mineral structure
  • Grow larger crystals - recrystallization
  • Hot rocks deform more easily than cold rocks

2) Pressure

• Confining pressure - squeezes rock - no change in shape
• Shearing Stress (or differential stress) - cause deformation of rocks
  • brittle deformation - rock breaks - grinding, pulverization
  • plastic deformation - rocks can be folded, reshaped

   

  Effects of Pressure

• Change minerals to high-pressure stable forms (denser minerals)
• Deforms rocks, minerals - causes alignment of minerals- foliation

3) Chemically active fluids

• Water, plus dissolved gases (mainly CO2) and dissolved ions and dissolved silica (SiO2)
• Sources of fluids
  • pores in rock
• structural water in certain minerals (micas, clays, amphiboles,...) is released at high temperatures
• circulating hydrothermal fluids from surface water percolating downward
• water released in last stages of cooling igneous pluton

Effects of fluids

• Enhance (speed up) rate of metamorphism
• Deposit into or dissolve away elements from the rock - many ore deposits form this way (hydrothermal deposits - much of the copper ore in Arizona is formed this way)

4) Composition of parent rock

Types of metamorphism

• Regional metamorphism - Large areas, associated with mountain-building, high pressure-low temperature metamorphism (convergent margins, subduction zones)
• Contact metamorphism - areas surrounding plutons - non-foliated high temperature, low pressure metamorphism
• Cataclastic metamorphism - fault zones - shallow, brittle deformation, fault breccia
• Burial metamorphism
• Hydrothermal metamorphism
• Plate tectonic settings of the different types of metamorphism - study Figure 8.3 carefully

Mineralogical & Textural changes - minerals change to become stable (in equilibrium) in new temperature, pressure, fluid environment

• For example - progressive metamorphism of shale, clays -> micas + H2O (fluid) -> amphiboles + more H2O (fluid)-> melt as metamorphic grade increases
• Another example - quartz + calcite -> wollastonite (CaSiO3 pyroxene) + CO2 (fluid) at 500°C
• Textural changes leading to foliation: shale -> slate -> phyllite -> schist -> gneiss

Types of Metamorphic rocks

• Foliated Rocks - slate, phyllite, schist, gneiss
  • Names given by types of minerals present, for example, garnet mica schist
• Non-foliated Rocks - quartzite, marble, hornfels, granulite
• Partially melted rocks - migmatites

Metamorphism and Plate Tectonics

Metamorphic Facies - can identify temperature and pressure of metamorphism by details of mineralogy

• Low temperature, high-pressure metamorphism - in subduction zones
• High temperature, high pressure metamorphism - deeper in subduction zones
• High-temperature, low pressure metamorphism (contact metamorphism) - surrounding plutons in crust
• Burial metamorphism, volcanic metamorphism - medium temperature, medium pressure
• Continental Shields - large expanses of very old metamorphic and igneous rocks that form the stable cores of continental regions - for example the Canadian Shield

Products from metamorphic rocks

• Slate, marble, graphite, asbestos (serpentine and amphibole types), talc, abrasives (garnet and corundum), materials for porcelain and high-temperature refractories, metallic ore deposits (copper, tin, iron, lead, tungsten,...)