Lecture THREE 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 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.

migmatitesmigmatites

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)