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![Page 1: Description and Classification Introduction to Lab MR-1 Nesse Ch. 11, p. 194-200 Lab Manual:Introduction p. 30-49 Lab MR-1 p. 50-52 Metamorphic Rocks 1:](https://reader036.fdocuments.us/reader036/viewer/2022062423/5697bfba1a28abf838ca063c/html5/thumbnails/1.jpg)
Description and ClassificationIntroduction to Lab MR-1
Nesse Ch. 11, p. 194-200Lab Manual: Introduction p. 30-49
Lab MR-1 p. 50-52
Metamorphic Rocks 1:
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Rock-Mineral Associations?
Nesse, Fig. 5.1
mineral stabilityis a function of:
pressure (P)temperature (T)composition (X)
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..... any dominantly solid-state process that produces
a change in the physical, mineralogical, textural,
or chemical character of
a pre-existing igneous, sedimentary, or
metamorphic rock, in response to
changes in pressure (P), temperature (T), chemical
environment (X), or stress field ()
What is metamorphism?
(meta = change; morph = form)
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Because change is essential to metamorphism,
studying metamorphic rocks always involves 2 questions:
a) What is it? (now)
b) What was it? (before metamorphism)
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Because change is essential to metamorphism,
studying metamorphic rocks always involves 2 questions:
a) What is it? (now)
b) What was it? (before metamorphism)
observation, description, classification
observation, interpretation
igneous or sedimentary (or metamorphic) precursor is referred to as the protolith
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Metamorphic rocks can be classified according to:
a) protolith
b) bulk composition
c) mineral assemblage
d) texture
The choice of rock name depends on:
- which feature(s) dominate
- what the observer wants to emphasise
i.e., more than one name may be appropriate!
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Metamorphic rocks can be classified according to:
a) protolith
b) bulk composition
c) mineral assemblage
d) texture
This has led to considerable inconsistency/confusion in classifying and naming metamorphic rocks
• recently, an IUGS subcommittee has addressed this problem and proposed some possible solutions
• work still in progress, no general consensus (yet)• preliminary results on poster in lab and in handout
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Metamorphic rocks can be classified according to:
a) protolith determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock
b) bulk composition
c) mineral assemblage
d) texture
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Metamorphic rocks can be classified according to:
a) protolith determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock
b) bulk compositiondetermined by/from minerals present in rock (types,
compositions, modal %) and/or chemical analysis
c) mineral assemblage
d) texture
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Metamorphic rocks can be classified according to:
a) protolith determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock
b) bulk compositiondetermined by/from minerals present in rock (types,
compositions, modal %) and/or chemical analysis
c) mineral assemblagereflects bulk composition and metamorphic grade
(P-T conditions)determined by petrographic observation
d) texture
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Metamorphic rocks can be classified according to:
a) protolith determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock
b) bulk compositiondetermined by/from minerals present in rock (types,
compositions, modal %) and/or chemical analysis
c) mineral assemblagereflects bulk composition and metamorphic grade
(P-T conditions)determined by petrographic observation
d) texturemay be inherited from protolith and/or developed during
deformation and (re)crystallisationdetermined by petrographic observation
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Metamorphic rocks can be classified according to:
a) protolith
b) bulk composition
c) mineral assemblage
d) texture
in order to reduce the number of possible variables to be considered, Labs 4 and 5 will focus on specific bulk compositions (and corresponding protoliths)
limits the range of possible mineral assemblages and textures
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Metamorphic rocks can be classified according to:
a) protolith
b) bulk composition
c) mineral assemblage
d) texture
LAB 4: Metabasites (derived from mafic/intermediate igneous rocks)
LAB 5: Pelites (derived from mudstones, shales, siltstones)
LAB 6: Other (derived from felsic and ultramafic igneous rocks; limestones, sandstones)
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a) protolith - mafic or intermediate igneous rocks (basalts, gabbros,
andesites, diorites) and their pyroclastic or volcano- clastic equivalents (mafic tuffs, etc.)
b) bulk composition (= metabasite)- relatively rich in Mg, Fe, Ca- relatively poor in Si, Al, K
c) mineral assemblage- amphibole + plagioclase ubiquitous except for unusual
P-T conditions; compositions vary with grade- garnet, biotite, quartz, epidote, chlorite, pyroxene (cpx
+/- opx), titanite, ilmenite, apatite (depends on grade)
d) texture- ranges from massive to foliated- relict features may be well preserved
LAB MR-1: METABASITES
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a) protolith mafic or intermediate igneous rocks (basalts, gabbros, andesites, diorites) and their pyroclastic or volcano-clastic equivalents (mafic tuffs, volcanogenic sandstones etc.) if protolith obvious, rock name = meta + protolith name
LAB MR-1: METABASITES
IR-3: gabbro plag laths with interstitial cpx(2.5 mm, PPL) ERTH 2002 Lab IR-2
MR-5: metagabbro relict plag laths with relict cpx
(6.25 mm, PPL) ERTH 2002 Lab MR-1
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MR-1: ???? amphibole in foliated matrix
(6.25 mm, PPL) ERTH 2002 Lab MR-1
protolith?
