L8 - Metamorphic Rocks

Metamorphic Rocks and Mountain Building

Metamorphic Grade

• Metamorphic grade refers to the degree of metamorphism a rock has undergone.

  • Low grade: Little metamorphic change.

  • High grade: Significant metamorphic change.

Metamorphic Grade Examples

• Slate: Characterized by slatey cleavage, indicating a preferred orientation of minerals.

• Phyllite: Exhibits a shiny luster due to the presence of mica.

• Schist: Defined by the growth of porphyroblasts (large crystals).

• Gneiss: Shows segregation of mafic (dark, Mg and Fe-rich) and felsic (light, Si and Al-rich) components into bands.

• Migmatite: Represents a stage of partial melting of segregated bands; it is at the high end of the metamorphic grade scale.

Regional Metamorphism and Foliation

• Most metamorphic rocks formed via regional metamorphism are foliated.

• Foliation: A 2D fabric (a plane) in metamorphic rocks.

• Foliations form in several ways:

  • Cleavage: Rock splits along closely spaced, sub-parallel planes.

  • Schistosity: Planes defined by the preferred orientation of minerals.

  • Gneissosity: Planes defined by mineral segregation.

Metamorphic Textures

• Crenulation cleavage: Formed by multiple deformation events, causing folding and/or shearing of a previous foliation.

Porphyroblasts

• Porphyroblasts: Large crystals grown in a finer crystalline matrix within a metamorphic rock.

• Interaction: The interaction between porphyroblasts and the surrounding matrix provides insight into the history of deformation events.

Metamorphic Textures and Deformation

• Folding, flattening, and shearing are types of deformation that affect metamorphic textures.

• Timing:

  • Pre-tectonic: Before major deformation.

  • Syn-tectonic: During major deformation.

  • Post-tectonic: After major deformation.

Metamorphic Facies

• Facies Definition: Regions in pressure (P) and temperature (T) space where rocks of similar chemical composition, metamorphosed under similar P-T conditions, contain similar minerals in similar proportions.

Metamorphic Facies Examples

• Zeolite Facies: 100-200°C

  • Pelitic rocks: Interlayered smectite/chlorite

  • Calcareous rocks: Calcite

  • Mafic rocks: Laumonite, thompsonite, calcite, interlayered smectite/chlorite

• Prehnite-Pumpellyite Facies: 150-300°C

  • Pelitic rocks: Prehnite, pumpellyite, calcite, chlorite, albite

  • Calcareous rocks: Calcite, dolomite, quartz, albite, biotite, tremolite

  • Mafic rocks: Prehnite, pumpellyite, calcite, chlorite, albite

• Greenschist Facies: 300-450°C

  • Pelitic rocks: Muscovite, chlorite, calcite, quartz, albite, garnet

  • Calcareous rocks: Calcite, quartz, muscovite, biotite, garnet, albite, tremolite, epidote, diopside

  • Mafic rocks: Albite, chlorite, quartz, epidote, actinolite, sphene

• Epidote Amphibolite Facies: 450-550°C

  • Pelitic rocks: Andalusite

  • Calcareous rocks: Calcite, diopside, quartz, wollastonite

  • Mafic rocks: Albite, epidote, hornblende, quartz

• Amphibolite Facies: 500-700°C

  • Pelitic rocks: Garnet, biotite, muscovite, quartz, plagioclase, staurolite, kyanite, or sillimanite

  • Calcareous rocks: Calcite, quartz, plagioclase, diopside, hypersthene

  • Mafic rocks: Hornblende, plagioclase, garnet, quartz, sphene, biotite

• Granulite Facies: 700-900°C

  • Pelitic rocks: Garnet, Kspar, sillimanite or kyanite, quartz, plagioclase, hypersthene

  • Calcareous rocks: Aragonite, white mica

  • Mafic rocks: Plagioclase, augite, hornblende, garnet, hypersthene, olivine

• Blueschist Facies: 150-350°C, P > 5-8 Kb

  • Pelitic rocks: Jadeite, albite, quartz, lawsonite, aragonite, paragonite

  • Calcareous rocks: Aragonite, quartz, omphacite (px), plagioclase, diopside, hypersthene

  • Mafic rocks: Glaucophane, albite, lawsonite, sphene, ± garnet

• Eclogite Facies: 350-750°C, P > 8-10 Kb

  • Pelitic rocks: Coesite, Kspar, sillimanite, plagioclase

  • Mafic rocks: Aragonite, quartz, omphacite (px), pyrope garnet

Metamorphic Protoliths

• Mafic Rocks:

  • Primarily basalt and gabbro rocks

  • High in MgO, FeO, and CaO

• Shales (Pelites):

  • Fine-grained clastic clays and silts

  • Deposited in stable platforms or offshore wedges

  • High in $Al2O3$, $K2O$, and $SiO2$

• Carbonates:

  • Sedimentary limestones, dolostones, and marls

  • High in CaO, MgO, and $CO_2$

• Quartz:

  • Oceanic cherts and continental red beds

  • Nearly pure $SiO_2$

• Ultramafic:

  • Mantle rocks, komatiites, and cumulates

  • Very high in MgO, FeO, NiO, and $Cr2O3$

• Quartzo-feldspathic:

  • Arkose, granitoids, and rhyolites

  • High in $SiO2$, $Na2O$, $Al2O3$, and $K_2O$

Gibbs Phase Rule

• The Gibbs phase rule is given by: $f = c + 2 - \phi$

  • f = degrees of freedom

  • c = number of components

  • $\phi$ = number of phases

• In the context of metamorphic rocks:

  • Both P and T can vary.

