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/cm3
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