Description and Classification contd

1
Metamorphic Rocks 2

• Description and Classification (contd)
• Introduction to Lab 5
• 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
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LAB 5 PELITES
a) protolith - fine-grained sedimentary rocks
(mudstones, shales, siltstones) /- small
proportion of interlayered coarser-grained
sedimentary material (sandstone) b) bulk
composition ( pelite) - relatively rich in Si,
Al, K (protolith mainly clay quartz) -
relatively poor in Ca, Mg c) mineral
assemblage - mica (mus /- bt) quartz
ubiquitous - plagioclase, chlorite, garnet,
Al2SiO5, staurolite, cordierite, K-feldspar,
graphite, ilmenite ...etc... (depending on
grade and specific bulk composition) d)
texture - planar fabrics generally well
developed (/- lineation) - sedimentary layering
may be preserved - porphyroblasts very common
5
LAB 5 PELITES
a) protolith - fine-grained sedimentary rocks
(mudstones, shales, siltstones) /- small
proportion of interlayered coarser-grained
sedimentary material (sandstone) b) bulk
composition ( pelite) - relatively rich in Si,
Al, K (protolith mainly clay quartz) -
relatively poor in Ca, Mg c) mineral
assemblage - mica (mus and/or bt) quartz
ubiquitous - plagioclase, chlorite, garnet,
Al2SiO5, staurolite, cordierite, K-feldspar,
graphite, ilmenite ...etc... (depending on
grade and specific bulk composition) d)
texture - planar fabrics generally well
developed (/- lineation) - sedimentary layering
may be preserved - porphyroblasts very common
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LAB 5 PELITES
c) mineral assemblage (reflects bulk
composition and grade) - mica (mus and/or bt)
quartz ubiquitous - plagioclase, chlorite,
garnet, Al2SiO5, staurolite, cordierite,
K-feldspar, graphite, ilmenite
...etc... (depending on grade and specific bulk
composition)
lower P,T
greenschist facies chlorite muscovite
quartz albite (An0-10) biotite garnet
amphibolite facies muscovite biotite
quartz plagioclase (An20-40) garnet
staurolite kyanite sillimanite granulite
facies biotite quartz K-feldspar
plagioclase (An30-50) sillimanite garnet
cordierite orthopyroxene
higher P,T
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LAB 5 PELITES
c) mineral assemblage (reflects bulk
composition and grade) - mica (mus and/or bt)
quartz ubiquitous - plagioclase, chlorite,
garnet, Al2SiO5, staurolite, cordierite,
K-feldspar, graphite, ilmenite
...etc... (depending on grade and specific bulk
composition)
can also refer to metamorphic grade of pelites in
terms of index minerals that appear
sequentially as grade increases chlorite biotite
garnet staurolite kyanite sillimanite
lower P,T
greenschist facies amphibolite
facies granulite facies
higher P,T
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LAB 5 PELITES
a) protolith - fine-grained sedimentary rocks
(mudstones, shales, siltstones) /- small
proportion of interlayered coarser-grained
sedimentary material (sandstone) b) bulk
composition ( pelite) - relatively rich in Si,
Al, K (protolith mainly clay quartz) -
relatively poor in Ca, Mg c) mineral
assemblage - mica (mus /- bt) quartz
ubiquitous - plagioclase, chlorite, garnet,
Al2SiO5, staurolite, cordierite, K-feldspar,
graphite, ilmenite ...etc... (depending on
grade and specific bulk composition) d)
texture - planar fabrics generally well
developed (/- lineation) - sedimentary layering
may be preserved - porphyroblasts very common
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LAB 5 PELITES
d) texture - planar fabrics generally well
developed (/- lineation) - sedimentary layering
may be preserved - porphyroblasts very common
types of planar fabrics in pelites -
sedimentary layering (relict, inherited from
protolith) - parallel alignment of sheet
silicates slaty cleavage (fine-grained, rock
splits easily) schistosity (medium-grained,
rock splits less easily) micro-scale
folds common crenulations - compositional
banding (other than sedimentary
layering) formed by deformation (of some
pre-existing feature) formed by injection of
veins formed by metamorphic segregation
(chemical migration) formed by in situ melting
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LAB 5 PELITES
types of planar fabrics in pelites -
sedimentary layering (relict, inherited from
protolith)
MR-6 relict sedimentary layering in slate (PPL,
6.25 mm)
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LAB 5 PELITES
types of planar fabrics in pelites -
sedimentary layering (relict, inherited from
protolith)
slate
originally mudstone
relict bedding
originally siltstone
metasiltstone
MR-6 relict sedimentary layering in slate (PPL,
6.25 mm)
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LAB 5 PELITES
types of planar fabrics in pelites - parallel
alignment of sheet silicates slaty cleavage
(fine-grained, rock splits easily) schistosity
(medium-grained, rock splits less easily)
micro-scale folds common crenulations
MR-7 schistosity with crenulations (PPL, 6.25 mm)
MR-6 slaty cleavage cutting relict bedding (PPL,
2.5 mm)
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LAB 5 PELITES
types of planar fabrics in pelites - parallel
alignment of sheet silicates slaty cleavage
(fine-grained, rock splits easily) schistosity
(medium-grained, rock splits less easily)
micro-scale folds common crenulations
crenulations
slaty cleavage
schistosity
MR-7 schistosity with crenulations (PPL, 6.25 mm)
MR-6 slaty cleavage cutting relict bedding (PPL,
2.5 mm)
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LAB 5 PELITES
types of planar fabrics in pelites -
compositional banding (other than sedimentary
layering) formed by deformation (of some
pre-existing feature) formed by injection of
veins formed by metamorphic segregation
(chemical migration) formed by in situ melting
MR-9 gneissic banding defined by biotite-rich
quartzfeldspar-rich layers (formed by
deformation melting) (PPL, 6.25 mm)
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LAB 5 PELITES
d) texture - planar fabrics generally well
developed (/- lineation) - sedimentary layering
may be preserved - porphyroblasts very common
porphyroblasts in pelites note the
following - mineral(s) (garnet? kyanite?
