Metamorphism: Fundamental Questions

• What are the subsolidus changes in fabric and composition that occur during metamorphism?

• What transfers mass and energy to cause these changes?

• In what tectonic settings does metamorphism occur?

• How does the study of metamorphic rocks and processes help us understand plate tectonics and Earth’s evolution?

Metamorphism: Simple Definitions

• Metamorphism is defined as physical and chemical changes that occur in pre-existing rocks (igneous, metamorphic, or sedimentary) in the solid state that yield in a lower free energy state as a result changes in conditions (e.g. T and P).– Prograde: Increasing T and P– Retrograde: Decreasing T and P

• Diagenesis occurs at relatively low T and P and grades into metamorphism.

• Melting or migmatization occurs at the most intense, i.e. highest T an P limits of metamorphism and therefore grades into igneous processes.

General P/T Conditions of Metamorphism

From Spear, 1993

Equilibration in Metamorphic Rocks

• Parent rock is called the protolith and pathway to a new equilibrium state may result in a different changes:– Crystallization of new minerals with preservation of

relic textures

– Recrystallization under hydrostatic conditions yielding a newly imposed granoblastic fabric

– Increase in grain size without changes in chemistry or mineralogy

– Crystallization of new and new fabrics

– Recrystallization under deviatoric stress yielding tectonite fabrics

Before and After Metamorphism: Volcanic Tuff

From: Best, 2003; Wilkinson & Whetten, 1964

Fresh Rhyolite Tuff

Relic Vitroclastic Texture

Incipient Burial Metamorphism

Simplified Scheme for

Hydrothermal Breakdown of

Primary Igneous Minerals

Difficult to write stoichiometricallycorrect rxn’s because of complexity

Liberated ions can mobilein an aqueous fluid phase:metasomatism

Relic Phenocrysts in Meta-andesite

Pyroxene pseudomorphically replaced by epidotePlagioclase pseudomorphically replaced by epidote, albite, and sericite

Ostwald Ripening

• Process illustrated using soap bubbles

• 120° grain boundaries mimic those found in granoblastic textures

• Metamorphic recrystallization likely requires 105 to 106 years

• Similar process seen in volcanic bubbles during tephra eruptions

Increasing time -> Increasing grain size

Prograde Thermal Metamorphism

Weakly metamorphosedDiabase -Greenstonemagmatic pyx’sreplaced by actinolites

Greenstoneor fine grainedAmphibolite;same mins;better graingrowth

Amphibolite -well developedgranoblasticTexture; noremaining vestigesof magmatic texture

GranoblasticPlag-pyxGranofels; closeto gabbro solidusbut clear meta.texture

More Textural Definitions• Porphyblastic: similar to the porphyritic texture seen in

magmatic rocks; but larger grains, referred to as porphyroblasts, grew under sub-solidus conditions.

• Poikiloblasts: porphyroblasts containing inclusions of other minerals.

• Epitaxial growth: a secondary phase grows on a crystalline substrate that has a similar atomic structure and thus influences the orientation of the overgrowth.

• Cataclasis: Occurs when briitle rocks are broken, crushed, and pulverized to form a dilatant, unconsolidated fault breccia or fine-grained gouge.

• Tectonites: rocks with fabrics formed by dutile deformation. Fabrics are strongly anisotropic

Epitaxial Growth of Secondary Minerals

Magmatic Pyroxene

Epitaxial Prismatic Amphibole

Tectonite Fabrics: Foliations and Lineations

Finite strainellipse: derivedfrom an originallyspherical reference

Foliation planeis perpendicularto the maximumshorteningdirection

Lineation isparallel to c or maximumelongationdirection

Fractal Nature of Deformation

mmscale

cmscale

mscale

km scale

Development of Tectonic Fabric in Graywacke

InitialIsotropicsandstonefabric

Foliatedmeta-graywackeor phyllite; NBdevelopment ofslip surfaces &relict qtz.

Aphanitic PhylliteZone; recryst. of new grainsobliterates orig.sandstone fabric yielding welldeveloped foliation

Fine grainedschist; coarsergrains andfoliationenhanced bysegration layersof qtz + plag. &musc. + biotite

Cleavage Formation

• Slaty cleavage: Defined by the alingment of aphanitic platy, phyllosilicate minerals (e.g. micas and chlorite) and graphite. Qtz lenses may remain and locally are sub-parallel to the cleavage planes

• Crenulation cleavage: Secondary cleavage formation that overprints and folds the primary cleavage. Example of polymetamorphism.

• Transposition: Shearing of existing sedimentary or compositional layers into a new oblique orientation during ductile deformation.

Examples of Ductile Metamorphism

Archean Pillow Basalts - Yellow Knife, NWT Canada

Undeformed but recrystallizedpillow basalts

From Lambert & Baragar

Highly deformed and transposedPillows (lighter colors)

Recognition of Metamorphic Protoliths

• Relict Fabrics: Low grade metamorphic rocks often retain outlines of sedimentary features (e.g. bedding) or igneous features (e.g. pillows).

• Field Relations: Some cases allow one to trace prograde metamorphism from the protolith through increasing grade. Contact metamorphism in a plutonic setting is a good example.

• Bulk chemical composition: Original chemical composition may be retained to some degree. Often one can use geochemical ratios of immobile (i.e. conservative) elements.

Global Average Shale CompositionShales are dominatedby clays (Al-rich)and are more aluminousthan common igneous rock types

Ca & Na aremobile elementsIn aqueous fluids.Deposited inLimestones.

Shales compriseabout 1/2 of allsedimentary rocks;Sandstones ~1/4 &Limestones of therest.

End-member Protoliths• Ultramafic: Derived from high-Mg-Fe magmatic rocks

(e.g. peridotites, pyroxinites, and dunites.• Mafic: Derived from basalts and gabbros. High

concentrations of Mg, Fe, and Ca and Al. Usually called metabasalts (e.g. greenstones and greenschists). Also related are spillites (contain cordierite and anthophyllite), derived from metasomatic alteration at the ocean ridges.

• Quartzo-feldspathic: Dominiated by qtz and fsp. And derived from qtz-bearing meta. Rx. And lithic sandstones. Also called psammites.

• Calc-silicate and Calcareous: Derived from “dirty” and pure limestones and dolostones. Recrystallized carbonates and Ca-Fe garnet, epidote, cpx, wollastonite, and tremolite are common.

• Ferruginous: Enigmatic Fe-rich rocks including banded iron formations and associated meta-cherts.

Controlling Factors in Metamorphism

Note that metamorphicequilibration is alsostrongly affected bykinetic factors, whichare not illustrated.

Four Factors:1) Temperature2) Pressure3) Fluid activity4) Deviatoric stress

Schematic Continental Convergent Margin

From Ernst, 1976

High P/Moderate TFacies Series:zeolite -> prehnite-pumpellyite ->glaucophene schist (blueschists).

Adjacent to themagmatic arc seetypical Barrovian-style metamorphism:moderate P and inc.T culminating inpartial melting andmigmatization.