metamorphism: new rocks from old chapter 10 geology today barbara w. murck, brian j. skinner n....

Metamorphism: New Rocks from OldMetamorphism: New Rocks from Old

Chapter 10Geology Today

Barbara W. Murck, Brian J. Skinner

N. Lindsley-Griffin, 1999Disharmonic Folding in Gneiss

Rock CycleRock Cycle

Metamorphic Metamorphic RockRock

Rock formed in the Rock formed in the solid state by solid state by alteration of alteration of preexisting rock preexisting rock deep within the deep within the Earth.Earth.

Heat, pressure, and Heat, pressure, and chemically active chemically active fluids are the fluids are the agentsagents..J. R. Griffin, 1999

N. Lindsley-Griffin, 1999

MetamorphismMetamorphism

Mineralogical, chemical, and structural changes in solid rocks, in response to physical and chemical conditions at depths below regions of sedimentation and diagenesis.

Fig. 10.2, p. 273


MetamorphismMetamorphism

Pressure, temperature are the most important factors.


PORE FLUIDSSmall amounts of gases or liquids between grains

Facilitate solution, migration, and precipitation of ions to speed up recrystallization

Provide a reservoir for ions during the growth of new minerals

Speed up reactionsFig. 10.3, p. 274

Metamorphic FactorsMetamorphic Factors

Quartz veins in slate


PRESSURE1. Confining pressure:greater density, prevents fracture, plastic deformation

2. Differential stress: Non-uniform pressure producespreferred orientation, rock cleavage, foliation

Fig. 10.4, p. 275:


Granite formed in uniform stress

Gneiss formed in differential stress


HEATEnhances recrystallization

Speeds up chemical reactions

At deepest crustal levels, some of the rock melts

Migmatite - part metamorphic and part igneous

(Fig. 10.9, p. 281)



TIME - enhances all other metamorphic factorsLong periods of time allow larger grains to grow

Slate Gneiss

Protolith: shale Protolith: shale

Low grade High grade


LARGE AMOUNTS OF FLUIDS = METASOMATISMComposition changes greatly by: - addition of new material - removal of old material - combination of both

Fig. 10.20, p. 291:Limestone changed into red garnet, green pyroxene,and calcite rock.If pure, would be marble.



Preferred orientation:Preferred orientation:


All the seagulls are facing into or away from the wind

This alignment produces a foliation

From conglomerate tometaconglomerate - flattened cobbles parallel to each other

Foliation:Foliation:

N. Lindsley-Griffin, 1998Metaconglomerate, Fig. 10.5, p. 276

Alluvial sandstone and conglomerate

Slaty cleavage - tendency to break along planes that form perpendicular to maximum stress.

In folded layers the cleavage parallels the axial plane



Slaty cleavage at angle to bedding

Fig. 10.7, p. 277

Schistosity - planar minerals like mica crystallize perpendicular to maximum stress.



Garnet Schist

Thin section view of schistosity in phyllite

Gneiss - micaceous schist alternating with coarsely crystalline bands

Pre-existing layers

High-grade metamorphism



Progressive changes to shale as higher T and P over time allow different index minerals to form.

Fig. 10.8, p. 280

Low Grade to High GradeLow Grade to High Grade



Types of MetamorphismTypes of Metamorphism

CONTACT METAMORPHISM

Rocks are heated and chemically changed by intrusion of hot magma.

Concentric zones or aureoles


BURIAL METAMORPHISM - Deep sedimentary basins

REGIONAL METAMORPHISM - Subduction and plate collision; most intense where continents collide

Affects broad regions

Mountain ranges and continental interiors

Fig. 10.12, p. 283


METASOMATISMVery large water-rock ratios

Composition changes greatly by: - addition of new material - removal of old material - combination of both

Fig. 10.20, p. 291:Limestone changed into red garnet, green pyroxene,and calcite rock.If pure, would be marble.




INDEX MINERAL - appears at certain P-T conditions in the progression from lower grade to higher grade: Chlorite Biotite Garnet Kyanite Sillimanite

ISOGRADS - lines on map showing where a particular index mineral first appears.

Fig. 10.13, p. 284

Metamorphic FaciesMetamorphic Facies

Assemblage of minerals typical of a set of metamorphic conditions



Fig. 10.14, p. 285

Polymorphs of Al2SiO5 reveal P-T conditions


Kyanite

Andalusite

Sillimanite


Fig. 10.14, p. 285, with additions

Continental core regions --

Canadian Shield

Regional metamorphismRegional metamorphism


Finely foliated rocks: slate and phyllite.

Slate, with slaty cleavage at high angle to bedding



Coarsely foliated rocks: Schist

Micaceous minerals, formed from shale or siltstoneN. Lindsley-Griffin, 1998


Coarsely foliated rocks: Gneiss

Bands of micaceous minerals alternating with bands of granular minerals (usually quartz and feldspar)



Shear MetamorphismShear MetamorphismAlong faults - grinding and crushing at shallow crustal levels,

stretching and recrystallization at deeper levels. Also known as cataclastic metamorphism (not in textbook).

Mylonite Thin sectionN. Lindsley-Griffin, 1998

Metamorphic RocksMetamorphic Rocks

Protolith + Process = Product

Basalt + moderate T and P = greenschist (green chlorite)

Basalt + high T and P = amphibolite (black amphibole)

N. Lindsley-Griffin, 1999Fig. 10.16, p. 288

AmphiboliteGreenschist

Foliated



Basalt + low T, high P = Blueschist (blue amphibole)

Blueschist + high T and P = Eclogite (green pyroxene, red garnet)

(Fig. 10.17, p. 288) N. Lindsley-Griffin, 1999

Foliated


Protolith + Process = ProductShale Slate Phyllite Schist, Gneiss


Fig. 10.8, p. 280

Increasing metamorphic grade

Foliated

Shale + heat, pressure Slate Phyllite

with progressive growth of foliation, grain size(Fig. 10.15, p. 287)




Foliated



Phyllite Schist (clay-rich)

Gneiss (quartz + feldspar rich) N. Lindsley-Griffin, 1999

Foliated



Quartz sandstone + Recrystallization = Quartzite


Thin section of QuartziteHand specimen of Quartzite

Fig. 10.19, p. 290

Nonfoliated



Limestone + Recrystallization = Marble


Thin section of MarbleHand specimen of Marble

Fig. 10.19, p. 290

Nonfoliated

© Houghton Mifflin 1998. All rights reserved

High pressure, low temperature in subduction zoneGreenschist and Blueschist facies

High temperature, low pressure in volcanic arcsGreenschist and Amphibolite facies

Metamorphic Facies at Convergent BoundariesMetamorphic Facies at Convergent Boundaries


Regional

Contact

Shear

Metamorphism at Convergent BoundariesMetamorphism at Convergent Boundaries


Sea water circulates in fractures

Water is heated, hydrothermally changes basalt

Metals are concentrated near hot vents

Metamorphism at Mid-Ocean RidgesMetamorphism at Mid-Ocean Ridges


Divergent:

Hydrothermal

Shear

Transform: Shear

Metamorphism at Divergent and Transform Boundaries

Metamorphism at Divergent and Transform Boundaries

metamorphism: new rocks from old chapter 10 geology today barbara w. murck, brian j. skinner n....

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