XI. Metamorphic Rocks

A. Controlling Factors

1. Heat
1. intrusions
2. burial - geothermal gradient is ~25C/km
2. Pressure
1. burial - lithostatic pressure
2. tectonics - differential pressure
3. fluids
1. speeds up reactions/ mineral changes
2. hydrothermal mineralization

B. Effects

1. change minerals - affected by composition, temperature and pressure
1. recrystallization (quartz, calcite)
2. growth of new minerals (clay >> mica, garnet, etc.)
2. change texture
1. foliation develops when minerals grow under directed pressure

C. Types of Metamorphism

1. contact metamorphism
1. conditions
• high Temperature dominates
2. cause
• contact with hot magma
3. effects
• main change is recrystallization and growth of new minerals; texture is not greatly changed
• metamorphic aureole - country rock around an intrusion is heated
• effects decrease with distance from intrusion
• hot fluids flow out from magma forming hydrothermal minerals and veins
2. regional metamorphism
1. conditions
• high Temperature and high Pressure
2. cause
• burial
• mountian building (convergent plate boundaries)
3. effects
• recrystallization and growth of new minerals;
• texture is greatly changed - foliation develops in rocks which originally contained many clay minerals - the clay minerals are often converted to micas
• metamorphic zones - presence of certain index minerals reflects variations in intensity of metamorphism
3. dynamic metamorphism
1. conditions
• high Pressure dominates
2. cause
• movement along fault zones
3. effects
• there is some recrystallization and growth of new minerals;
• minerals become elongate parallel to fault

D. Classification

Type of metamorphism

non-foliated rocks	foliated rocks
contact	mudrocks >>> hornfels limestone >>> marble quartz sandstone >> quartzite
regional	limestone >>> marble quartz sandstone >> quartzite	mudrocks >>slate, phyllite,  or schist mud/sand or felsics >>> gneiss
dynamic		mylonites

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