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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 planarfabric 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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Shear stress & accompanying brittle to ductile deformation mylonites

& cataclastic textures.

In a fault zone environment, a layered rock consisting of bands of hard, brittlerocks in a matrix of softer, clay-rich layers will develop lenses of brittle mineral

boudins (from French word meaning sausage)

Boudins

Augen structure

Normal lava block from ChaosCrags, N. California

Cataclasitelava block

with stretched

phenocrysts from shear

zone in Chaos Crags

dome

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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 metamorphicpetrology). 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:

1. Chlorite zone: Chlorite + Muscovite + Quartz + Albite

2. Biotite zone: Biotite + Chlorite + Muscovite + Albite + Quartz3. 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

Barrows interpretation that the metamorphic grade/intensity increased from

Chlorite to the Sillimanite zones was based on observation that grain sizealso increased. These zones Barrovian Zonesand are

recognized as representative of intermediate P-T metamorphism.

Tilley extended this study and introduced the concept of an isogradwhich

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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8/14Metamorphic facies + tectonic associations.

Mid-ocean ridges

Orogenic belts

Subduction zones

Note: White lines

are isograds

Glaucophane amphibole

Garnet & Omphacite pyroxene

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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 4or 5of 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 faciesis 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 bulkcomposition 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 (retrogradeduring exhumation process) set of

P-T conditions, the assignment of a rock to a facies is always based on the peak

metamorphic conditionsit 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-typesequences 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.

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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 beltmirrors the location of thesubduction zone where thesubducting plate moves tothe NW. The low P-T belt

is an ancient island arc thathas been thrust against thehigh P-T belt.

Thrustingis common insubduction zones.