Tectonic processes lead to rocks being exposed to changes in temperature + pressure, making them unstable eg deep burial in collision zones heating by rising magma above a subduction zone
METAMORPHISM = breakdown of previously stable minerals + formation of new ones
SOLID STATE process - no melting involved
Usually in a CLOSED SYSTEM ie no elements added/removed nb H2O, CO2 may be released
May be RECRYSTALLIZATION of existing minerals or growth of completely new metamorphic minerals
If differential pressure is involved during recrystallization, new platy minerals eg clay minerals + micas, become aligned at 90' to the direction of maximum stress, leading to an
ANISOTROPIC rock with a FOLIATION
Generally, temperature + pressure increase with depth
Temperature
Continental geothermal gradient 25'C/km
Base of continental crust 900'C
Rocks = poor heat conductors
Heat transfer by convection more efficient - high heat flow in regions of rising magma
Oceanic geothermal gradient very variable: very high at M.O.R. low (10'C/km) at trenches
Pressure
LITHOSTATIC PRESSURE increases @30MPa/km (calculation using rocks of average density)
Pressure also increased by continental collision = DIFFERENTIAL STRESS
Metamorphism occurs in the zone between diagenesis + melting
8km @200'C / 220MPa - 35-40km @650'C / 1100MPa
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Sedimentary rocks more easily affected because formed at low 'C/pressure
Igneous rocks less susceptible because formed at high 'C/pressure
Factors controlling effects of metamorphism
1 Original rock composition
Amount of 'C
Amount of pressure (CONFINING PRESSURE + HYDROSTATIC PRESSURE)
Presence of pore fluids (mainly water + CO2 - promotes crystallization)
TIME during which 'C/pressure experienced
(Changes are slow enough for metamorphic minerals to be preserved in rocks exposed by erosion eg Garnets grow 1.4mm/10million years nb The size of the 'C/pressure stability fields varies between minerals

CONTACT or THERMAL METAMORPHISM
Low pressure/high 'C
High heat flow at/near surface due to magmatic activity
Zone of COUNTRY ROCKS surrounding intrusion affected by contact metamorphism =
METAMORPHIC AUREOLE
Most prominent around plutons intruded into shallow crustal rocks @ 1000'C
Width of metamorphic aureole may be cm-km, depending on:
1) rock type eg Limestone is reactive, so wide aureole
2) size of intrusion; bigger intrusion-longer period of heating - wider aureole
3) angle of intrusion margin - shallow dip - wide aureole
Contact metamorphism of sandstone
Simple recrystallization of quartz to form interlocking crystals: METAQUARTZITE
Equigranular texture (may be SACCHAROIDAL = looks like sugar)
Contact metamorphism of limestone
Calcite + aragonite recrystallise to calcite to form MARBLE
Equigranular texture (may be SACCHAROIDAL = looks like sugar)
If limestone is impure, new metamorphic minerals formed
1 eg sandy limestone:
Calcite
CaCO3
2
+
+ eg Dolomite
Silica >
SiO2 >
Wollastonite +
CaSiO3
Carbon Dioxide
+ CO2 (lost to system)
Dolomite >
CaMg(CO3)2
Marble
CaCO3
+ Periclase
MgO
+ Carbon Dioxide
CO2 (lost to system)
SKARNS = banded rocks formed by contact metamorphism of impure limestones + dolomites
Metamorphism may be accompanied by addition/removal of new elements/ions from late stages of crystallisation of intrusion:
CONTACT METASOMATISM by liquids eg Copper porphyry deposits
PNEUMATOLYSIS by gases eg Tourmalinisation

