METAMORPHIC ROCKS
•
Definition of metamorphism: When magmatic, sedimentary or old metamorphic
rocks are subjected for long periods to new stability conditions that significantly
differ from their original formation conditions, the rocks react towards a new state of
chemical and mechanical equilibrium. These reactions lead to changes in
mineralogy, fabric and sometimes rock composition. The processes leading to such
changes are collectively termed "metamorphism"; rocks that show evidence of such
changes are termed "metamorphic"; the original, pre-metamorphic rocks are termed
"protoliths"
•
According to this definition, the process of diagenesis is simply a form of
metamorphism. However, scientific convention sets a semantic boundary at the burial
temperature at which clay minerals react to micas (150 ± 50 °C). In nature the
transition between diagenesis and metamorphism is continuous.
Five parameters that govern metamorphism
1) Temperature changes affect mineral stabilities + deformation mechanisms. Increasing
temperature favours higher entropy states (H2O and CO2 are driven out of crystal
lattices to form free fluids), volume expansion and ductile deformation (e.g. quartz is
brittle below ca. 350°C but ductile at higher T)
2) Pressure changes affect mineral stabilities and deformation mechanisms. Increasing
pressure favours high-density (low molar volume) mineral lattices
Einführung in die Mineralogie und Petrographie: Metamorphite
5 PARAMETERS THAT GOVERN METAMORPHISM
3) Deviatoric stress: Changes in orientation and magnitude of the deviatoric stress field
influences rock fabrics and deformation mechanisms. Recrystallization in response to
deviatoric stress often produces parallel alignment and layering of the silicate
minerals that have platey and acicular crystal habits (e.g. micas, amphiboles).
Metamorphic rocks with fine mineral layering (mm thickness) are termed schist
(Schiefer); rocks with coarse layering (cm thickness) are termed gneiss (Gneis)
4) Compositional changes: Contact of rocks with circulating hydrothermal fluids or with
rocks of different composition may cause chemical gradients which drive fluidmineral and mineral-mineral reactions. The resulting changes in mineralogy also
influence deformation mechanisms
5) Time – a precondition for metamorphism: Any change in parameters 1-4 must be
sustained for long enough to allow reactions to occur. High temperatures accelerate
reactions and deformation; the presence of water in rock pores also accelerates
reactions by providing a medium for fast transport of matter (diffusion in aqueous
solution is faster than diffusion in solid-state); water may also react with some
minerals and accelerate deformation (hydrolytic weakening)
Einführung in die Mineralogie und Petrographie: Metamorphite
METAMORPHIC REACTIONS (1)
• Mostly solid-state reactions (cf. dissolution and precipitation in sedimentary and
diagenetic processes; cf. melting and crystallization in magmatic processes). These
reactions are accelerated by presence of water on grain-boundaries
• Devolatilization reactions (loss of H2O, CO2, H2S, etc.) are driven by increasing
temperature and decreasing pressure (most dehydration reactions have positive
slopes in T-P space)
• Stability of minerals is determined by T-P conditions and by bulk rock composition
(if all elements in rock can communicate with each other); rocks of a given
composition always develop the same assemblages of metamorphic minerals when
subjected to the same set of T-P conditions
• The T-P stability regions of mineral assemblages are determined quantitatively by
experiments and by thermodynamic calculations for specified bulk rock
compositions
Einführung in die Mineralogie und Petrographie: Metamorphite
METAMORPHIC REACTIONS (2)
•
T-P space of metamorphism is divided into characteristic facies of mineral
assemblages (zeolite facies, amphibolite facies, etc.)
•
In the same way as a sedimentary facies is characteristic of the paleo-environment
of deposition, a metamorphic facies is characteristic of the P-T conditions of
metamorphic recrystallization.
Einführung in die Mineralogie und Petrographie: Metamorphite
METAMORPHIC REACTIONS (3)
•
•
Metamorphism at high T is termed high grade; metamorphism at low T is termed
low grade; reactions that take place during heating of the protolith are termed
prograde; reactions that take place during cooling (principally re-hydration) are
termed retrograde
•
Unless abundant water is present, retrograde reactions are extremely slow. Thus,
high-grade mineral assemblages are often found in metamorphic rocks exposed at
the Earth's surface!
•
During a reaction, product minerals may coat (armour) some of the reactants,
thereby isolating the remaining reactants and stopping the reaction (preserving a
reaction texture). In other cases, products may replace a reactant but preserve the
reactant's original crystal habit. Thus several steps in the reaction history of a rock
may be preserved for examination by the petrologist
A
A
B
B
C
C
B
A
A
A
A
A
B
A
A
B
(1) Protolith (e.g. sediment)
(2) Reaction texture
(3) Reaction complete
Schematic progress of a metamorphic reaction of the type A + B → C
Cc
Do
Di
Ol
Di
Do
Cc
Microscopic view of reaction texture preserved in a siliceous metacarbonate:
olivine (Ol) + calcite (Cc) → pyroxene (Di) + dolomite (Do)
Einführung in die Mineralogie und Petrographie: Metamorphite
METAMORPHIC P-T-t PATHS
•
Petrographic identification of mineral assemblages and the sequence of reaction
steps allows the P-T-t (t = time) path of the protolith to be reconstructed
•
Reconstructed P-T-t paths allow rocks to be fitted into a past plate-tectonic
environment. Alternatively, the paths provide information about the mechanisms
of plate tectonics
•
Reconstructed P-T-t paths of rocks collected over a geographic region reveal the
spatial extent of past plate-tectonic environments
Einführung in die Mineralogie und Petrographie: Metamorphite
4 REGIMES OF METAMORPHISM AND THEIR
PLATE TECTONIC SETTING
Plate movements and accompanying processes (e.g. generation of magmas, burial of
sediments, heat flow) drive changes in the parameters that govern metamorphism (P, T
and deformation). Because plate movements are slow relative to most reaction rates,
sufficient time is usually available (millions of years) for metamorphism to visibly affect
the protoliths
1) REGIONAL (OROGENIC) METAMORPHISM:
• Causes: Changes in T, P and deviatoric stress of protoliths due to burial to extreme
depths during compressional tectonics (e.g. European Alps, Himalaya). Heating
occurs along low geothermal gradients. Minor compositional changes occur via
breakdown of volatile-bearing minerals (e.g. dehydration of hydrates; decarbonation
of carbonates; desulphidation of sulphides and sulphates, etc.).