a) protolith mafic or intermediate igneous rocks (basalts, gabbros, andesites, diorites) and their pyroclastic or volcano-clastic equivalents (mafic tuffs, volcanogenic sandstones etc.) if protolith obvious, rock name = meta + protolith name
LAB MR-1: METABASITES
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b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, Kreflected in minerals present in rock (types, compositions, %)
classification based on bulk composition:
- find appropriate special name - modify textural root term with compositional term
LAB MR-1: METABASITES
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b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, Kreflected in minerals present in rock (types, compositions, %)
some special rock names that apply specifically to metabasites:
greenschist – fine- to medium-grained metabasite with a distinct foliation, containing some combination of chlorite +/- actinolite +/- epidote
greenstone – fine- to medium-grained massive metabasite, typically preserving some relict features, containing some combination of chlorite +/- actinolite +/- epidote
amphibolite – medium- to coarse-grained, massive to foliated metabasite consisting largely of hornblende + plagioclase
granulite – medium- to coarse-grained, massive to foliated metabasite containing metamorphic opx + cpx + plag
others (e.g., blueschist, eclogite) will not be encountered this term
LAB MR-1: METABASITES
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MR-1: ???? amphibole in foliated matrix
(6.25 mm, PPL) ERTH 2002 Lab MR-1
b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, K
reflected in minerals present in rock (types, compositions, %)
greenschist? greenstone? amphibolite? granulite?
picking appropriatespecial rock namerequires knowing
mineralogy and texture
conversely, special rock namesconvey information about
both mineralogy and texture
LAB MR-1: METABASITES
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c) mineral assemblage (reflects bulk composition and grade)
amphibole + plagioclase ubiquitous except for unusual P-T conditions; compositions vary with grade
garnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx), titanite, ilmenite, apatite (depends on grade)
amphibole:
actinolitehornblende: blue-green green brown
lower P,T
higher P,T
plagioclase:
albite (An0-10)
oligoclase (An20-30)andesine (An30-50)
LAB MR-1: METABASITES
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lower P,T
higher P,T
greenschist facies: chlorite ± epidote ± albite (An0-10) ± actinolite + quartz + titaniteamphibolite facies: hornblende + plagioclase (An20-40) + quartz ± biotite ± garnet ± titanite ± ilmenitegranulite facies: orthopyroxene + clinopyroxene + plagioclase (An30-50) + quartz + ilmenite ± rutile ± garnet ± hornblende ± biotite
metamorphic facies: a set of mineral assemblagesindicative of metamorphic P-T conditions (grade)
c) mineral assemblage (reflects bulk composition and grade)
amphibole + plagioclase ubiquitous except for unusual P-T conditions; compositions vary with grade
garnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx), titanite, ilmenite, apatite (depends on grade)
LAB MR-1: METABASITES
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MR-5: metagabbroplag + gnt + px + amph + ......
(6.25 mm, PPL) ERTH 2002 Lab MR-1
MR-1: ???? amph + plag + qtz + ......
(2.5 mm, PPL) ERTH 2002 Lab MR-1
c) mineral assemblageamphibole + plagioclase ubiquitous except for unusual P-T
conditions; compositions vary with gradegarnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx),
titanite, ilmenite, apatite (depends on grade)
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
- relict = inherited (from protolith or earlier stage of metamorphism)
- foliation = planar fabric (layering, alignment of platy minerals, etc.)
- lineation = linear fabric (stretching, alignment of elongated minerals, etc.)
- porphyroblasts = coarser-grained metamorphic minerals in a finer-grained matrix (cf. phenocryst)
- reaction texture = replacement of one (or more) mineral by another mineral (or minerals) (pseudomorphs, reaction rims, coronas, etc.)
