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:
Rock-Mineral Associations?
Nesse, Fig. 5.1
mineral stabilityis a function of:
pressure (P)temperature (T)composition (X)
..... 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)
Because change is essential to metamorphism,
studying metamorphic rocks always involves 2 questions:
a) What is it? (now)
b) What was it? (before metamorphism)
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
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!
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
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
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
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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