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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 LineationsFinite 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 MetamorphismArchean 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.