LAB MR-1: METABASITES
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IR-3: gabbro plag laths with interstitial cpx(2.5 mm, PPL) ERTH 2002 Lab IR-2
MR-5: metagabbro relict plag laths with relict cpx
(6.25 mm, PPL) ERTH 2002 Lab MR-1
d) texture: common textures of metabasites
relict = inherited (from protolith or earlier stage of metamorphism)
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
foliation = planar fabric (layering, alignment of platy minerals, etc.)
special types of foliation include:
cleavage – tendency for rock to split along closely spaced,parallel planes
schistosity – defined by parallelalignment of platy minerals(e.g., chlorite, mica)
gneissosity – compositionallayering in medium- to coarse-grained rocks defined by variable proportions of light and dark minerals
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
foliation = planar fabric (layering, alignment of platy minerals, etc.)
special types of foliation include:
cleavage – tendency for rock to split along closely spaced,parallel planes
schistosity – defined by parallelalignment of platy minerals(e.g., chlorite, mica)
gneissosity – compositionallayering in medium- to coarse-grained rocks defined by variable proportions of light and dark minerals
MR-1: (6.25 mm, PPL) amphibole in foliated matrix
ERTH 2002 Lab 4
S1
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
lineation = linear fabric (stretching, alignment of elongated minerals, etc.)
special types of lineation include:
stretching lineation – defined by stretched minerals or mineral aggregates
mineral lineation – defined by parallel alignment of long axes of elongated minerals
intersection lineation – defined by intersection of two planar fabric elements
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
porphyroblasts = coarser-grained minerals in a finer-grained matrix (cf. phenocrysts in igneous rocks)
MR-1: (6.25 mm, PPL)amphibole porphyroblasts in finer-grained foliated matrix
porphyroblast shapes can bedescribed as:
euhedral or idioblasticsubhedral or subidioblasticanhedral or xenoblastic
because porphyroblasts growfrom a solid matrix, they generally contain inclusionsthat may preserve an earlier stage of the metamorphichistory (minerals +/- textures) = poikiloblastic texture
LAB MR-1: METABASITES
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d) texture: common textures of metabasites
reaction texture = replacement of one (or more) minerals (reactants) by one (or more) new minerals (products)
MR-5: metagabbroreaction rim (corona) betweenplagioclase + another mineral (6.25 mm, PPL) ERTH 2002 Lab 4
reaction textures include:
pseudomorph – replacement byone or more minerals, where shapeof the original grain is preserved
reaction rim – reactant rimmed by one or more product minerals
corona – reaction rim where reaction products form concentric zones around/between original (reactant) minerals
LAB MR-1: METABASITES
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Classification/nomenclature according to IUGS:
a) protolith – do features inherited from protolith dominate?if so, add meta- to original rock name
LAB MR-1: METABASITES
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Classification/nomenclature according to IUGS:
a) protolith – do features inherited from protolith dominate?if so, add meta- to original rock name
b) bulk composition – is there a special rock name that fits this bulk composition? if so, use it!
(alternatively, modify textural root name with compositional term)
LAB MR-1: METABASITES
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Classification/nomenclature according to IUGS:
a) protolith – do features inherited from protolith dominate?if so, add meta- to original rock name
b) bulk composition – is there a special rock name that fits this bulk composition? if so, use it!
(alternatively, modify textural root name with compositional term)
c) mineral assemblage – are one or two minerals in the rock particularly important? if so, add mineral name(s) to root name based on texture (see below)
LAB MR-1: METABASITES
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Classification/nomenclature according to IUGS:
a) protolith – do features inherited from protolith dominate?if so, add meta- to original rock name
b) bulk composition – is there a special rock name that fits this bulk composition? if so, use it!
(alternatively, modify textural root name with compositional term)
c) mineral assemblage – are one or two minerals in the rock particularly important? if so, add mineral name(s) to root name based on texture (see below)
d) texture – is there a special rock name that fits this texture?
if so, use it! if not....is the rock foliated? if not, use term “granofels”does the rock have a foliation defined by alignment of
sheet silicates? if so, use term “schist”is the rock medium- to coarse-grained, with pronounced cm-scale compositional banding? if so, use term “gneiss”
LAB MR-1: METABASITES
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Classification/nomenclature of MR-1 and MR-5 based on:
MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)
protolith:
bulk composition: = metabasite
mineral assemblage:
texture:
LAB MR-1: METABASITES
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Classification/nomenclature of MR-1 and MR-5 based on:
MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)
protolith: ???? metagabbro
bulk composition: greenschist metagabbro (=metabasite)mineral assemblage: greenschist gnt-px metagabbro
texture: greenschist coronitic metagabbro
LAB MR-1: METABASITES
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MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)
Classification/nomenclature of MR-1 and MR-5 based on:
protolith: ???? metagabbro
bulk composition: greenschist metagabbro (=metabasite)mineral assemblage: greenschist gnt-px metagabbro
texture: greenschist coronitic metagabbro
LAB MR-1: METABASITES
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1. Fill in mineral compositions on p.50 of lab manual
2. Work through MR-1, MR-5 with partner using
petrographic description sheets
3. Fill in checklist for all 5 samples
have this checked before you leave!
4. Fill out petrographic description sheets for
any 1 of the other 3 samples
hand this in with your checklist at the
beginning of the next lab
LAB MR-1: METABASITES