Einführung in die Mineralogie und Petrographie: Metamorphite
4 REGIMES OF METAMORPHISM AND THEIR
PLATE TECTONIC SETTING
• Protoliths: any sediments, volcanic rocks and older metamorphic rocks dragged down
subduction zones (and returned to Earth's surface in orogenic belts)
• Rock fabrics: Fine-grained, unstrained, partial reaction fabrics at low grade (e.g.
zeolite facies); strongly strained schist and gneiss fabrics at medium to high grade:
weakly strained granoblastic fabrics in mica- and amphibole-free rocks (e.g.
quartzites); unstrained granoblastic fabric in some eclogites; folds common at all
scales
Metaconglomerate with well-preserved
clasts. Matrix is weakly foliated
Augengneiss with deformed K-feldspar
crystals (Augen), demonstrating that
the protolith of this rock was a granite
• Typical regional-metamorphic rocks: pelitic- and psammitic-fschists and gneisses,
quartzites, marbles, greenschists, amphibolites, blueschists, eclogites
(2) BURIAL METAMORPHISM:
• Causes: Extensional subsidence of basins allows thick sequences of sediments and
volcanic rocks to accumulate. Progressive burial drives increase in T, P and deviatoric
stress of protoliths along normal geothermal gradients through the realm of
diagenesis into the realm of low-grade, low-pressure metamorphism.
• Protoliths: Any sedimentary and volcanic rocks deposited in sedimentary basins
(fore- and back-arc basins; incipient rift zones)
• Rock fabrics: same as regional metamorphism; no folding
• Typical burial-metamorphic rocks: metagreywackes; metapelites, metapsammites,
marbles, quartzites, evaporites
Einführung in die Mineralogie und Petrographie: Metamorphite
4 REGIMES OF METAMORPHISM AND THEIR
PLATE TECTONIC SETTING
(3) CONTACT (THERMAL) METAMORPHISM:
• Causes: Changes in rock temperature due to transfer of heat from cooling magmas
(very high geothermal gradient); locally changes in rock composition occur due to
reactions with hydrothermal fluids exsolved from magmas (km-scale) and due to
reactions with adjacent wall rocks (dm-scale). Minor compositional changes occur via
thermal breakdown of volatile-bearing minerals. Intrusion of large volumes of magma
into base of crust is thought to cause regional granulite-facies metamorphism and
partial melting
• Protoliths: Any metamorphic, sedimentary or older magmatic rocks that are intruded
by magmas (e.g. above subduction zones, in intraplate magmatic settings and at
oceanic and continental spreading centres)
• Rock fabrics: very fine- to fine-grained granoblastic in contact aureoles; mediumgrained granoblastic in granulites; folds near intrusive contacts
• Typical contact-metamorphic rocks: hornfels, spottet phyllite, marble
Map and cross section through contact aureole around Skiddaw Granite, UK.
Einführung in die Mineralogie und Petrographie: Metamorphite
4 REGIMES OF METAMORPHISM AND THEIR
PLATE TECTONIC SETTING
(4) SEAFLOOR METAMORPHISM:
• Causes: Shrinkage-fractures and transform faults in actively forming oceanic crust
(MOR) allow seawater to penetrate into cooling magmatic rocks. Presence of abundant
hot water drives metamorphic hydration reactions, e.g. (greatly simplified):
olivine + H2O → serpentine
pyroxene + H2O → chlorite
• Protoliths: Magmatic rocks at submarine mid-oceanic spreading centres (MOR)
• Rock fabrics: Fine-grained, sequential crystallization fabrics which copy the magmatic
fabrics of the protoliths (hence granoblastic fabric is rare); Strained fabrics at active
transform faults, e.g. serpentinite- and chlorite-schists
• Typical seafloor-metamorphic rocks: zeolite-, greenschist- and amphibolite-facies
metabasalts, metagabbros, metaperidotites, serpentinites.
;;;
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4°
dikes
BASALT
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Flux
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Tiefdruck-Faziestypen: Beispiel Troodos Massif, Zypern
0°
Kissenlaven
Dikes, Ganggesteine
Zeolith
Fazies
Grüschiefer
Fazies
Einführung in die Mineralogie und Petrographie: Metamorphite
1500 m
700 m
400°C