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 minerals 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 !stoichiometrically!correct rxn’s because !of complexity.!Liberated ions can become mobile in an aqueous fluid phase causing metasomatism.!Relic Phenocrysts in Meta-andesite!Pyroxene pseudomorphically replaced by epidote Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)!Plagioclase pseudomorphically replaced by epidote, albite, and sericite!(K,Na)Al 2[(OH)2AlSi3O10]!!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 brittle 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 strain!ellipse: derived!from an !originally!spherical !reference!Foliation plane!is perpendicular!to the maximum!shortening!direction!Lineation is!parallel to “a” !or maximum!elongation!direction!Cleavage Formation!• Slaty cleavage: Defined by the alignment 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.!Development of Tectonic Fabric in Graywacke !Initial!Isotropic!sandstone!fabric!Foliated!meta-graywacke!or phyllite; NB!development of!slip surfaces &!relict qtz.!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!L-S Tectonite Fabric Development!Crenulation cleavage development!Inclusions and Schistosity in in Garnet Porphyroblasts!Observation: Crenulated schistosity within garnet porphyroblast!Interpretation: Garnet grew after formation of!S-C fabric!Observation: Z-shaped schistosity within garnet porphyroblast!Interpretation: Garnet preserved earlier foliation – note difference with matrix!Observation: spiral traces of foliation within garnet porphyroblast!Interpretation: Garnet “rolled” or rotated during growth – note matrix!Pressure Solution and Volume Loss!Pressure solution!removes volume!Formation of Spaced Cleavage!Examples of Ductile Metamorphism!Archean Pillow Basalts - Yellow Knife, NWT Canada!Undeformed but recrystallized!pillow basalts!From Lambert & Baragar!Highly deformed and transposed!Pillows (lighter colors)!Deformation in Mixed Strength Rocks: Boudinage!From Hyndman, 1985!Boudins of biotite-rich qtz-fsp schist enclosed in granite from Idaho batholith!Rotated boudins of amphibolite (fine grained amph-rich meta-basalt) in granite augen gneiss from Idaho batholith!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 Composition!Shales are dominated!by clays (Al-rich)!and are more aluminous!than common igneous !rock types!Ca & Na are!mobile elements!In aqueous fluids.!Deposited in!Limestones.!Shales comprise!about 1/2 of all!sedimentary rocks;!Sandstones ~1/4 &!Limestones the!rest.!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.!Metamorphic Grade!• Prograde: Refers to a metamorphic P-T-time path that progresses in toward a maximum final temperature. Reactions liberate volatiles with increasing T. !– Dehydration rxns, i.e. muscovite breakdown, liberate H2O !– Decarbonation rxns, i.e. calcite breakdown, liberate CO2!• Retrograde: Refers to a metamorphic pathway with decreasing T, which would be expected after attaining peak metamorphic temperatures. Since volatiles were liberated and migrated away during the prograde path, retrogression is often kinetically inhibited w/o re-introduction of water.!P-T-time Paths!Ostwald Ripening!• Process