Metamorphic Textures Summary - Myweb @ CW Post

Metamorphic Textures and Classification 

(based primarily on Chapter 17, Hefferan & OʼBrien, 2010) 

Grain Textures 

grain shape (important in metamorphism under differential stress) 

equant: approximately equal dimensions in all directions 

inequant: tabular, platy, bladed, prismatic, acicular 

grain size 

aphanitic to phaneritic 

! in general, grain size increases with metamorphic grade 

! but depends on other factors such as protolith grain size 

porphyroblasts 

! large, metamorphic crystals surrounded by matrix of finer metamorphic crystals 

! examples: garnet or staurolite porphyroblasts in schist 

porphyroclasts 

! relict crystals surrounded by finer (typically crushed) matrix 

! ! typically in shear zones 

! augen gneiss - eye-shape feldspars surrounded by finer matrix 

! ! augen are the result of pressure solution from high stress locations 

! ! and growth of “tails” in “stress shadows”! ! 

grain orientation - depends on grain shape and type of metamorphism 

! random (non-foliated) - equant grains or no differential stress 

! preferred orientation - non-equant grains and differential stress 

! foliation - tabula, platy, and prismatic crystals all oriented in a common plane 

! lineation - elongate/prismatic crystals aligned in same direction 

Non-Foliated Textures 

hornfels 

fine-grain metamorphic rocks 

! contact metamorphism of fine-grain protolith (mudrocks, volcanic rocks) 

granoblastic rocks 

coarse-grain metamorphic rocks composed of equant crystals 

can preserve relict sedimentary features at less extreme degrees of metamorphism 

! quartzite (metaquartzite): intergrown quartz, from metamorphism of sandstone 

! marble: intergrown calcite, from metamorphism of limestone & dolomite 

! skarn: intergrown calc-silicate minerals, from contact metamorphism of 

! ! limestone and dolomite 

! ! carbonates reacted with silica in hydrothermal fluids from the magma

cataclastic & non-crystalline textures 

metabreccia 

! metamorphism of sedimentary or volcanic breccia 

! or by dynamic metamorphism of rocks in a fault zone or impact structure 

cataclasite 

! brittle cataclasis in fault zone or impact structure producing a cohesive matrix 

! fault breccia (metabreccia) is a coarse-grain catclasite 

pseudotachylite 

! mixed glass, devitrified glass, and sheared and brecciated rock in a fault 

! ! or impact zone 

! glass formed by rapid frictional heating, partial melting, and rapid solidification 

impactite - several related features of an extraterrestrial impact 

! impact breccia: rock broken up by the impact 

! shatter cones: downward and outward opening cones of fractured rock 

! tektites: glass droplets (often devitrified) formed from melt ejected from impact 

! shocked quartz: two sets of deformation lamella due to the intense pressure 

! ultra-high pressure minerals: high pressure quartz (coesite & stishovite) 

transitional: non-foliated to foliated 

metaconglomerate 

! metamorphosed conglomerate 

! recrystallization of matrix means rock will now fracture across large clasts 

stretched pebble metaconglomerate 

! flattened by compression or stretched by shearing 

! elongate pebble may form either a foliation (flattening) or a lineation (shearing) 

serpentinite 

! formed by hydrothermal metamorphism of ultramafic rocks at midocean ridges 

! as well as in subduction zone and accretionary wedge settings 

! produces hydrous serpentine group minerals (phylosilicates) 

soapstone 

! fine-grain rock formed by hydrothermal metamorphism of Mg-rich ultramafic rock 

! or Mg-rich carbonate (remember, ultramafic mantle rock is rich in Mg) 

! contains talc (hydrated Mg silicate) plus serpentine 

! very soft because of the talc (softest mineral) 

greenstone 

! formed by hydrothermal metamorphism of midocean ridge basalt and gabbro 

! Precambrian age greenstone belts formed from mafic and ultramafic crust 

! contains chlorite, epidote, prehnite, pumpellyite, talc, serpentine, actinolite, albite 

! ! all but albite are hydrous minerals - mostly amphiboles & phyllosilicates 

! chlorite & epidote are green 

amphibolite 

! form by high pressure & temp. metamorphism of mafic rocks (e.g., basalt) 

! hornblende (amphibole) is dominant, with plagioclase, garnet plus others 

! some foliated (e.g., Central Park amphibolite), some non-foliated (our big lump)

! granulite 

! medium to coarse grain; granoblastic or foliated 

! form by high pressure & temp metamorphism (hotter than for amphibolites) 

! ! in lower continental crust 

! dehydration reactions change hydrous amphiboles & micas 

! ! into non-hydrous minerals pyroxene, kspar, kyanite & garnet 

eclogite 

! form by very high pressure and temp metamorphism of basalt and gabbro 

! ! from thickening of continental crust by collision 

! ! crystallization of basaltic magma in the deep lower crust 

! ! subduction of oceanic crust 

! contain green jadeite (pryoxene) omphacite, and red garnet 

! very high density, 3.5-4 g/cm 3 

textures of foliated metamorphic rocks 

slaty cleavage 

! low grade metamorphism, 150-250 °C, of clay-rich rocks 

! relatively shallow burial combined with compressive stress 

! clays re-orient along with neocrystallization of micas 

! at higher grades all clays recrystallized into micas 

! microscopic clays and micas aligned in preferred orientation 

! ! due to compression possibly combined with shearing 

! the rock cleaves along this preferred alignment direction 

phyllitic cleavage 

! a little higher grade metamorphism at 250-300 °C, of clay-rich rocks 

! wavy foliation 

! sheen due to small, just visible micas 

schistosity 

! intermediate to high grade regional metamorphism, > 300 °C, of clay-rich rocks 

! coarse-grained, wavy foliation 

! as grade increases, phyllosilicates become less abundant 

! ! and anhydrous minerals more abundant 

! ! garnet then staurolite then kayanite form at progressively higher temperature 

! the rock breaks roughly along the foliation 

gneissic banding 

! high grade regional metamorphism to temperatures that may exceed 600 °C 

! alternating bands of felsic quartz and feldspar 

! ! with bands of mafic biotite and amphiboles like hornblende 

! at higher grade amphiboles (hydrous) are replaced by pyroxenes (anhydrous) 

! gneissic banding develops from various igneous and sedimentary protoliths 

! ! orthogneiss forms from the metamorphism of an igneous protolith 

! ! paragneiss forms by metamorphism of a sedimentary protolith

migmatite 

! at temperatures above ~700-800 °C partial melting of “wet” felsic rocks 

! a composite of foliated (especially the mafic bands) and igneous (felsic) 

! some migmatites my form by partial melting in place 

! ! or by partial melting of deeper rocks and injection into higher rocks 

mylonite 

! fine-grained matrix with elongated porphyroclasts 

! formed in ductile shear zones 

! both brittle grinding and plastic flow may occur

Metamorphic Textures Summary - Myweb @ CW Post

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