Welcome to GLG101C Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Chapter 9

Chapter 9. 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 - lead to the idea of 'metamorphic grade'.

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 (alos called directional or differential stress) - cause deformation of rocks
  • brittle deformation - rock breaks - grinding, pulverization
  • plastic deformation - rocks can be folded, reshaped - foliation

  Effects of Pressure

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

3) Chemically active fluids (process is called metasomatism)

• 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)

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
• Seafloor or Hydrothermal metamorphism (metasomatism) - At mid-ocean ridges, hot seawater percolates through rocks and alters them. On continents, hot fluids circulate near igneous intrusions
• Burial Metamorphism - Deposition of sediments in a basin leads to bending (subsidence) of the crust and increased pressure on the rocks at the bottom of the sedimentary pile. The rocks at the bottom of the pile also heat up (because of the geothermal gradient), resulting in metamorphism.
• High pressure and ultra high pressure metamorphism - very high pressure metamorphism. Occurs mostly in subduction zones where rocks are pushed down to great depths (30 km or even much deeper). In subduction zones, the characteristic minerals indicate high pressure accompanied by low temperature conditions
• Shock Metamorphism - occurs when a meteorite collides with the Earth. Results in particular minerals being formed that are characteristic of the very high pressures (and high temperatures) created during a shock event.
• Plate tectonic settings of the different types of metamorphism - study Figure 9.3 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 (Granular or granoblastic) Rocks - quartzite, marble, hornfels, granulite
• Partially melted rocks - migmatites

Regional Metamorphism and Metamorphic grade

Metamorphic Facies or Metamorphic grade - can identify temperature and pressure of metamorphism by details of mineralogy (index minerals). Then can map these regions - see where regional metamorphism has reached certain grades (isograds)

Parent Rock Composition - different parent rocks result in different types of metamorphic rocks

• shale (fine grained sediments) go to slate, phyllite, schist, gneiss - metasediments
• sandstone becomes quartzite
• limestsone becomes marble
• basalt metamorphoses to greenschist rock (contains chlorite - a greenish clay) then amphibolite (amphibole rich)
• in subduction zones - basalt metamorphoses to blueschist rock - hi P,low T metamorphism (because the minerals are blue-ish)

Metamorphism and Plate Tectonics - Orogeny = 'mountain making' - folding and faulting

• 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
• Seafloor metapmorphism - at or near mid-ocean ridges
• Continental Shields - large expanses of very old metamorphic and igneous rocks that form the stable cores of continental regions - for example the Canadian Shield

Metamorphic P-T-t paths - Pressure-Temperature-time paths. From analysis of minerals in metamorphic rocks

• Continent - ocean convergence - sediments are deposited in the oceanic trench, then are scraped off (folded, distorted), mixed up ("melange"). Blueschist rocks - hi P, low T - Coast Range of California (Franciscan Formation)
• Continent-continent collision - neither side will subduct because they are both low density. Form a collision zone, as in the Himalaya Mts today - high P, high T metamorphism

How do Metamorphic Rocks formed at depth get back up to the surface?

• Uplift due to rapid erosion. A link between orogeny and climate (Plate Tectonic System and Climate System)

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,...)