etc....) - shape (euhedral, subhedral,
anhedral) - size (lt 5 mm, gt 1 cm, etc......) -
abundance (rare, common, abundant, etc....) -
inclusions (if any) - what minerals? - what
kind of pattern (if any)?
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LAB 5 PELITES
porphyroblasts in pelites note the following
mineral(s) staurolite shape euhedral size up
to 1 cm abundance abundant inclusions very
abundant quartz, ilmenite, garnet,
biotite define internal fabric equivalent to
matrix fabric
MR-7 staurolite porphyroblast in pelitic schist
(PPL, 6 mm)
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LAB 5 PELITES
porphyroblasts in pelites note the following
mineral(s) staurolite shape euhedral size up
to 1 cm abundance abundant inclusions very
abundant quartz, ilmenite, garnet,
biotite define internal fabric equivalent to
matrix fabric
MR-7 inclusions in staurolite porphyroblast
(PPL, 2.5 mm)
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LAB 5 PELITES
porphyroblasts in pelites note the following
mineral(s) staurolite shape euhedral size up
to 1 cm abundance abundant inclusions very
abundant quartz, ilmenite, garnet,
biotite define internal fabric equivalent to
matrix fabric
MR-7 inclusions in staurolite porphyroblast (XN,
2.5 mm)
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LAB 5 PELITES
some special rock names that apply mainly to
pelites slate - fine-grained metasedimentary
rock with a strong cleavage, defined by
alignment of fine-grained platy
minerals (chlorite /- graphite /-
muscovite) phyllite fine- to medium- grained
metasedimentary rock with a strong schistosity,
defined by alignment of platy minerals (chlorite
/- muscovite /- biotite) schist medium- to
coarse-grained metasedimentary rock with a
strong schistosity, defined by alignment of
platy minerals (chlorite /- muscovite /-
biotite)
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LAB 5 PELITES
some special rock names that apply mainly to
pelites slate - fine-grained metasedimentary
rock with a strong cleavage, defined by
alignment of fine-grained platy
minerals (chlorite /- graphite /-
muscovite) phyllite fine- to medium- grained
metasedimentary rock with a strong schistosity,
defined by alignment of platy minerals (chlorite
/- muscovite /- biotite) schist medium- to
coarse-grained metasedimentary rock with a
strong schistosity, defined by alignment of
platy minerals (chlorite /- muscovite /-
biotite)
other names that may apply to some samples in
Lab 5 gneiss medium- to coarse-grained
metamorphic rock with well defined, regular,
cm-scale compositional layering defined by
variable light- and dark-coloured
minerals migmatite mixed rock with granitic
metamorphic components irregular or
cross-cutting granitic bands or veins in a
darker, somewhat finer-grained, well foliated
host
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LAB 5 PELITES
gneiss medium- to coarse-grained metamorphic
rock with well defined, regular, cm-scale
compositional layering defined by variable
light- and dark-coloured minerals migmatite
mixed rock with granitic metamorphic
components irregular or cross-cutting granitic
bands or veins (leucosome) in a darker, somewhat
finer-grained, well foliated host (mesosome)
MR-10 irregular banding formed by deformation
melting (PPL, 6.25 mm)
MR-9 regular gneissic banding formed by
deformation melting (PPL, 6.25 mm)
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LAB 5 PELITES
gneiss medium- to coarse-grained metamorphic
rock with well defined, regular, cm-scale
compositional layering defined by variable
light- and dark-coloured minerals migmatite
mixed rock with granitic metamorphic
components irregular or cross-cutting granitic
bands or veins (leucosome) in a darker, somewhat
finer-grained, well foliated host (mesosome)
these textures best observed in hand
sample commonly gradational (gneissic migmatite,
migmatitic gneiss)
MR-10 irregular banding formed by deformation
melting (PPL, 6.25 mm)
MR-9 regular gneissic banding formed by
deformation melting (PPL, 6.25 mm)
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LAB 5 PELITES
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