Metamorphic Rock Grades

Grades of Metamorphism

• Metamorphic grade is the general temperature and pressure during metamorphism.
• Rocks metamorphose to a higher grade as temperature and pressure increase.
• Prograde metamorphism occurs when rocks change to another as they experience higher metamorphic grades.

Low-Grade Metamorphism

• Low-grade metamorphism occurs at approximately 200–320 ºC and relatively low pressure.
• Conditions are not far beyond those of lithified sedimentary rocks.
• Low-grade metamorphic rocks often resemble their protolith.
• Abundant hydrous minerals characterize low-grade metamorphic rocks.
• Examples of hydrous minerals include clay, serpentine, and chlorite.
• Metamorphic minerals often remain too small to be seen without a microscope in low-grade metamorphism.

Medium-Grade Metamorphism

• Medium-grade metamorphism takes place at approximately 320–450 ºC and at moderate pressures.
• Micas like biotite and muscovite replace low-grade hydrous minerals.
• Non-hydrous minerals such as garnet may grow.
• Garnet can form porphyroblasts, which are larger and more equant grains that stand out.

High-Grade Metamorphism

• High-grade metamorphism occurs at temperatures above about 450 ºC.
• Micas tend to break down at this grade.
• Hornblende, stable at higher temperatures, forms.
• At even higher grades, hydrous minerals like hornblende may break down.
• Higher-temperature, non-hydrous minerals such as pyroxene may replace hydrous minerals.

Index Minerals

• Index minerals indicate metamorphic grade.
• Lower-grade index minerals are replaced by higher-grade minerals during prograde metamorphism in rocks of a specific chemical composition.
• In metamorphosed shale, index minerals prograde in the following sequence:
  • Chlorite characterizes the lowest regional metamorphic grade.
  • Biotite replaces chlorite at the next metamorphic grade, considered medium-low grade
  • Garnet appears at the next metamorphic grade, medium grade
  • Staurolite marks the next metamorphic grade, which is medium-high grade
  • Sillimanite characterizes high-grade metamorphic rocks

Mapping with Index Minerals

• Geologists use index minerals to map metamorphic grades in metamorphic rock regions.
• Geologists map, collect rock samples, and mark the geologic map with sample locations and index mineral types
• Geologists delineate metamorphic grade zones by drawing lines around areas with the same index minerals.
• These lines are known as isograds.

Regional Metamorphism

• Regional metamorphism affects large rock areas subjected to differential stress over long periods.
• This commonly occurs during mountain building at subduction and continental collision zones.
• Foliated metamorphic rocks like slate, phyllite, schist, and gneiss form during regional metamorphism.
• Rocks become ductile when heated deep in the Earth in regional metamorphism.
• Folding and deformation can distort rock shapes, creating folded layers and mineral veins.

Contact Metamorphism

• Contact metamorphism occurs in solid rock near igneous intrusions, caused by the magma's heat.
• Contact metamorphic rocks do not become foliated because there are no changes in pressure or differential stress.
• The zone of contact metamorphism is relatively narrow in shallow intrusions (a few meters).
• Contact metamorphic zones around larger intrusions can be over 1000 m thick.
• The metamorphic aureole is the zone of contact metamorphism around an intrusion.
• The highest metamorphic grade is closest to the intrusion contact, decreasing with distance.
• Contact metamorphism is high-temperature, low-pressure, occurring at shallow to moderate depths.
• Hornfels, a hard metamorphic rock from fine-grained clastic sedimentary rocks, commonly forms.

Hydrothermal Metamorphism

• Hydrothermal metamorphism results from extensive interaction between rock and high-temperature fluids.
• Chemical reactions occur due to compositional differences between the existing rock and invading fluid.
• Hydrothermal fluid may come from nearby magma, hot groundwater, or ocean water.
• Metasomatism occurs if the fluid introduces or removes substantial amounts of ions, changing the rock's chemical composition.
• Ocean water circulating through hot, cracked oceanic crust creates hydrothermal metamorphism near mid-ocean ridges and volcanic zones.
• Basalt subjected to this metamorphism often turns into greenschist.
• Greenschist contains green minerals like chlorite, epidote, talc, Na-plagioclase, or actinolite.
• Fluids escape through black smokers, forming thick mineral deposits on the ocean floor.

Burial Metamorphism

• Burial metamorphism occurs in rocks buried beneath sediments, exceeding sedimentary rock formation conditions.
• Rocks experience uniform lithostatic pressure, so foliation does not develop.
• This is the lowest grade of metamorphism.
• Zeolite, a group of low-density silicate minerals, typically grows.
• A strong microscope is needed to see small zeolite grains.

Subduction Zone Metamorphism

• During subduction, a tectonic plate of oceanic crust and lithospheric mantle is recycled.
• The subducting plate is relatively cold compared to its formation temperature at a mid-ocean ridge.
• Subduction leads to high-pressure, low-temperature metamorphism deep in subduction zones.
• Oceanic basalts in a subducting plate develop distinctive minerals like glaucophane, a blue amphibole.
• Blueschist is a metamorphic rock type formed in subduction zones.
• Blueschist indicates a past subduction zone, even if plate boundaries have shifted.

Metamorphic Facies

• Minerals and textures in metamorphic rocks reveal pressure, temperature, fluids, and stress conditions.
• Specific metamorphic rocks form under specific pressure and temperature conditions.
• These conditions are determined by experiments, calculations, rock textures, and field relations.
• Metamorphic facies is a concept based on the temperatures and pressures at which metamorphic rocks form.
• Each metamorphic facies is represented by a specific metamorphic rock type formed under specific pressure and temperature conditions.
• The name of each metamorphic facies comes from a rock type that forms under those conditions, but other rock types can also form.
• Basalt protoliths become greenschist under greenschist facies conditions.
• Shale protoliths become muscovite-biotite schist, not green, under greenschist facies conditions.
• A muscovite-biotite schist formed at 350ºC and 400 MPa is in the greenschist facies, even if it is not greenschist itself.
• Earth's surface is at 15ºC and 0.1 MPa.
• Pressure inside the Earth comes from the weight of overlying rock, and MPa is the unit of pressure.
• 1 MPa is nearly 10 atmospheres, and 1000 MPa is about 35 km deep.
• Temperature depends on depth and heat flow.
• The geothermal gradient is the way temperature changes with depth and varies by location.
• High-pressure, low-temperature geotherms occur in subduction zones, leading to zeolite, blueschist, and eclogite facies.
• High-temperature, low-pressure geotherms occur near igneous intrusions, leading to hornfels and granulite facies.
• Blueschist and hornfels facies have unusual geothermal gradients.
• Most regional metamorphic rocks form between these extremes, passing through greenschist to amphibolite facies.
• At maximum pressures and temperatures, rocks enter granulite or eclogite facies.
• Regionally metamorphosed rocks with hydrous fluids begin to melt before passing the amphibolite facies.