Presentation on theme: "Metamorphism: Fundamental Questions"— Presentation transcript:
1Metamorphism: 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?
2Metamorphism: 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 PRetrograde: Decreasing T and PDiagenesis 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.
3General P/T Conditions of Metamorphism From Spear, 1993
4Equilibration 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 texturesRecrystallization under hydrostatic conditions yielding a newly imposed granoblastic fabricIncrease in grain size without changes in chemistry or mineralogyCrystallization of new and new fabricsRecrystallization under deviatoric stress yielding tectonite fabrics
5Before and After Metamorphism: Volcanic Tuff Relic Vitroclastic TextureFresh Rhyolite TuffIncipient Burial MetamorphismFrom: Best, 2003; Wilkinson & Whetten, 1964
6Simplified Scheme for Hydrothermal Breakdown of Primary Igneous Minerals Difficult to writestoichiometricallycorrect rxn’s becauseof complexityLiberated ions can mobilein an aqueous fluid phase:metasomatism
7Relic Phenocrysts in Meta-andesite Pyroxene pseudomorphically replaced by epidotePlagioclase pseudomorphically replaced by epidote, albite, and sericite
8Ostwald Ripening Increasing time -> Increasing grain size Process illustrated using soap bubbles120° grain boundaries mimic those found in granoblastic texturesMetamorphic recrystallization likely requires 105 to 106 yearsSimilar process seen in volcanic bubbles during tephra eruptions
10More 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
11Epitaxial Growth of Secondary Minerals Magmatic PyroxeneEpitaxial Prismatic Amphibole
12Tectonite Fabrics: Foliations and Lineations Finite strainellipse: derivedfrom anoriginallysphericalreferenceFoliation planeis perpendicularto the maximumshorteningdirectionLineation isparallel to cor maximumelongationdirection
13Fractal Nature of Deformation kmscalemscalemmscalecmscale
15Cleavage FormationSlaty 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 planesCrenulation 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.
16Examples of Ductile Metamorphism Archean Pillow Basalts - Yellow Knife, NWT CanadaUndeformed but recrystallizedpillow basaltsHighly deformed and transposedPillows (lighter colors)From Lambert & Baragar
17Recognition 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.
18Global Average Shale Composition Shales are dominatedby clays (Al-rich)and are more aluminousthan common igneousrock typesShales compriseabout 1/2 of allsedimentary rocks;Sandstones ~1/4 &Limestones of therest.Ca & Na aremobile elementsIn aqueous fluids.Deposited inLimestones.
19End-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.
20Controlling Factors in Metamorphism Four Factors:1) Temperature2) Pressure3) Fluid activity4) Deviatoric stressNote that metamorphicequilibration is alsostrongly affected bykinetic factors, whichare not illustrated.
21Schematic Continental Convergent Margin 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.From Ernst, 1976