Lecture THREELecture THREE

Types of Metamorphism

Types of metamorphismOn 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 orogenic metamorphism :- 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 ocean-floor metamorhism :- 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 of burial metamorhism :- 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 (Shale, limestones or igneous rocks)

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 could produced in such conditions.

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, mylonite, ultramylonite to pseudotachylite.

B2- Cataclastic or shear zone metamorphism

B3- Hydrothermal metamorphism Features of hydrothermal metamorphism :- 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)