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        Metamorphic rocks are materials of igneous, sedimentary, or metamorphic origin, that  have changed their physical form and/or mineralogical composition as a result of changes in  temperature, pressure, deviatoric stress, or the passage of a fluid phase. Rocks that have  recrystallized while undergoing deformation are called tectonites and are said to have suffered  dynamic metamorphism at lower temperatures and dynamothermal metamorphism at higher  temperatures. If the metamorphism takes place under static conditions involving no strain, the  metamorphism is called thermal metamorphism.
                                       TYPES OF METAMORPHISM

                          DEVIATORIC       TEMPERATURE,         TEMPERATURE
                          STRESS           PRESSURE, AND        AND PRESSURE 
                          ALONE            DEVIATORIC\          ALONE
                        |                   ROCK TYPES
DYNAMIC                 | cataclasites/                 
                        | mylonites                
DYNAMOTHERMAL           |                  slates/phyllites     
                        |                  /schists/gneisses    
                        |                  /migmatites
THERMAL                 |                                       hornfels (local)
                        |                                       granofels

      The 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.                                           Blastomylonite
        Dynamic 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.

Dynamothermal metamorphism.

      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.

Metamorphic facies.

       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.


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
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.


Figure 9. The Brittle-Ductile transition.

Figure 10. Metamorphic facies.

Figure 11. Dynamothermal metamorphism.

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