Metamorphism - Lecture3

Metamorphic facies; Equilibrium & the phase rule

Low, low grade

I. Zeolite Facies (anchizone metamorphism)

P = 0-5 kbar T= 100-250șC

Diagnostic assemblage: laumonite + quartz + chlorite

(+ other zeolites like heulandite, natrolite, wairakiite)

In pelitic rocks, commonly see white mica (phengite and sericite)

Low grade

II. Prehnite-Pumpellyite Facies - handout

-laumontite no longer stable

-mostly for mafic rocks (not in pelites or carbonates

Diagnostic assemblage: prehnite + pumpellyite + quartz

(± actinolite, chlorite, epidote, albite, quartz, sericite)

III. Blueschist Facies - Fig 4.13 of handout

Mafic rocks: schists of blue color with abundant glaucophane (amphibole). Also lawsonite, aragonite, jadeite (pyroxene)

Common assemblages:

Greywackes: not blue! (lawsonite, jadeite)

Carbonates: not blue! (aragonite)

Metamorphic reactions (all lithologies): (see fig 4.13)

Regional metamorphism

IV. Greenschist facies

Diagnostic assemblage: albite + epidote ± quartz

Common to all pelites, greywackes, and mafic rocks

Other common and characteristic minerals: (in addition to albite and epidote)

pelites: muscovite + chlorite + pyrophyllite + sphene + quartz ± chloritoid ± stipnomelane ± biotite at higher grades

EQUIV. TO CHLORITE AND BIOTITE ZONES OF BARROVIAN METAMORPHISM

metabasites: chlorite + actinolite + stipnomelane + calcite + quartz

ultramafic: serpentine + talc + tremolite

calcareous: calcite + dolomite + quartz + phlogopite + talc

 

Greenschist facies does not include zeolites, prehnite, pumpellyite, lawsonite

Defining reaction: kaolinite + quartz = pyrophyllite + H2O

(lower limit of greenschist facies)

V. Albite-Epidote Amphibolite facies

pelites - GARNET ZONE OF BARROVIAN METAMORPHISM

Reaction: chlorite + muscovite = garnet + biotite + quartz + H2O

metabasites - hornblende (Na-rich) + albite + epidote + chlorite + quartz

Reaction: actinolite = Na-hornblende

calcareous rocks - not found

VI. Amphibolite facies - Fig 4.4 of handout

Pelites: quartz + muscovite + biotite + almandine garnet ± staurolite ± kyanite

Reactions:

(1) Lower amphibolite facies (equivalent to staurolite zone of Barrovian suite)

Chlorite + Muscovite = Staurolite + Biotite + Quartz + H2O

Garnet + Muscovite + chlorite = Staurolite + Biotite + Quartz + H2O

(2) Medium amphibolite facies (equivalent to kyanite zone of Barrovian)

Muscovite + Staurolite + Chlorite = Biotite + Kyanite + Quartz + H2O

(3) Upper amphibolite facies (equivalent to sillimanite zone of Barrovian)

Kyanite = Sillimanite

Metabasites: hornblende + plagioclase ± quartz ± almandine garnet ± clinopyroxene

Hornblende + plagioclase make up at least 80% of rock

Reaction:

Actinolite = Hornblende

Calcareous rocks: calcite + diopsite + tremolite

calcite + grossular garnet + zoisite

Ultramafic rocks: tremolite + forsterite ± serpentine ± talc ± enstatite

VII. Granulite facies

Extreme dehydration at maximum grades. No hydrous minerals

No Muscovite, biotite, hornblende

pelites: quartz + kspar + sillimanite + almandine garnet ± cordierite ± kyanite

Reaction: 2nd sillimanite isograd

Muscovite + Quartz = Sillimanite + Kspar + H2O

metabasites: plagioclase + clinopyroxene + orthopyroxene ± garnet

calcareous: wollastonite + calcite + diopside + forsterite (± scapolite ± corundum)

Reaction:

Calcite + Quartz = Wollastonite + CO2

ultramafic: forsterite + enstatite + spinel

= mineralogy of the upper mantle. Mantle is granulite facies metamorphic rock!

Ultra-high pressure metamorphism

VIII. Eclogite facies

Metabasalts: omphacite (cpx) + garnet ± rutile ± quartz ± kyanite

Pelites: many minerals including diamond! and coesite

Contact metamorphism

1. Albite-epidote hornfels facies

2. Hornblende hornfels facies

pelitic: quartz + biotite + muscovite + andalusite + cordierite

calcareous: calcite + diopside + tremolite

calcite + diopside + grossular garnet (skarn)

3. Pyroxene hornfels facies