Metamorphic Rocks 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
Rock-Mineral Associations?
mineral stability
is a function of:
pressure (P)
temperature (T)
composition (X)
Nesse, Fig. 5.1
What is metamorphism?
(meta = change; morph = form)
..... 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 (s)
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)
observation, description, classification
b) What was it? (before metamorphism)
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 composition
determined 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 composition
determined by/from minerals present in rock (types,
compositions, modal %)
and/or chemical analysis
c) mineral assemblage
reflects 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 composition
determined by/from minerals present in rock (types,
compositions, modal %)
and/or chemical analysis
c) mineral assemblage
reflects bulk composition and metamorphic grade
(P-T conditions)
determined by petrographic observation
d) texture
may be inherited from protolith and/or developed during
deformation and (re)crystallisation
determined 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)
LAB MR-1: METABASITES
a) protolith
- mafic or intermediate igneous rocks (basalts, gabbros,
andesites, diorites) and their pyroclastic or volcanoclastic 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
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
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
protolith?
MR-1: ????
amphibole in foliated matrix
(6.25 mm, PPL) ERTH 2002 Lab MR-1
LAB MR-1: METABASITES
b) bulk composition (= metabasite)
relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, K
reflected 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, K
reflected 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
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 appropriate
special rock name
requires knowing
mineralogy and texture
conversely, special rock names
convey information about
both mineralogy and texture
MR-1: ????
amphibole in foliated matrix
(6.25 mm, PPL) ERTH 2002 Lab MR-1
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:
actinolite
hornblende:
blue-green
green
brown
lower P,T
plagioclase:
albite (An0-10)
oligoclase (An20-30)
andesine (An30-50)
higher P,T
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)
lower P,T
higher P,T
greenschist facies: chlorite ± epidote
± albite (An0-10) ± actinolite + quartz + titanite
amphibolite facies: hornblende +
plagioclase (An20-40) + quartz ± biotite
± garnet ± titanite ± ilmenite
granulite facies: orthopyroxene +
clinopyroxene + plagioclase (An30-50)
+ quartz + ilmenite ± rutile ± garnet
± hornblende ± biotite
metamorphic facies: a set of mineral assemblages
indicative of metamorphic P-T conditions (grade)
LAB MR-1: METABASITES
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)
MR-1: ????
amph + plag + qtz + ......
(2.5 mm, PPL) ERTH 2002 Lab MR-1
MR-5: metagabbro
plag + gnt + px + amph + ......
(6.25 mm, PPL) ERTH 2002 Lab MR-1
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
d) texture: common textures of metabasites
relict = inherited (from protolith or earlier stage of metamorphism)
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
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 parallel
alignment of platy minerals
(e.g., chlorite, mica)
gneissosity – compositional
layering in medium- to coarsegrained 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 parallel
S1
MR-1: (6.25 mm, PPL)
amphibole in foliated matrix
ERTH 2002 Lab 4
alignment of platy minerals
(e.g., chlorite, mica)
gneissosity – compositional
layering in medium- to coarsegrained rocks defined by variable
proportions of light and dark
minerals
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)
porphyroblast shapes can be
described as:
euhedral or idioblastic
subhedral or subidioblastic
anhedral or xenoblastic
MR-1: (6.25 mm, PPL)
amphibole porphyroblasts in
finer-grained foliated matrix
because porphyroblasts grow
from a solid matrix, they
generally contain inclusions
that may preserve an earlier
stage of the metamorphic
history (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)
reaction textures include:
pseudomorph – replacement by
one or more minerals, where shape
of the original grain is preserved
reaction rim – reactant rimmed
by one or more product minerals
corona – reaction rim where
MR-5: metagabbro
reaction rim (corona) between
plagioclase + another mineral
(6.25 mm, PPL) ERTH 2002 Lab 4
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-1:
(6.25 mm, PPL)
protolith:
bulk composition: = metabasite
mineral assemblage:
texture:
MR-5:
(6.25 mm, PPL)
LAB MR-1: METABASITES
Classification/nomenclature of MR-1 and MR-5 based on:
MR-1:
protolith:
bulk composition:
(6.25 mm, PPL)
????
greenschist
MR-5:
(6.25 mm, PPL)
metagabbro
metagabbro
(=metabasite)
mineral assemblage: greenschist
texture:
greenschist
gnt-px metagabbro
coronitic metagabbro
LAB MR-1: METABASITES
Classification/nomenclature of MR-1 and MR-5 based on:
MR-1:
protolith:
bulk composition:
(6.25 mm, PPL)
????
greenschist
MR-5:
(6.25 mm, PPL)
metagabbro
metagabbro
(=metabasite)
mineral assemblage: greenschist
texture:
greenschist
gnt-px metagabbro
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