Metamorphic Textures Summary - Myweb @ CW Post
Metamorphic Textures Summary - Myweb @ CW Post 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
- Page 2 and 3: cataclastic & non-crystalline textu
- Page 4: migmatite ! at temperatures above ~
<strong>Metamorphic</strong> <strong>Textures</strong> and Classification<br />
(based primarily on Chapter 17, Hefferan & OʼBrien, 2010)<br />
Grain <strong>Textures</strong><br />
grain shape (important in metamorphism under differential stress)<br />
equant: approximately equal dimensions in all directions<br />
inequant: tabular, platy, bladed, prismatic, acicular<br />
grain size<br />
aphanitic to phaneritic<br />
! in general, grain size increases with metamorphic grade<br />
! but depends on other factors such as protolith grain size<br />
porphyroblasts<br />
! large, metamorphic crystals surrounded by matrix of finer metamorphic crystals<br />
! examples: garnet or staurolite porphyroblasts in schist<br />
porphyroclasts<br />
! relict crystals surrounded by finer (typically crushed) matrix<br />
! ! typically in shear zones<br />
! augen gneiss - eye-shape feldspars surrounded by finer matrix<br />
! ! augen are the result of pressure solution from high stress locations<br />
! ! and growth of “tails” in “stress shadows”! !<br />
grain orientation - depends on grain shape and type of metamorphism<br />
! random (non-foliated) - equant grains or no differential stress<br />
! preferred orientation - non-equant grains and differential stress<br />
! foliation - tabula, platy, and prismatic crystals all oriented in a common plane<br />
! lineation - elongate/prismatic crystals aligned in same direction<br />
Non-Foliated <strong>Textures</strong><br />
hornfels<br />
fine-grain metamorphic rocks<br />
! contact metamorphism of fine-grain protolith (mudrocks, volcanic rocks)<br />
granoblastic rocks<br />
coarse-grain metamorphic rocks composed of equant crystals<br />
can preserve relict sedimentary features at less extreme degrees of metamorphism<br />
! quartzite (metaquartzite): intergrown quartz, from metamorphism of sandstone<br />
! marble: intergrown calcite, from metamorphism of limestone & dolomite<br />
! skarn: intergrown calc-silicate minerals, from contact metamorphism of<br />
! ! limestone and dolomite<br />
! ! carbonates reacted with silica in hydrothermal fluids from the magma
cataclastic & non-crystalline textures<br />
metabreccia<br />
! metamorphism of sedimentary or volcanic breccia<br />
! or by dynamic metamorphism of rocks in a fault zone or impact structure<br />
cataclasite<br />
! brittle cataclasis in fault zone or impact structure producing a cohesive matrix<br />
! fault breccia (metabreccia) is a coarse-grain catclasite<br />
pseudotachylite<br />
! mixed glass, devitrified glass, and sheared and brecciated rock in a fault<br />
! ! or impact zone<br />
! glass formed by rapid frictional heating, partial melting, and rapid solidification<br />
impactite - several related features of an extraterrestrial impact<br />
! impact breccia: rock broken up by the impact<br />
! shatter cones: downward and outward opening cones of fractured rock<br />
! tektites: glass droplets (often devitrified) formed from melt ejected from impact<br />
! shocked quartz: two sets of deformation lamella due to the intense pressure<br />
! ultra-high pressure minerals: high pressure quartz (coesite & stishovite)<br />
transitional: non-foliated to foliated<br />
metaconglomerate<br />
! metamorphosed conglomerate<br />
! recrystallization of matrix means rock will now fracture across large clasts<br />
stretched pebble metaconglomerate<br />
! flattened by compression or stretched by shearing<br />
! elongate pebble may form either a foliation (flattening) or a lineation (shearing)<br />
serpentinite<br />
! formed by hydrothermal metamorphism of ultramafic rocks at midocean ridges<br />
