Metamorphic Rocks
Metamorphic Rocks Recrystallization in the solid state Caused by changes in T, P or fluids New environment = new minerals Growing minerals create a new texture
Sources of metamorphic change
Metamorphism Recrystallization T, P or change in pore fluids initiate change in the mineral assemblage Reaction occur entirely in the solid state Shale Schist Initiate: وارد کردن assemblage:اجتماع
Textural changes occur during metamorphism New minerals grow during metamorphism Precursor rock textures are modified Precursor rock:سنگ جدید
Protolith The parent rock subjected to Metamorphism -can be any rock type: igneous, sedimentary, or metamorphic Shale Schist Protolith: سنگ منشا دگرگونی
Protolith For example, limestone is the protolith of marble
Origin of Metamorphic Rocks Metamorphism Temperature exceeds 200OC Pressure exceeds 300 M Pa (approx. 10,000 ft deep) Metamorphism ends when melting begins Melting begins at ~700oC
Stability ranges for Al2 SiO5 minerals
Source of heat Proximity to igneous intrusions Depth of burial Contact metamorphism Different metamorphic grade ring the intrusion Depth of burial 15-30oC increase per km Geothermal Gradient
Zone of Contact Metamorphism Magma Body -source of heat
-equal in all directions: uniform stress Pressure and Stress Lithostatic Pressure- pressure increases with depth due to the weight of the overlying rocks. -equal in all directions: uniform stress -a cubic foot of granite weighs 167.9 pounds.
Tectonic Pressure/Stress -directional/non-uniform -referred to as deviatoric stress Deviator: منحرف شونده
Folded Metamorphic Rocks -subjected to non-uniform deviatoric stress uniform stress: تنش همه جانبه ، تنش یک نواخت subjected to: در معرض Folded Metamorphic Rocks -subjected to non-uniform deviatoric stress
T & P determine degree of metamorphism Low-grade metamorphism- 200 to 350 OC Intermediate-grade metamorphism-350 to 550 OC High-grade metamorphism - very high temperatures, above 550OC
Progressive metamorphism of shale Conspicuous foliation: برگوارگی آشکار
Fig. 6.5. Stability ranges for Al2O5 minerals
Index Minerals
Metamorphic Rocks Classified by texture and composition Texture is divided into foliated & nonfoliated rocks Mineral composition qualifies textural name Qualifies: توصیف کردن
Foliated Nonfoliated textures
Foliation: -preferred orientation or location of minerals
Slate
Phyllite Similar to slate More Recrystallization Common Protolith: Crystals are larger Common Protolith: shales
Schist Strongly foliated rock Medium to coarse grained -dominated by platy minerals Protoliths: multiple??
[Garnet] Schist w/porphyroblasts
Gneiss Coarse grained granular rock Foliation: alternating bands of light & dark minerals Quartz, feldspar, amphibole, biotite Protoliths: ??
Nonfoliated Rocks Marble Interlocking, coarse grained calcite Recrystallization of limestone or dolostone Sedimentary features destroyed
Quartzite Metamorphism of quartz sandstone Entire rock is recrystallized
Amphibolites & Eclogites Metamorphism of mafic rocks Mineral assemblage gives green color Amphibolite [pillow basalt] Eclogite from Norway
Regional Metamorphic Zones Index minerals A mineral that forms within a specific,often narrow range of conditions Identifies a specific grade of metamorphism Allows further subdivision of rock types
Metamorphic Index Minerals
Protolith controls final mineral assemblage Different mineral assemblages form at a given P & T for various parent rocks Metamorphic facies describe a group of minerals formed under similar conditions May be found in different rocks Assemblage: اجتماع
Regional Metamorphic Gradients
Metamorphic facies
Metamorphic Rocks & Tectonics Most metamorphic rocks develop due to plate collisions Processes are not observable in the field Conditions at various plate boundaries define possible P & T range Parent rocks present control mineral assemblages plate collisions: برخورد صفحات