Metamorphism

• Metamorphism
• process whereby preexisting rock undergoes pronounced changes in texture and minerology at T > 200-300ºC while in the solid state
• Agents of metamorphism are heat, pressure, chemically active fluids

• Heat - temperature increases with depth
• Geothermal Gradient - variable; 30ºC/km on continent
• Effect of Heat
• increased ductility
• change in mineral assemblage
• Pressure (Stress) - Increases with depth (0.3 kbars/km)
• Confining pressure - felt equally in all directions
• Directed pressure - added pressure from tectonic activity (tension, compression, shear)
• Effect of pressure
• favors dense, compact structures
• minerals grow perpendicular to maximum pressure - produces a parallel arrangement of platy minerals called foliation.  Gives rock a layered or banded appearence.
• Chemically Active Fluids
• ion transport

• Foliation
• rock cleavage - slaty cleavage
• schistosity - parallel alignment of visible platy minerals
• gneissic - banded; layers of light and dark minerals
• Lineation - parallel alignment of needle-like grains
• Porphyroblastic - larger crystals (porphyroblasts) in a finer matrix

• Foliated Metamorphic Rocks
• Slate - very fine grained, clay rich
• slaty cleavage
• bedding often present
• metamorphic grade: low
• Phyllite
• cleavage surface has sheen
• mica and/or chlorite grains
• Schist
• > 50% visible platy minerals; may have layers or qtz and/or feldspar
• bedding rare
• grade: medium to high
• rock name based on predominant mineral
• Ex: muscovite schist (clay rich sed. rock commonly parent)
• Gneiss
• light bands of qtz and feldspars, dark bands of biotite and hornblende (amphibole)
• grade: high
• parent: igneous rock, shale 
• Non-Foliated Metamorphic Rocks
• Low Fe-Mg
• Marble
• fine to coarsely crystalline calcite
• Parent: limestone or dolomite
• Quartzite
• metamorphosed qtz sandstone; almost all qtz
• Metaconglomerate 
• High Fe-Mg (probably basalt parent)
• Hornfels
• uniform, fine-grained
• varied composition
• associated with contact metamorphism
• Greenstone - dark green, generally aphanitic
• Amphibolite - amphibole, plag.
• Granulite - pyx, gt, sillimanite
• Eclogite - pyx, gt

• Types of Metamorphism - Contact, Burial, Regional, Hydrothermal
• Burial
•  low grade metamorphism of deeply buried sediments; bedding preserved

• Contact
• magma comes in contact with cooler rock
• aureole - zone of metamophosed rock around pluton; non-foliated

• l Regional
• Extensive area

• Associated with subduction (compression)
• Majority of metamorphic rocks
• Can map pattern of regional metamorphism using index minerals which give clues to pressure and temperature of metamorphism

• Hydrothermal
• frequently associated with MOR
• hot seawater alters basalts
• on continent, alteration of country rocks by fluids from pluton at depth