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Transcript
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
affected by hot fluids
if the bulk chemical composition is
metasomatized.
NOTE: Hot inter-granular fluids (commonly H2O & CO2 ) speed
up metamorphic reactions and fluids are heated by geothermal
gradient or igneous intrusion.
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
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.
Shear stress & accompanying brittle to ductile deformation
& cataclastic textures.
mylonites
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
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.
These zones are:
1.
Chlorite zone: Chlorite + Muscovite + Quartz + Albite
2.
Biotite zone: Biotite + Chlorite + Muscovite + Albite + Quartz
3.
Garnet zone: Garnet + Quartz + Biotite + Muscovite + Albite
4.
Staurolite zone: Staurolite + Garnet Quartz + Muscovite + Biotite +
Plag
5.
Kyanite zone: Kyanite + Garnet + Muscovite + Biotite + Quartz + Plag
+ K-feldspar
Barrow’s 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.
Buchan zones
Barrovian zones
Subduction zones
Garnet & Omphacite pyroxene
Glaucophane amphibole
Orogenic belts
Mid-ocean ridges
Note: White lines
are isograds
Metamorphic facies + tectonic associations.
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
Limestone
Basalt
Granite
Shale
Peridotite
-
quartzite
marble
amphibolite
garnet-gneiss
sillimanite gneiss
olivine-tremolite schist
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.
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.
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-amphibolite-granulite-(INTERMED. P-T)
3/ Greenschist-amphibolite-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.
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.
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.