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Contact metamorphism of mudstones
Metamorphism characterised by appearance of INDEX MINERALS with rise in 'C
Increasing change = increasing METAMORPHIC GRADE
Low grade SPOTTED ROCK with MACULOSE TEXTURE spots = recrystallised micas
Medium grade ANDALUSITE HORNFELS large white euhedral acicular PORPHYROBLASTS of andalusite
(Low pressure/high temperature aluminosilicate polymorph) square cross section + cross shape of inclusions = CHIASTOLITE
High grade HORNFELS totally recrystallised to GRANOBLASTIC TEXTURE randomly arranged equidimensional interlocking crystals quartz, andalusite,mica
Bedding usually destroyed but sometimes a trace is left
= RELICT BEDDING + GHOST STRUCTURES

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REGIONAL OR DYNAMOTHERMAL METAMORPHISM
At destructive plate margins high differential stress combined with high temperatures due to deep burial at plate boundaries lead to REGIONAL METAMORPHISM
Can affect belts few hundred km wide, thousands km long
Metamorphic recrystallisation under differential pressure
New minerals aligned at 90' to stress = FOLIATION
Platy minerals eg micas + clays most sensitive to developing foliation
Fine grained mudrocks have large surface area : volume ratio
Increases reaction rates > rapid changes in mineralogy
Sequence of changes with increasing pressure + 'C ie increasing METAMORPHIC GRADE
1
ISOGRADS = lines joining points of first appearance of INDEX MINERALS characteristic of increasing metamorphic grade
Low grade
SLATY CLEAVAGE
High pressure @200MPa / low 'C@150'C
Partial recrystallisation of small quartz grains + growth of new platy clay minerals at 90' to maximum stress direction
Traces of bedding may be preserved = RELICT BEDDING
Fossils may indicate amount of deformation of known shapes
Low/Medium grade @300MPa / @200'C
PHYLLITE
Clay minerals recrystallise to form aligned platy crystals of muscovite + chlorite
Green-grey silky sheen due to light reflected off muscovite crystals
Medium Grade
SCHIST
@500MPa / @300'c
Medium - coarse grained recrystallisation: individual crystals visible
Different minerals appear with increasing 'C
Muscovite schist - Chlorite schist - Biotite schist
May get porphyroblasts of high pressure mineral GARNET
High grade
GNEISS
@700MPa / @500'C
Alternate light/dark layers = GNEISSOSE BANDING
Light layers = coarse grains of pale FELSIC minerals; quartz + feldspar
Dark layers = coarse grains of dark MAFIC minerals; Biotite + garnet
MIGMATITE
Partial melting ie on boundary between metamorphic + igneous processes
Swirling patterns of deformed light + dark bands

METAMORPHIC MINERALS
Minerals formed during metamorphism depend on 'C, Pressure, Parent rock type
Different minerals stable under different conditions
Some have same composition but different structure under different conditions
= POLYMORPHS eg Aluminium Silicates Hi P/Lo'C
HiP/Hi'C
KYANITE
SILLIMANITE
Stability fields illustrated on a graph
LoP/Hi'C ANDALUSITE (contact)
3 polymorphs can only coexist at triple point where 3 stability fields meet
SUBDUCTION ZONE METAMORPHISM
Characterized by paired metamorphic belts in overriding plate
Next to trench low geothermal gradient low 'C
High directional stress
BLUESCHIST FACIES
(Called blueschist due to Na rich amphibole GLAUCOPHANE - navy blue associated with v high pressure/Low 'C ECLOGITE - pyroxene + garnet
Away from trench high geothermal gradient due to partial melting of descending oceanic crust at depth > rising magma > efficient transfer of heat
High 'C/Low pressure
AMPHIBOLITE FACIES
GRANULITE FACIES
DYNAMIC METAMORPHISM
Hornblende
Pyroxene, Kyanite, Sillimanite
Faults + Shear zones - rock broken up = CATACLASIS
Shallow Brittle fracture > FAULT BRECCIA + FAULT GOUGE
Deep
Mineralisation by precipitation from circulating groundwater common ductile deformation > MYLONITE lens shaped fragments in a streaky fgr matrix eg in basal thrust zone of nappes
May be large sheared feldspar porphyroblasts = AUGEN GNEISS
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Contributor: Ben Church Establishment: Monmouth Comprehensive School Date:24:05:05