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Metamorphism and Metasomatic Fluids
• Metamorphism is "changing the form" of rocks
while they are still largely solid, including changes
that happen while strictly solid (simple phase
changes, going from a low pressure to a high
pressure crystal structure) and changes involving
solutions and short-range transport.
• Metamorphism grades into diagenesis on the low
end and generation of magmas on the high end.
• Metamorphism mainly involves changes in
temperature, pressure, differential stress or
chemistry, where the rock moves towards higher
PT and presence of hydrothermal fluids.
• Hydrothermal/metasomatic fluids are high
temperature aqueous liquids, usually very
concentrated and often very acidic: they can be
associated with metamorphism, igneous intrusions,
loss of water from subducting rocks, or deep
circulation of groundwater.
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Metamorphism and Metasomatic Fluids
3 Metamorphic Processes
Products
4 Products > Metamorphic Textures
5 Products > Minerals and Metamorphic Grade
Circumstances
6 Circumstances of Metamorphism
7 Circumstances of Metamorphism Figures
8 Metamorphism and Plate Tectonics
9 Map of Metamorphic Regions
10 Classification of Metamorphic Rocks
Hydrothermal/Metasomatic Fluids
11 Hydrothermal or Metasomatic Fluids > Sources
12 Hydrothermal/Metasomatic Fluids > Processes and Products
Metamorphic Processes
• Changes in mineral stability at high P, T => new minerals
Minerals formed under one set of conditions (P, T, and chemistry) might not be stable under a different set
of conditions. Just as some minerals are more stable in the low-temperature, low-pressure, oxidized,
slightly acidic conditions at the surface, other minerals are favored at high temperatures, pressures, and
bizarre chemistry found at depth and near intrusions. Metamorphism is a kind of opposite to
weathering.
• Re-crystallization => bigger crystals
Box 7.2.1
• Pressure solution and re-deposition
=> orientation of minerals and foliated fabrics
• Hydrothermal alteration of chemistry and mineralogy => new minerals
Fig. 7.5
• Partial melting, dissolution
From http://www.geologyrocks.co.uk/forum/q_and_a/anybody_able_describe_how_diagenisis_occurs_sandstone%3F
Products > Metamorphic Textures
Metamorphism involves growth or formation of new mineral grains, usually under high
P,T conditions. Because these conditions often include differential stress, the minerals
can develop preferred orientations and fabric.
Planar Textures and
Foliation ( an oriented fabric marked by the alignment of platy minerals, usually micas)
–Slaty cleavage Slate breaks in parallel sheets, which have earthy luster Foliated
–Phyllitic Phyllite breaks in parallel sheets, which have a more vitreous luster, sort of
like silk F
–Shistose Schist Doesn’t break into sheets, but is composed of aligned micas F
–Gneissic Gneiss alternating bands of light and dark colored minerals
–Migmatitic Mixed metamorphic rock and not-far-travelled magma.
• Lineation: oriented fabric marked by the alignment of needle-like (acicular) minerals,
often amphibole.
• Mylonitic: "milled" rock. Rock is crushed and smeared out in a fault zone. Like
foliation without big xtls.
• Massive none of the above. Hornfels, quartzite, marble, veins
Fig. 7.8
Fig. 7.10b
Fig. 7.11
Fig. 7.12
Fig. 7.13
Products > Minerals and Metamorphic Grade
Box 7.3.1
• Metamorphic Grade is the assemblage of
minerals stable under the conditions of
metamorphism, usually the peak PT conditions.
This is usually given as a hyphenated group of
minerals that have a limited range, or a rock name.
Biotite-garnet, biotite-staurolite, eclogite,
blueschist, …
• The combination of grade (PT conditions + others)
and protolith (precursor rock) determines the
metamorphic facies (properties of actual rock,
including fabric and mineralogy)
Box 7.2.1
Box 7.2.1
Protolith
Lowest grade
Shale
Slate
Basalt
Greenstone
Quartz sandstone
Quartzite
Limestone
Granite
Marble
Granite
Highest grade
Phyllite
Schist
Gneiss
Amphibolite
Gneiss
quartzite
Quartzite
Marble
Granite
Marble
Schist
Igneous
Migmatite
Magma
Quartzite
Quartzite
Dissolved
Dissolved
Gneiss
Dissolved
Migmatite
Dissolved
Magma
Circumstances
of
Metamorphism
• Contact: Rocks altered by heat and fluid, with little or no differential stress, so no foliation, often rather
hard and dense, unfoliated like brick. When associated with igneous intrusions or hydrothermal fluids, they
typically form a halo around an igneous intrusion. Submarine volcanics can be metamorphosed immediately
after/during eruption. The contact metamorphic rocks and veins are often important ore sites. Products:
hornfels, quartzite, marble, skarn, greenstone [basalt altered to produce chlorite (a green, magnesian mica) +
assorted green minerals], veins of almost any composition, clay-altered rocks, sulfide deposits. Common.
