Lecture THREE Types of Metamorphism

1. Lecture THREETypes of Metamorphism

2. 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)

3. 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.

4. migmatites

5. 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)

6. 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

7. 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

8. 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.

9. 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.

10. 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.

11. B2- Cataclastic or shear zone metamorphism

12. 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

13. 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

14. 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)

15. 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)