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TYPES OF METAMORPHISM DEVIATORIC TEMPERATURE, TEMPERATURE STRESS PRESSURE, AND AND PRESSURE ALONE DEVIATORIC\ ALONE STRESS --------------------------------------------------------------------------------------- | ROCK TYPES DYNAMIC | cataclasites/ | mylonites | DYNAMOTHERMAL | slates/phyllites | /schists/gneisses | /migmatites | THERMAL | hornfels (local) | granofelsThe rock from which a metamorphic rock is derived is known as the protolith of the metamorphic rock. The protolith can often be predicted from the mineralogy or rock composition of the metamorphic rock. For example, it can be assumed that the protolith of serpentinite is dunite, a rock largely composed of olivine, on the basis of the reaction relationship:
1) 2 Mg3Si2O5(OH)4 = 3 Mg2SiO4 + SiO2 + 4H2O
OR
2) Mg3Si2O5(OH)4 + MgO = 2 Mg2SiO4 + 2H2O
In the first case the conversion of
dunite (olivine) to serpentinite (serpentine) requires the addition
of dissolved silica (SiO2) in the hydrous fluid reacting with the olivine,
whereas in the second case the conversion of olivine to serpentine
necessitates the removal of MgO by the
fluid.
The reaction Ca(Mg,Fe)2Al2Si3O12
= CaAl2Si2O8 + (Mg,Fe)2SiO4, implies that the protolith
of a garnet rich rock should contain calcic-plagioclase (anorthite) and
olivine, a rock known as troctolite, whereas
the protolith of a rock composed of amphibole is likely to be a tholeiitic
basalt:
Ca2(Mg,Fe)4Al2Si7O22(OH)2 = CaAl2Si2O8 + Ca(Mg,Fe)Si2O6 + 1.5
(Mg,Fe)2Si2O6 + H2O
What would be the protolith of garnet-amphibolite?
Muscovite schist?
Pressure, deviatoric stress and deformation
If the compressional stresses
acting from all directions on a rock are equal in value, the pressure is
said to be hydrostatic, and the rocks will undergo a volume
change only. On the other hand, if the complementary
orthogonal (normal) stresses are unequal the rock material will
undergo both a volume change, reflecting the action of the average hydrostatic
pressure, and a distortion (dislocations in the crystallographic
lattice) which reflects the action of the deviatoric
stresses. For example, if the complementary
orthogonal stresses have values of 30 and 20 stress units, the average
hydrostatic pressure will be 25 units [(30+20)/2], and the deviatoric stresses
will have values of +5 (30-25) and -5 (20-25) units, respectively.
Note that one of the deviatoric stresses will be compressive (positive
= push) and the other tensile (negative = pull), and that the larger the
difference between the complementary orthogonal stresses, the larger
will be the deviatoric stress.
The deviatoric
distortion can be accomodated as a recoverable elastic
strain in the case of small strains. However at larger strains,
the distortions in the crystal lattice are dissipated either by fragmentation
(fracture, crushing) of the crystals, or by
the strained minerals recrystallizing into a number of smaller grains,
a process known as polygonization.
In the former case the crushed rocks are massive and are known as
cataclasites,
whereas in the latter case the deformed minerals grains will tend tend
to take on a preferred shape or lattice orientation, thus giving the rock
a foliated appearance, and are known as mylonites.
At higher temperatures, where the rate of formation of lattice dislocation
is balanced by the rate of removal of the dislocations by recrystallization,
the crystal will simply change in shape without polygonization. The
fracture mode of deformation takes place at lower pressures and temperature,
whereas the recrystallization mode is favoured by higher pressures
and temperatures.
DYNAMO-METAMORPHIC ROCKS Nature of matrix | Proportion of granulated or polygonized matrix 0-10% 10-50% 50-90% 90-100% Crushed massive | Protocataclasite Cataclasite Ultracataclasite Crushed foliated |Tectonic breccia Protomylonite Mylonite Ultramylonite. Glassy | Tachylite/Hyalomylonite Little recryst. | Hartschiefer Mostly recryst. BlastomyloniteDynamic and Dynamothermal Metamorphism
Rocks materials are therefore said to be either brittle or ductile (plastic), to have brittle strength or ductile (plastic) strength, and to deform by brittle loss of cohesion or ductile change in shape. These two concepts can be illustrated on a plot of depth versus strength of a common rock material. The parameter 'depth' incorporates two variables, 'pressure' and 'temperature', and the diagram assumes that the geothermal gradient is 20 degree C /km.
The Brittle-Ductile transition.
The brittle strength (deviatoric)
of a rock increases with depth because rocks get stronger with increasing
hydrostatic pressure whereas plastic strength is influenced more
by temperature. Therefore, at shallow depths within the Earth's crust,
as depth increases the brittle strength of a rock material increases,
whereas at depths greater than about 12 km, the mode of deformation
changes and the strength of a rock material decreases within increasing
depth. Consequently, where the brittle and ductile strength lines cross
one another, the nature of the rock strain may reflect both modes
of deformation. At depths greater than about 12 km, almost any value
of deviatoric stress would cause rocks to undergo ductile deformation.
