Metamorphic Facies:

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Metamorphic Facies Reminder of principle
metamorphic changes 1/ Recrystallization changes
in grain size responding to T P changes.
Coarsening of grains is common e.g. Quartzite. 2/
Neomineralization Growth of new minerals is
common. 3/ Development of oriented fabric A
pervasive planar fabric defined by parallel
structural planes lineation of minerals. 4/
Metasomatism if the bulk chemical composition is
affected by hot fluids metasomatized. NOTE
Hot inter-granular fluids (commonly H2O CO2 )
speed up metamorphic reactions and fluids are
heated by geothermal gradient or igneous
intrusion.
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Porphyroblasts - Commonly one metamorphic mineral
grows much larger than other constituent
minerals porphyroblasts which may grow over an
extended period or at late-stage of a metamorphic
event. In a foliated rock, foliation may wrap
around a porphyroblast and slightly coarser
grains may develop in pressure shadows on
either side of porphyroblast.
porphyroblast
pressure shadow
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The evidence of shear stress and resulting
rotation of porphyroblasts is often preserved as
subparallel S-shaped inclusions within them.
The line of inclusions appear continuous with the
surrounding groundmass foliation.
When porphyroblast growth foliation occurs at a
late stage of metamorphism then the
porphyroblasts simply overprints the general
fabric.
Porphyroblasts may grow over long period of time
and record a history of temp pressure change
therefore the study of porphyroblasts is of
importance to metamorphic petrologists.
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Shear stress accompanying brittle to ductile
deformation mylonites cataclastic textures.
In a fault zone environment, a layered rock
consisting of bands of hard, brittle rocks in a
matrix of softer, clay-rich layers will develop
lenses of brittle mineral boudins (from
French word meaning sausage)
Augen structure
Boudins
Cataclasite lava block with stretched phenocrysts
from shear zone in Chaos Crags dome
Normal lava block from Chaos Crags, N. California
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Metamorphic Facies A metamorphic petrologist
can decipher the times at which a metamorphic
rocks of a region were subjected to different P-T
conditions. In other words, the evolutionary
history of such a region in terms of
pressure-temp-time (P-T-t). 1893 George Barrow
carried out field-based study in Scotland using
mineralogical changes as a function of
metamorphic intensity in mudrock protolith (or
pelite in metamorphic petrology). Barrow showed
that distinct zones or boundaries are marked by
appearance/disappearance of a specific mineral
(or index mineral) and this can be mapped at
outcrop scale, not just in Highlands but
globally.
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• These zones are
• Chlorite zone Chlorite Muscovite Quartz
  Albite
• Biotite zone Biotite Chlorite Muscovite
  Albite Quartz
• Garnet zone Garnet Quartz Biotite
  Muscovite Albite
• Staurolite zone Staurolite Garnet Quartz
  Muscovite Biotite Plag
• Kyanite zone Kyanite Garnet Muscovite
  Biotite Quartz Plag K-feldspar
• Barrows interpretation that the metamorphic
  grade/intensity increased from Chlorite to the
  Sillimanite zones was based on observation that
  grain size also increased. These zones
  Barrovian Zones and are recognized as
  representative of intermediate P-T metamorphism.
• Tilley extended this study and introduced the
  concept of an isograd which is a contour on a
  geological map that marks the first appearance
  disappearance of an index mineral.

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Buchan zones
Barrovian zones
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Subduction zones
Garnet Omphacite pyroxene
Glaucophane amphibole
Orogenic belts
Mid-ocean ridges
Note White lines are isograds
Metamorphic facies tectonic associations.
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Later studies found different types of
metamorphic zonation in rocks of pelitic
composition worldwide. Close to Barrows study
area, in the Buchan area of eastern Dalradians, a
very different sequence of metamorphic zones
occurs in a pelitic protolith. Here, the index
mineral sequence is Buchan sequence
Staurolite - Cordierite - Andalusite -
Sillimanite Bulk composition (inc. fluid
composition) has an important control on type of
mineral reactions in a protolith thus affecting
the mineral-based isograds. Bulk composition
also dictates what minerals may form at specific
P-T fluid composition. The list below
highlights the different rocks forming under
similar P-T conditions Sandstone -
quartzite Limestone - marble Basalt
- amphibolite Granite - garnet-gneiss Shale
- sillimanite gneiss Peridotite -
olivine-tremolite schist
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Metamorphic Facies Despite a wide variation in
bulk composition of protoliths, these rocks
develop 1/ Metamorphic assemblages with simple
mineralogies where each rock has 4 or 5 of
following minerals quartz, K-feldspar,
plagioclase, cordierite, wollastonite, diopside,
hypersthene and garnet. 2/ For a particular bulk
composition, the mineral assemblage is the same.
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IMPORTANT A metamorphic facies is not a single
rock-type but a wide range of minerals that form
under similar P-T and fluid composition
conditions. A general facies diagram was
developed and names of each facies are based on
those mineral assemblages that develop when a
mafic bulk composition undergoes various P-T
conditions. These facies are Zeolite facies -
zeolites Prehnite-Pumpellyite - Prehnite
pumpellyite Blueschist facies - glaucophane
lawsonite or epidote ( albite
chlorite) Greenschist Facies - hlorite albite
epidote actinolite Epidote-Amphibolite facies -
plagioclase hornblende /- garnet Amphibolite
facies - plagioclase hornblende
garnet Granulite facies -orthopyroxene
clinopyoxene plag hornblende
garnet Eclogite facies - omphacitic pyroxene
garnet Boundaries between 2 facies is
gradational.
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IMPORTANT Although rocks undergo metamorphism
over increasing (or prograde) as well as
decreasing (retrograde during exhumation process)
set of P-T conditions, the assignment of a rock
to a facies is always based on the peak
metamorphic conditions it reached. The
retrograde conditions are generally incapable of
obliterating the peak P-T conditions
reached. Metamorphic Facies Series Plate
Tectonics Miyashiro noted the consistent
differences between Barrovian Buchan-type
sequences in his study of Japan metamorphic
belts. He noted 3 sequences, mainly formed due
to a variation in pressure. 1/ Zeolite -
prehnite-pumpellyite-blueschist-eclogite (HIGH
P-T) 2/ Greenschist-epidote-amphibolite-amphibolit
e-granulite-(INTERMED. P-T) 3/ Greenschist-amphibo
lite-granulite (LOW P-T) Even prior to the
concept of plate tectonics, Miyashiro recognized
sub-parallel belts of high P-T adjacent to low
P-T metamorphic rocks parallel to the Trench
called them paired metamorphic belts.
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Paired metamorphic belts of Japan
The low P-T belt is composed of
andalusite-sillimanite facies assemblages,
occuring to the NW of a major tectonic
discontinuity and the high P-T belt occuring to
SE of it. High P-T belt consists of zeolite
facies to blueschist / greenschist facies and
some amphibolite rocks. Miyashiro also noted
paired metamorphic belts around the entire
Pacific Rim.
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In Japan, the high P-T belt mirrors the location
of the subduction zone where the subducting plate
moves to the NW. The low P-T belt is an ancient
island arc that has been thrust against the high
P-T belt. Thrusting is common in subduction
zones.