Transcript Lecture notes on Metamorphic Petrology

Lecture THREE
Types of Metamorphism
Types of metamorphism
On the basis of (i) Geological setting, and (ii) agents of
metamorphism, the type of metamorphism includes:
- Regional extent (over a wide area)
- Orogenic metamorphism (T, P, active fluids)
- Ocean floor metamorphism (T)
- Subduction zone metamorphism (HP/LT)
- Burial metamohism (LT/LP)
- Local extent (local area)
- Contact or thermal metamorphism (T)
- Cataclastic or shear zone metamorphism (P)
- Hydrothermal metamorphism (active fluids)
- Impact or shock metamorphism (extreme P-T)
A1: Orogenic metamorphism
(Regional or dynamothermal metamorphism)
Features
of
metamorphism :
orogenic
- Where?: Restricted to orogenic belts and
extent over distance of hundreds to
southlands Kms, e.g. East-African orogen
- The agents of metamorphism: include T, P
& active chemical solution
-Time duration is long (million or tens of
millions years)
- The yielded rocks suffered deformation and
recrystallization, and exhibit penetrative
fabric with preferred orientation of mineral
grains. They could suffered phases of
crystallization and deformation
- At higher P-T conditions, partial to
complete melting may accompanied and
both
migmatites
and
granites
may
associates, or granulite could be develop.
migmatites
A2: Ocean-floor metamorphism
Features
of
metamorhism :
ocean-floor
- where?: Restricted to transformation
of the oceanic crust at the vicinity of
mid-ocean ridge
- Occur in the upper part of the oceanic
crust, typically in sheeted dykes
- The agents of metamorphism include
T & sea water percolation
- The yielded rocks are mostly basic
(sheated dykes) in composition, with no
penetrative fabric (non-foliated texture)
A3: Subduction zone metamorphism
Features of subduction zone
metamorphism :
- where?:
At
convergence plate
margins, where subduction of cold
oceanic lithosphere and overlying
sediments
against
an
adjacent
continental or oceanic plate.
- The agents of metamorphism include
higher pressure, low temperature
conditions
- The yielded rocks contain high
pressure mineral assemblage such
glucophane, and kyanite should
formed
-To preserve such environment , the
rock requires rapid uplift
A4: Burial metamorphism
Features
metamorhism :
of
burial
- Where?: in subsidence basins, where
sediments and interlayered volcanics
suffered low temperature regional
metamorphism
- Agent of metamorphism include low
temperature-low pressure conditions
due to burial affect without any
influence of orogenesis or magmatic
intrusions.
- The yielded rocks lack schistosity
and the original fabrics are largely
preserved. So, the yielded rocks are
distinguished only in thin section
- In Extensional regime, Diatathermal
metamorphism is used
B1: Contact or thermal metamorphism
Features of Contact or thermal
metamorphism :
- Where
?: At vicinity of contacts with
intrusive or extrusive igneous rock bodies
-Agent of metamorphism is the higher
temperature resulted from heat emanating
from the magma, and sometimes by
deformation
connecting
with
the
emplacement of the igneous bodies.
- The zone of the contact metamorphism is
known as contact aureole, various from
meter to few kms.
-The width of the zone depend up on:
1- volume of the magmatic bodies
2- nature of the magmatic bodies (basaltic
or granitic composition)
3- The intrusion depth of magmatic bodies.
B1: Contact or thermal metamorphism, cont.
4- Type of country rocks
limestones or igneous rocks)
(Shale,
5- structures of the country rocks (cracks
and fissures)
- Duration of metamorphism is short time
(up to hundred years)
- The yielded rocks are generally fine
grained and lack schistosity (hornfels)
- In case of higher temperature influence,
Pyrometamorphism, is used.
- Migmatites
conditions.
could
produced
in
such
B2- Cataclastic or shear zone metamorphism
Features of cataclastic or shear
zone metamorphism :
- where?: Restricted to the vicinity of faults
of overthrusts in the upper crust level
(brittle deformation)
-Agents of metamorphism is pressure in
form of mechanical forces.
-The yielded rocks suffered crushing,
granulation and pulverization (reducing in
grain size).
- The yielded rocks are non-foliated and
braccia-like,
cataclasite,
ultramylonite to pseudotachylite.
mylonite,
B2- Cataclastic or shear zone metamorphism
B3- Hydrothermal metamorphism
Features
of
metamorphism :
hydrothermal
- where?: Localized at interaction of hot, largely
aqueous fluids (from igneous source) with
country rocks.
- Similar to regional ocean-floor metamorphism
- the aqueous hydrothermal fluids usually
transported via fractures and shear zones at
some distance either near or far from their source
- The yielded rocks are mineralogically and
chemically changed than the protolith and ore
deposits are occasionally originated
-If the gases instead the
aqueous fluids,
Pneumatolytic
metamorphism, is used
B4- Impact or shock metamorphism
Features of impact metamorphism :
- Where?: Impact of fall meteorites with different size on the Earth’s crust.
- This impact yielded shock waves with extreme higher P-T conditions, up to
1000 kbar and 5000 °C
- Duration time is very short, microsecond.
- The impacted rocks were vaporized, but in less condition, they melted to
produce vesicular glass containing coesite and stishovite, as well as minute
diamond
Metamorphism and plate tectonic
1- Divergent plate margin:
-Ocean floor metamorphism (HT/LP & seawater fluids)
- Diatathermal metamorphism (HT/LP)
- Contact metamorphism (HT/LP)
- Hydrothermal metamorphism
(Hydrothermal fluids)
Metamorphism and plate tectonic
2- Convergent plate metamorphism
-Orogenic condition (various P-T)
- Cataclastic and Subduction zone metamorphism (LT/HP)
3- Transform plate boundaries
- Cataclastic or Subduction zone metamorphism (LT/HP)