From the surface downwards therefore, with increasing depth the nature
of faulting changes from a mode dominated by cataclastic
fracture to a mode involving brittle/ductile grain polygonization
and the formation of mylonite.
At high
levels in the crust under conditions of low temperature, low pressure,
and high rates of strain, the kinds of structures produced are therefore
those associated with loss of cohesion in rock material - in other
words joints, faults, and low
amplitude buckle folds. At low levels
in the crust, material tends to deform in a ductile manner and involve
flattening
and
shape change.
At the crust-mantle boundary, because olivine is so much stronger than quartz and plagioclase, it is commonly thought that deformation tends once again to be brittle-ductile, such that only at some depth of about 100 km does the mantle behave in a fully ductile manner. In other words, the continental lithosphere consists of a weak lower crust sandwiched between a relatively strong upper crust and uppermost mantle - the 'jelly sandwich' concept. In this context the upper part of the mantle beneath continental areas is known as the mantle 'elastic lid'. However, recent studies of the Himalayan-Tibetan orogenic system has led the proposition that the behaviour of the continental lithosphere is dominated by the strength of the upper seismogenic layer, with the continental mantle having no significant long-term strength. As a corollary, tectonic phenomena "are related to forces on the edges of the lithosphere that arise from plate motions, or from within the lithosphere that arise from crustal thickness contrasts." The detailed patterns of deformation on a scale of 100-400 km are likely "to be controlled predominantly by the strength of the crustal blocks and the faults that bound them."
Metamorphic Grade
The total range of metamorphic conditions represented in the Earth's crust and upper mantle is subdivided into a set of subsidiary P/T ranges named according to the typical mineral assemblages found to form under those conditions. For example, 'the greenschist facies' refers to rocks containing green minerals such as chlorite or actinolite or epidote, 'the blueschist facies' to rocks containing the blue amphibole glaucophane.
During crustal collision, metamorphism
commonly involves recrystallization at elevated temperatures and
pressures of rock undergoing plastic strain. Minerals growing under such
conditions will commonly exhibit a preferred shape
or crystallographic orientation, e.g. mica schists; acicular
amphibolites. At high pressure, olivine,orthopyroxene, and calcic-plagioclase
(anorthite)are converted to garnet or garnet and quartz, respectively,
whereas the sodic-plagioclase mineral albite is converted to the pyroxene
mineral jadeite. Rocks of mafic composition (basalt, gabbro) are thus converted
to eclogite. At lower temperatures where water
is an available component, mafic rocks are converted to a distinctive rock
composed of the bluish coloured sodic amphibole glaucophane.
These rocks are known as 'blueschists' and
the P/T field is called the 'blueschist facies'.
At temperatures high enough
to cause melting of quartz-plagioclase-K-feldspar aggregates the
resulting melt may segregate into patches veins or layers to form migmatites.
The residual rock material is known as restite
and is commonly enriched in biotite and garnet. The melt material
may inject other unmelted schists to form other varieties of migmatite.
In the following figure migmatites are classified according to texture
at the scale of the rock outcrop.
MIGMATITES Agmatite Diktyonitic Folded Ptygmatic or breccia or net (Folded (Veins folded (skialiths) (thin veins) veins) small wave length) ------------------------------------------------------------------------------------------------------ Schollen Phlebitic Ophthalmic Stictolithic or raft or vein or augen or flecked (Disoriented) (Migmatitic) (Eyed; Ollo) (Recryst. mat in veins) ------------------------------------------------------------------------------------------------------ Stromatic Surreitic Schlieren Nebulitic or layered or dilational or streaked or ghost (Lit par lit) (Boudinage) (Complex veins) ------------------------------------------------------------------------------------------------------Thermal Metamorphism
In the absence of deviatoric
stress, recrystallization as a result of heating can be provoked
by the convective intrusion of hot igneous
melts into lower temperature crustal rocks, or by the conductive
transfer of heat over a large area as a result of the subcrustal
ponding of basaltic melt, or by the thinning of the lithosphere
during crustal rifting. Heat can also be transferred by the convective
passage of hot fluids through the crust. Sometimes the two- dimensional
structure of the path followed by the conductive heat transfer is imaged
by the presence of concentric zones (metamorphic nodes) of rocks
of varying metamorphic grade, with the highest grade (temperature)
occurring at the centre of the node. If the metamorphism is localized
adjacent to intrusive melt, the metamorphic rocks tend to have a fine grained
flinty appearance and are consequently known as 'hornfels'.
Coarser grained rocks formed under regional conditions of thermal
metamorphism are known as 'granofels'. At
very high temperatures, solid solution becomes important in the feldspar
minerals and the rocks take on the greenish
greasy look of rocks of the granulite metamorphic facies.
Thermal metamorphism associated with
intrusive rocks often involves not only crystallization of new minerals
but also a reconstitution of the rock as as result of the addition of new
material (metasomatism) derived from the intrusive material. Such rocks
are known as 'skarns', and are commonly
formed from a carbonate protolith. Skarns
are usually very coarse grained and contain minerals (Pb, Zn, Cu)
of economic importance.
FIGURES
Figure 9. The Brittle-Ductile transition.
Figure 10. Metamorphic facies.
Figure 11. Dynamothermal metamorphism.
