Metamorphic Rocks

 

•METAMORPHISM:  a process that occurs typically at elevated temperature and pressure to change the texture and assemblage of minerals present in the original, or parent rock

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•Includes recrystallization:  making new minerals from original minerals, or changing the texture of the rock

•

•Metamorphism is a solid state transformation.

 

Factors of Metamorphism

•High temperature:  lower limit ~150ฐC (diagenesis) and upper limit ~ 700 to 900ฐC (melting of granite or basalt)

•High pressure:  commonly due to overlying rock or force applied during mountain building

•Presence of fluids (especially H20):  active in making and breaking chemical bonds

•Shear stress:  deformation of the rock by slippage along countless parallel planes

 

Types of Metamorphism

•Contact metamorphism:

–Achieved by application of heat to rock surrounding a cooling body of magma

–High temperature and (typically) low pressure

–Normally affects a small area.

•Regional metamorphism:

–Associated with mountain-building

–High temperature, high pressure, and shear stress

–Affects a large area.

 

Grade of Metamorphism

•Low-grade (mild) metamorphism:  small changes in texture and/or mineralogy of parent rock (T = 150-200ฐC)

•High-grade (extreme) metamorphism:

         

radical changes in original texture and/or mineral assemblage (T as high as 600-700ฐC)

 

Metamorphic Texture

 

•Foliation: parallel alignment of platy or elongate mineral grains (mica, amphibole) in a rock caused by application of directed stress.

•Foliated textures:

–slaty cleavage:  parallel alignment of microscopic platy minerals (chiefly mica).  LOW GRADE METAMORPHISM

–phyllitic texture:  parallel, but wavy, foliation of fine-grained platy minerals (mainly mica and chlorite) imparting a shiny or glossy luster.  LOW GRADE METAMORPHISM

–schistosity:  parallel to sub-parallel foliation of medium to coarse-grained platy minerals, especially mica.  INTERMEDIATE TO HIGH GRADE METAMORPHISM

–gneissic layering:  discontinuous light and dark layering due to mineral segregation.  INTERMEDIATE TO HIGH GRADE METAMORPHISM

 

•Non-foliated texture:

–absence of parallel layers of platy minerals

–may exhibit stretched grains (ductile deformation.

–normally composed of stubby, interlocking grains of approximately the same size

 

 

Textural Changes

•Metamorphism can also produce these changes:

–Crystals grow in size.

–Minerals can become segregated from one another to form compositional layering (as in gneiss).

–Crystals shapes can become distorted by ductile deformation.

–New minerals can form:

•polymorphic transformation

•reshuffling of atoms to form new minerals with no change in bulk chemical composition

Mineral Assemblages

Depend upon:

•chemical composition of parent rock

•intensity of metamorphism (involving temperature, pressure, shear stress)

Mineral assemblage can change with no change in bulk chemical composition.

Shear Stress (directed stress)

•Distortion or deformation (change in shape or size, or both)

•Development of lineation:  preferred orientation of elongated grains (hornblende)

•Development of foliation:  parallel arrangement of minerals with platy or elongate habit (such as mica or hornblende)

Index Minerals

•Diagnostic minerals indicate a limited (restricted) range of pressure, temperature (P,T) conditions of metamorphism.

•General appearance with increasing metamorphism:

–Low grade--------------------------------------------เhigh grade

–Mica appears (clay disappears)เgarnet and staurolite appear; amphibole increasesเpyroxene increases (mica disappears)

–H2O-rich-------------------------------------------------เ no H2O.

Increasing Metamorphic Grade

Mudstone/shale

 ญญญเ slate  เ phyllite  เ schist  เ gneiss

    (fine-grained)              เ (medium-coarse grained)

 

Bulk Composition

•Although a mineral assemblage may change with an increasing grade of metamorphism, the bulk chemical composition of the original parent rock commonly does not change (except for loss of water).

•Examples:

–Quartz sandstone --------------- quartzite

–Limestone/dolomite ------------- marble

–Basalt ------------------------------ amphibolite

–Granite ---------------------------- granite gneiss