Metamorphic Zones and Facies
(based primarily on sect. 18.1-18.3, Hefferan & OʼBrien, 2010)
Metamorphic Zones
metamorphic grade
The degree of metamorphic recrystallization can range from almost no change to the
protolith to changes that make the protolith unrecognizable, from the growth of new
minerals in a shale to complete recrystallization as schist or gneiss. This variation in
metamorphic grade is due to variations in the temperature and pressure (burial
depth) that the protoliths experienced.
Barrovian metamorphic zones and index minerals
late 19th century geologist were aware that specific metamorphic minerals
formed at or above certain temperatures and pressure
they could be used as index minerals for temperature and burial depth
Barrows (1912) and Tilley (1925) mapped regional metamorphic rocks in Scotland
based on the first appearance of several index materials from lowest to
highest grade in pelitic (mud-rock) protoliths
chlorite zone lowest grade
rocks: chlorite-bearing slate, phyllite, schist
minerals: chlorite, sericite (phyllosilicate), muscovite
! quartz, albite (Na plag.) and pyrophyllite (phyllosilicate)
biotite zone growth of biotite mica, chlorite mica unstable!
rocks: biotite-bearing phyllite and schist
minerals: remnant chlorite, muscovite, sericite, quartz, Na-rich plagioclase
garnet zone: growth of almandine (red, Fe-garnet)
rocks: garnet-mica schist
! minerals: almandine, biotite, muscovite, magnetite, quartz, Na-rich plagioclase
staurolite zone: growth of staurolite (nesosilicate related to kyanite & silimanite)
! rocks: staurolite-mica schist, staurolite-garnet-mica schist
minerals: staurolite, biotite, muscovite, almandine, quartz, k-spar
kyanite zone: growth of kyanite (Al2SiO5), staurolite not stable
rocks: kaynite schist, kyanite-mica schist
minerals: kyanite, biotite, muscovite, almandine, quartz, cordierite
silimanite zone: growth of silimanite (Al2SiO5), kyanite not stable
! !
Barrowsʻ & Tilleyʼs highest temperature zone
rocks: silimanite schist, silimanite gneiss, cordierite gneiss
minerals: silimanite, biotite, muscovite, almandine, quartz, oligoclase and
! orthoclase feldspars, cordierite (cyclosilicate related to emerald & tourmaline)
migmatites: at even higher temperature, partial melting can begin in wet protoliths
migmatites are an intermingling of gneissic banding and felsic igneous layers
with increasing grade (temp & pressure) low-temperature, hydrous phyllosilicates
are replaced by minerals stable at higher temp & pressure
a problem with Barrovian metamorphic zones is that they were defined based on
single index minerals that form from only one kind of protolith
Metamorphic Facies
Eskola (1920, 1939), Turner (1958) developed the concept of metamorphic facies based
on assemblages of minerals that should form together in a metamorphic rock based
on specific protoliths and at particular temperatures & pressures
hornfels facies - high temp low pressure metamorphism
the product of contact metamorphism
most common in the upper crust (< 8-10 km) where the country rocks are
relatively “cold” compared to the intruding magma
albite-epidote hornfels facies
< 450 °C contact metamorphism at < 6 km depth
meta Rx of this facies contain albite (Na-plag) and/or epidote (a hydrous sorosilicate)
! regardless of protolith
includes other hydrous minerals, e.g., chlorite & actinolite
hornblende hornfels facies
450-600 °C contact metamorphism at < 8 km depth
meta Rx of this facies contain hornblende and/or another amphibole (all hydrous)
! regardless of protolith
pyroxene hornfels facies
600-800 °C contact metamorphism at < 8 km depth
! found in direct contact with high temp, mafic intrusions
these Rx contain one or more pyroxenes (hypersthene, diopside, augite, enstatite)
! regardless of prototype
biotite is the only hydrous mineral in these high temperature metamorphic rocks
sanidinite hornfels facies
> 800 °C contact metamorphism at < 8 km depth
rare, found around mafic & ultramafic intrusions
sanidine is common where protolith was felsic silicate composition
hydrous minerals are absent
increasing temp & pressure facies
greenschist through granulite facies correspond approximately to Barrovian regional
metamorphic zones
zeoloite facies
low pressure-low temperature, ~150-300 °C, < ~15 km depth
all but ultramafic protoliths produce zeolites (hydrous framework alumino-silicates)
! zeolites commonly form via hydrothermal metamorphism of volcanic rocks
pelitic Rx: altered mudrocks to low-grade slate
prehnite-pumpellyite facies
~250-350 °C, < ~20 km depth
hydrothermal & burial metamorphism
prehnite (hydrous phyllosilicate), pumpellyite (hydrous sorosilicate)
rich in other hydrous minerals
pelitic Rx: slate to phyllite
greenschist facies
regional metamorphism at ~350-550 °C, ~10-30 km depth
corresponds approximately to the Barrovian chlorite through lower garnet facies
key minerals: epidote, chlorite, actinolite (a green amphibole)
! pelitic Rx: slate, phyllite, schist
dehydration reactions release volatiles
! water released by breakdown of kaolinite clay
! CO2 liberated by breakdown of dolomite into calcite
amphibolite facies
regional metamorphism at ~550-750 °C, ~12-40 km depth
corresponds approximately to the Barrovian upper garnet through silimanite facies
pelitic Rx: schist, gneiss
dehydration reactions release volatiles
! water released by breakdown of talc
! CO2 liberated by breakdown of dolomite into diopside (pyroxene)
! both water and CO2 liberated by reaction of tremolite and calcite to form diopside
granulite facies
regional metamorphism at ~700-900 °C, ~10-50 km depth
corresponds approximately to the Barrovian upper garnet through silimanite facies
hydrous minerals are absent from upper granulite facies rocks
! quartz & kspar are common in pelitic and quartzofeldspathic rocks
! pelitic Rx: gneiss, granulite
dehydration reactions release water
! amphiboles undergo dehydration reactions into pyroxenes
! phyllosilicates (clays & micas) dehydrate to various (e.g., muscovite - kspar)
high pressure facies
form in environments with much lower temperature to pressure (depth) relationship than
for the greenschist - granulite sequence
associated with subduction zones (sinking crust remains cool, well down into mantle)
and very deep lower crustal and mantle conditions
blueschist facies - high pressure low temperature
subduction zone metamorphism at ~150-500 °C, but ~13-66 km depth
often contains the blue amphibole, glaucophane
eclogite facies
rocks from the deep lower crust or from the mantle
metamorphism at ~400-900 °C, and ~40 km to >80 km depth
! key minerals: garnet & omphacite (pyroxene)
very dense rocks
very deep eclogite facies rocks contain ultra-high pressure minerals
! that are evidence for the depths the rocks were at before being brought to the
! surface
! ! coesite (high pressure polymorph [crystal form] of SiO2)
! ! diamond (high pressure polymorph of carbon)
! ! majorite (high pressure nesosilicate - garnet)
ultra-high pressure minerals are also found in impactites