! as well as in subduction zone and accretionary wedge settings<br />
! produces hydrous serpentine group minerals (phylosilicates)<br />
soapstone<br />
! fine-grain rock formed by hydrothermal metamorphism of Mg-rich ultramafic rock<br />
! or Mg-rich carbonate (remember, ultramafic mantle rock is rich in Mg)<br />
! contains talc (hydrated Mg silicate) plus serpentine<br />
! very soft because of the talc (softest mineral)<br />
greenstone<br />
! formed by hydrothermal metamorphism of midocean ridge basalt and gabbro<br />
! Precambrian age greenstone belts formed from mafic and ultramafic crust<br />
! contains chlorite, epidote, prehnite, pumpellyite, talc, serpentine, actinolite, albite<br />
! ! all but albite are hydrous minerals - mostly amphiboles & phyllosilicates<br />
! chlorite & epidote are green<br />
amphibolite<br />
! form by high pressure & temp. metamorphism of mafic rocks (e.g., basalt)<br />
! hornblende (amphibole) is dominant, with plagioclase, garnet plus others<br />
! some foliated (e.g., Central Park amphibolite), some non-foliated (our big lump)
! granulite<br />
! medium to coarse grain; granoblastic or foliated<br />
! form by high pressure & temp metamorphism (hotter than for amphibolites)<br />
! ! in lower continental crust<br />
! dehydration reactions change hydrous amphiboles & micas<br />
! ! into non-hydrous minerals pyroxene, kspar, kyanite & garnet<br />
eclogite<br />
! form by very high pressure and temp metamorphism of basalt and gabbro<br />
! ! from thickening of continental crust by collision<br />
! ! crystallization of basaltic magma in the deep lower crust<br />
! ! subduction of oceanic crust<br />
! contain green jadeite (pryoxene) omphacite, and red garnet<br />
! very high density, 3.5-4 g/cm 3<br />
textures of foliated metamorphic rocks<br />
slaty cleavage<br />
! low grade metamorphism, 150-250 °C, of clay-rich rocks<br />
! relatively shallow burial combined with compressive stress<br />
! clays re-orient along with neocrystallization of micas<br />
! at higher grades all clays recrystallized into micas<br />
! microscopic clays and micas aligned in preferred orientation<br />
! ! due to compression possibly combined with shearing<br />
! the rock cleaves along this preferred alignment direction<br />
phyllitic cleavage<br />
! a little higher grade metamorphism at 250-300 °C, of clay-rich rocks<br />
! wavy foliation<br />
! sheen due to small, just visible micas<br />
schistosity<br />
! intermediate to high grade regional metamorphism, > 300 °C, of clay-rich rocks<br />
! coarse-grained, wavy foliation<br />
! as grade increases, phyllosilicates become less abundant<br />
! ! and anhydrous minerals more abundant<br />
! ! garnet then staurolite then kayanite form at progressively higher temperature<br />
! the rock breaks roughly along the foliation<br />
gneissic banding<br />
! high grade regional metamorphism to temperatures that may exceed 600 °C<br />
! alternating bands of felsic quartz and feldspar<br />
! ! with bands of mafic biotite and amphiboles like hornblende<br />
! at higher grade amphiboles (hydrous) are replaced by pyroxenes (anhydrous)<br />
! gneissic banding develops from various igneous and sedimentary protoliths<br />
! ! orthogneiss forms from the metamorphism of an igneous protolith<br />
! ! paragneiss forms by metamorphism of a sedimentary protolith
migmatite<br />
! at temperatures above ~700-800 °C partial melting of “wet” felsic rocks<br />
! a composite of foliated (especially the mafic bands) and igneous (felsic)<br />
! some migmatites my form by partial melting in place<br />
! ! or by partial melting of deeper rocks and injection into higher rocks<br />
mylonite<br />
! fine-grained matrix with elongated porphyroclasts<br />
! formed in ductile shear zones<br />
! both brittle grinding and plastic flow may occur