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Metamorphic Rocks
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What causes metamorphism?•Heat•Pressure•Reaction with fluids
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What changes during metamorphism?
• Mineralogy changes– New minerals form that are stable under the
new metamorphic conditions• Polymorphic transformations • Mineral reactions
• Rock texture changes – Foliation can develop in response to stress– Minerals recrystallize
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• Polymorphic transformations Kyanite === sillimanite
Calcite === Aragonite
• Mineral reactions
Chlorite + muscovite === staurolite + biotite + quartz + H2O (dehydration)
Staurolite + muscovite + quartz === sillimanite + biotite + H2O (higher T & P)
Calcite + quartz === wollastonite + CO2 (decarbonation)
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For a given parent rock, the sequence of new minerals with increasing temperature and pressure indicates the metamorphic grade – these are index minerals
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Index minerals can be mapped over large areas, to show regions of similar metamorphic grade. The boundaries are called isograds.
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Metamorphic Grade = intensity of metamorphism
LowIntermediate
High
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Recrystallization – minerals grow and develop an interlocking texture
Quartzite – metamorphosed sandstone
Marble –Metamorphosed limestone
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Metamorphic rocks are classified by:Parent rockTemperature and pressure conditions
Parent rocks (protoliths) control the elements that are available to form minerals:
• Pelitic (shales and siltstones, Al rich)• Mafic (basalts, greywackes, Mg and Fe rich)• Carbonate or calc-silicate (limestones, Ca and Mg
rich)
See Appendix C in textbook for other associations
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Metamorphic grade
Metamorphic facies:
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Minerals in calc-silicate rocks:
Grade:Low: talc, calcite, dolomite
Medium: tremolite, calcite, dolomite
High: diopside, grossular and andradite garnets, calcite, dolomite
Very high (contact): wollastonite
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