  • Unlikely to be on a univariant line (f = 1) or an invariant point (f = 0).

  • Mostly considering a divariant field (f = 2).

• Quartz example: Quartz oceanic cherts, continental red beds nearly pure $SiO_2$. The protolith is sandstone, and the metamorphic rock is quartzite.

Fractional Crystallization and Mineral Composition

• Feldspars: Framework silicate

• Pyroxene: Single chain silicate

• Olivine: Isolated tetrahedra silicate

• Amphibole: Double chain silicate

• Biotite: Sheet silicate

• Quartz: Framework silicate

• Major elements in basalt: $c + \phi = 6$

Metamorphic Protoliths: Basalt Example

• Mafic rocks like basalt and gabbro are high in MgO, FeO, and CaO.

• Basalt metamorphoses to greenschist, containing albite, chlorite, quartz, epidote, actinolite, and sphene.

• ACF compositional diagram:

  • A = $[Al2O3 + Fe2O3] – [Na2O + K2O]$

  • C = $[CaO] – 3.33[P2O5]$

  • F = $[FeO + MgO + MnO]$

• Ingredients: Pyroxene, plagioclase, and olivine.

• Product: Epidote, albite, and actinolite.

Metamorphic Protoliths: Mudstone Example

• Shales (pelites) are fine-grained clastic clays and silts, deposited in stable platforms or offshore wedges, and are high in $Al2O3$, $K2O$, and $SiO2$.

• Mudstone metamorphoses to greenschist, containing kyanite, chloritoid, chlorite, biotite, K-feldspar, quartz, and muscovite.

• AFM compositional diagram:

  • A = $Al2O3$

  • F = FeO

  • M = MgO

• Ingredients: Kaolinite, illite, quartz, orthoclase.

• Product: Biotite, muscovite, phlogopite, andalusite/kyanite/sillimanite.

Metamorphic Protoliths: Limestone Example

• Carbonates (sedimentary limestones, dolostones, marls) are high in CaO, MgO, and $CO_2$.

• Limestone metamorphoses to greenschist, containing calcite, dolomite, quartz, epidote, and tremolite.

  • A = $[Al2O3 + Fe2O3] – [Na2O + K2O]$

  • C = $[CaO] – 3.33[P2O5]$

  • F = $[FeO + MgO + MnO]$

• Ingredients: Calcite and dolomite.

• Product: Epidote and tremolite.

Metamorphism and Mountain Building

• Regional metamorphism occurs during mountain building.

• Contact (thermal) metamorphism is associated with volcanic arcs and collisional mountain belts.

• Metamorphic facies vary with depth and temperature, ranging from zeolite and greenschist at lower temperatures to amphibolite and granulite at higher temperatures.

Barrow's Zones in Regional Metamorphism

• Barrow's zones: Mapping metamorphic zones based on index minerals.

• Index minerals (increasing grade): chlorite, biotite, garnet, staurolite, kyanite, and sillimanite.

Regional Metamorphism Reactions

• Progressive metamorphic reactions with increasing grade:

  • biotite + chlorite + muscovite + quartz + albite

  • garnet + biotite + chlorite + muscovite + quartz + albite

  • staurolite + garnet + biotite + muscovite + quartz

  • kyanite + biotite + quartz

  • sillimanite + garnet + biotite + muscovite + quartz

Subduction Metamorphism

• Occurs at convergent boundaries with subduction zones.

• High pressure and low temperature conditions lead to the formation of blueschist and eclogite facies.

Metamorphosing Oceanic Crust

• Basalts composed of anhydrous minerals won't undergo significant metamorphism in a dry state.

• Metasomatism (alteration by fluids) of oceanic crust creates hydrous phases and generates heat (exothermic reaction).

Blueschist Facies

• Characterized by minerals like lawsonite (Lws), epidote (Ep), paragonite (Prg), jadeite (Jd), glaucophane (Gln), quartz, and albite.

Eclogite Facies

• Eclogite is a rock type valued for its beauty.

High P Metamorphism

• High-pressure phases are associated with significant depths.

Ultra-High P Metamorphism

• The presence of coesite implies these rocks were taken to extreme depths during metamorphism *Garnet Composition:

  • Pyrope: $Mg3Al2Si3O{12}$

  • Almandine: $Fe3Al2Si3O{12}$

  • Grossular: $Ca3Al2Si3O{12}$

  • Uvarovite: $Ca3Cr2Si3O{12}$

  • Andradite: $Ca3Fe2Si3O{12}$

  • Spessartine: $Mn3Al2Si3O{12}$

Return of Subducted Rocks to the Surface

• Subducted rocks may be thrust upwards by later collisional tectonics or by isostatic rebound as sediments are added to the accretionary wedge.

Contact Metamorphism

• Occurs around igneous intrusions, such as granites.

• Isograds mark the degree of contact metamorphism.

Contact Metamorphism Examples

• Forsterite: $Mg2SiO4$ (a type of olivine) is formed from the reaction: dolomite + tremolite → calcite + forsterite + $H2O + CO2$

• Periclase: MgO (a type of oxide), often altered to brucite $Mg(OH)2$, is formed from the reaction: dolomite → calcite + periclase + $CO2$

• Ferropericlase: $(Fe,Mg)O$ makes up about 20% of Earth’s lower mantle and transforms from olivine at high P and T.

Characteristics of Contact-Metamorphosed Rocks

• Non-foliated (hornfels).

• Form under lithostatic conditions (no driving force for a preferred orientation).

• Angles between grains tend to 120° to minimize surface tension.
  *In Bowen's reaction series, high temperatures (~1200°C) are associated with ultramafic rocks, while low temperatures (~750°C) are associated with felsic rocks.