Description and Classification contd

1
Metamorphic Rocks 3

• Description and Classification (contd)
• Introduction to Lab 6
• Nesse Ch. 11, p. 194-200
• lab class handouts

2

• 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)

3
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
4
LAB 6 OTHER

• protolith
• quartzite
• (arenaceous sandstone)
• carbonate
• (limestone or dolostone)
• ultramafic rock
• (peridotite or pyroxenite)
• granitoid rock
• (diorite, granodiorite, or granite)

b) bulk composition silicic calcareous ult
ramafic quartzo-feldspathic
5
LAB 6 OTHER

• protolith
• quartzite
• (arenaceous sandstone)
• carbonate
• (limestone or dolostone)
• ultramafic rock
• (peridotite or pyroxenite)
• granitoid rock
• (diorite, granodiorite, or granite)

c) minerals quartz /- feldspar,
mica, chlorite, epidote, garnet, Fe-Ti oxides,
carbonate calcite /- dolomite /- amphibole,
mica, olivine, clinopyroxene, wollastonite,
clinohumite, talc, chlorite, serpentine,
titanite, plagioclase olivine /- serpentine /-
amphibole, mica, spinel, talc, pyroxene,
magnetite, chlorite quartz feldspar mica /-
amphibole, pyroxene, garnet, chlorite, Fe-Ti
oxides, titanite
6
LAB 6 OTHER

• protolith
• quartzite
• (arenaceous sandstone)
• carbonate
• (limestone or dolostone)
• ultramafic rock
• (peridotite or pyroxenite)
• granitoid rock
• (diorite, granodiorite, or granite)

d) textures (highly variable!) massive to
foliated massive to foliated compositional
banding massive to foliated foliated, banded,
gneissic, migmatitic
7
LAB 6 OTHER

• protolith
• quartzite
• (arenaceous sandstone)
• carbonate
• (limestone or dolostone)
• ultramafic rock
• (peridotite or pyroxenite)
• granitoid rock
• (diorite, granodiorite, or granite)

e) special names metaquartzite marble,
calc-silicate (see Fig. C1) metaperidotite,
serpentinite, metapyroxenite, hornblendite metag
ranitoid, orthogneiss, migmatite
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LAB 6 OTHER
e) special names marble, calc-silicate (see
Fig. C1) marbles contain gt 50 calcite and/or
dolomite calc-silicates contain mainly
Ca-bearing silicate minerals orthogneiss
gneissic rock with plutonic protolith (typically
granitoid) paragneiss gneissic rock
with sedimentary or volcanic protolith (not in
this lab) migmatite gneiss or schist
with irregular or cross-cutting light- coloured
layers (leucosome)

• protolith
• carbonate
• (limestone or dolostone)
• granitoid rock
• (diorite, granodiorite, or granite)

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LAB 6 OTHER
possible mineral identification problems in this
lab 1. calcite vs dolomite note twinning vs
rhombic cleavage 2. Mg-rich
varieties of Fe-Mg minerals that are normally
strongly coloured and/or pleochroic (in Mg-rich
rocks) since Fe is generally the main
chromophore, these varieties may be faintly
coloured (if at all) and display weak pleochroism
(if any) e.g., hornblende, biotite, chlorite
calcite twins // long diagonal or rhomb
face edges
dolomite twins // short or long
diagonal
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LAB 6 OTHER
mineral assemblage (reflects bulk composition
and grade)
Given the variety of bulk compositions
represented in this lab, there will be no attempt
to relate specific mineral assemblages to
specific metamorphic grades. However, please note
the presence in some samples of pseudomorphs of
lower-grade minerals (e.g., chlorite,
serpentine) after higher-grade minerals
(e.g.,olivine, biotite).
11
LAB 6 OTHER
mineral assemblage (reflects bulk composition
and grade)
Given the variety of bulk compositions
represented in this lab, there will be no attempt
to relate specific mineral assemblages to
specific metamorphic grades. However, please note
the presence in some samples of pseudomorphs of
lower-grade minerals (e.g., chlorite,
serpentine) after higher-grade minerals
(e.g.,olivine, biotite).
this type of replacement texture typically forms
during retrograde metamorphism
(T-decreasing) peak T mineral assemblages formed
during prograde metamorphism (T-increasing) may
no longer be stable but can survive because
there is not enough H2O in the system to
completely rehydrate the rock
dehydration
peak
P
prograde
retrograde
rehydration
T
12
LAB 6 OTHER

• protolith
• quartzite
• (arenaceous sandstone)

13
LAB 6 OTHER

• protolith
• carbonate
• (limestone or dolostone)

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LAB 6 OTHER

• protolith
• ultramafic rock
• (peridotite or pyroxenite)

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LAB 6 OTHER

• protolith
• granitoid rock
• (diorite, granodiorite, or granite)