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Metamorphism: Fundamental Questions

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1 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?

2 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.

3 General P/T Conditions of Metamorphism
From Spear, 1993

4 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

5 Before and After Metamorphism: Volcanic Tuff
Relic Vitroclastic Texture Fresh Rhyolite Tuff Incipient Burial Metamorphism From: Best, 2003; Wilkinson & Whetten, 1964

6 Simplified Scheme for Hydrothermal Breakdown of Primary Igneous Minerals
Difficult to write stoichiometrically correct rxn’s because of complexity Liberated ions can mobile in an aqueous fluid phase: metasomatism

7 Relic Phenocrysts in Meta-andesite
Pyroxene pseudomorphically replaced by epidote Plagioclase pseudomorphically replaced by epidote, albite, and sericite

8 Ostwald Ripening Increasing time -> Increasing grain size
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

9 Prograde Thermal Metamorphism
Weakly metamorphosed Diabase - Greenstone magmatic pyx’s replaced by actinolites Greenstone or fine grained Amphibolite; same mins; better grain growth Amphibolite - well developed granoblastic Texture; no remaining vestiges of magmatic texture Granoblastic Plag-pyx Granofels; close to gabbro solidus but clear meta. texture

10 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

11 Epitaxial Growth of Secondary Minerals
Magmatic Pyroxene Epitaxial Prismatic Amphibole

12 Tectonite Fabrics: Foliations and Lineations
Finite strain ellipse: derived from an originally spherical reference Foliation plane is perpendicular to the maximum shortening direction Lineation is parallel to c or maximum elongation direction

13 Fractal Nature of Deformation
km scale m scale mm scale cm scale

14 Development of Tectonic Fabric in Graywacke
Foliated meta-graywacke or phyllite; NB development of slip surfaces & relict qtz. Initial Isotropic sandstone fabric Aphanitic Phyllite Zone; recryst. of new grains obliterates orig. sandstone fabric yielding well developed foliation Fine grained schist; coarser grains and foliation enhanced by segration layers of qtz + plag. & musc. + biotite

15 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.

16 Examples of Ductile Metamorphism
Archean Pillow Basalts - Yellow Knife, NWT Canada Undeformed but recrystallized pillow basalts Highly deformed and transposed Pillows (lighter colors) From Lambert & Baragar

17 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.

18 Global Average Shale Composition
Shales are dominated by clays (Al-rich) and are more aluminous than common igneous rock types Shales comprise about 1/2 of all sedimentary rocks; Sandstones ~1/4 & Limestones of the rest. Ca & Na are mobile elements In aqueous fluids. Deposited in Limestones.

19 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.

20 Controlling Factors in Metamorphism
Four Factors: 1) Temperature 2) Pressure 3) Fluid activity 4) Deviatoric stress Note that metamorphic equilibration is also strongly affected by kinetic factors, which are not illustrated.

21 Schematic Continental Convergent Margin
High P/Moderate T Facies Series: zeolite -> prehnite- pumpellyite -> glaucophene schist (blueschists). Adjacent to the magmatic arc see typical Barrovian- style metamorphism: moderate P and inc. T culminating in partial melting and migmatization. From Ernst, 1976

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