• Cataclastic: Mylonitic texture crushing and shearing of rock in a fault zone or shear zone. High
differential stress, but low temperature and pressure. Associated with faults (where rock bodies slide past
one another) or shear zones (where rock bodies slide internally). Products: are mylonites (they look a lot
like schist but are very smeared out or abraded) and augen gneiss (gneiss with almond-shaped chunks of
feldspar (augen, from the German for eyes) that result from the shearing of large feldspar crystals. Rare
• Regional: High temperature and pressure, with differential stress, resulting in a foliation. Very Common
–Occurrence
• Mountain building Cores of mountain belts (fairly easy to map and apply grade concepts)
• Mid to lower crust probably has conditions that lead to regional metamorphism.
• Shields/basement Basement complex: igneous and metamorphic rocks under sediments or exposed at
the surface. Usually complex enough to be very difficult/impossible to map. Includes most crust
formed more than 1 billion years ago. Canada and upper Midwest, Africa, India… Very extensive
–Distribution of metamorphic grade (Get rocks, find PT, plot on graph, and they often define trajectory in
PT space called a geotherm)
• Normal continental
• Normal oceanic
• Batholith (Upper plate in collision zone)
• Blueschist (lower plate in subduction zone)
• Archean (older than 2.5 Ga, steeper increase of temperature with depth.
• Plate Tectonics and Metamorphism
Circumstances of Metamorphism Figures
Box 7.3.2
Metamorphism and Plate Tectonics
Map of Metamorphic Regions
From http://www.bbc.co.uk/news/science-environment-12249909 Report: Urgent
action needed to avert global hunger (bad article but good map)
Classification of Metamorphic Rocks
•Most metamorphic rock types are named for a texture, often with a
string of hyphenated mineral names in front that describe the rock and
indicate the metamorphic grade: Hornfels, Slate, phyllite, schist,
gneiss, mylonite, migmatite
•Some are named based on mineralogy: Quartzite, Marble, Amphibolite
•You can prefix Meta-(rocktype) to emphasize the protolith metaconglomerate, meta-basalt, meta-granite…
•Some get special names, like blueschist and greenschist, which are
both fine-grained schists with distinctive mineralogy and colors.
Hydrothermal or Metasomatic Fluids > Sources
• Magmatic Segregation: A magma can become over-saturated with respect to water. When
this happens, the magma separates into a silica-rich and a water-rich liquid. Each species
of ion will tend to concentrate into one or the other liquid. (Same process can occur, very
early on with a basaltic magma, yielding sulfide or oxide and silica-rich liquids) Process is
called exsolution.
–Exsolution is often accompanied by a volume increase or decrease (and thus pressure or
stress increase) and shattering of surrounding rocks.
–Porphyries usually are the result of segregation into a siliceous and aqueous phase, with
large, early-formed crystals as phenocrysts; the groundmass usually forms when the
water leaves and raises the melting point (all the minerals are now below their freezing
point with the water gone, so they crystallize).
–The resulting aqueous phase may have high concentrations of odd ions, or may have
only quartz and feldspar dissolved. In either case, you get veins, sometimes pegmatites
(common in Colorado), sometimes disseminated mineral deposits, such as copper
porphyries from the Andes,
• Metamorphic: Clays => feldspar liberates water, Squeezing and loss of pore space
liberates water, Result is much the same as for water coming off a melt.
• Deep circulation of groundwater
Hydrothermal/Metasomatic Fluids > Processes+Products
• Processes
–Hydrothermal fluids are generally hot, acidic, and very ion-rich. The acidity makes it very easy for
feldspars and mafics to be hydrolyzed, resulting in big clay-altered zones. The acids can leach any metal
ions—especially ions that don’t quite fit into their crystal structure (substitutions)—out of minerals, so
these can extract trace constituents, like Cu from surrounding rocks.
–Where the circulating fluids undergo a sudden change of pressure, temperature, or chemistry, precipitates
often form as the mineral-stability changes with PT. For example, where hydrothermal fluids are
expelled into cold sea water, sulfides, that had been stable in solution, are suddenly over-saturated and
precipitate, giving black smokers. Where boiling occurs due to decrease in pressure, many ions will
partition into the vapor phase and others in the liquid become over-saturated, and you get precipitates.
• Products
–Clay (and other) alteration zones
–Veins
–Disseminated ores
–Smokers, black and white
–Porphyries
–Geysers, fumaroles, paint pots, etc.
Metamorphic Rocks
• Metamorphism is the result of
temperatures and pressures intermediate
between diagenesis and melting,
associated with
–mountain-building,
–igneous intrusions and extrusions, and
–deep crustal conditions.
• Metamorphic rocks develop new textures
and minerals in response to their
metamorphic grade.
• Metamorphic rocks are classified based
on texture and somewhat on mineralogy
and origin.
• Hydrothermal/metasomatic fluids are
associated with metamorphism, igneous
activity, and even sedimentary diagenesis.
They are hot fluids that contain lots of
ions and alter surrounding rocks and can
deposits ores and other vein-fillings.