Geoscience Reference
In-Depth Information
Table 9.1 Key minerals in common metamorphic rock compositions: L, low; M, medium; H, high; P, pressure;
T, temperature.
Pressure/
temperature
conditions
Pelitic
(mudstone)
Mafi c
(basalt)
Felsic
(granite)
Ultramafi c
(peridotite)
Calc-silicate
(impure
limestone)
LT
Chlorite
Chlorite
Chlorite,
epidote
Serpentine
Talc
9
LP
Andalusite,
cordierite
No garnet
Pyroxenes, olivine
No garnet
Andalusite
MP/T
Chloritoid,
staurolite
Actinolite, epidote,
zoisite
Talc
(abundant)
Tremolite
HP
Kyanite, talc,
rutile
No plagioclase
Lawsonite,
Na-pyroxene,
rutile, glaucophane
No plagioclase
Na-pyroxene,
kyanite
No plagioclase
Zoisite
HT
Sillimanite,
spinel,
orthopyroxene
No muscovite
Clinopyroxene,
orthopyroxene
Orthopyroxene,
cordierite,
sillimanite
Orthopyroxene Wollastonite,
Mg-olivine,
Ca-pyroxene,
spinel
Wide P and T
ranges
Muscovite,
biotite, garnet,
quartz,
plagioclase
Garnet,
hornblende,
plagioclase,
biotite, quartz,
titanite
Quartz, biotite,
K-feldspar,
plagioclase,
muscovite
Olivine,
chlorite,
magnesite
Calcite, dolomite,
plagioclase,
Ca-garnet,
hornblende,
chlorite, epidote
schist or 'andalusite hornfels'. Another convention, adding the
prefi x 'meta-' to a protolith name (e.g. metagabbro, meta-
limestone), requires interpretation that may have to be revised
following laboratory analysis. Any interpretation or inference
on composition or protolith should be clearly noted, and
justifi ed by supporting evidence (e.g. preserved igneous
textures; colour; mineralogy). For fi ne-grained rocks where
mineral identifi cation is impossible in the fi eld, use textural
names with other descriptive terms (e.g. purple slate, kinked
grey phyllite, spotted hornfels, banded mylonite).
9.4 Unravelling metamorphism and
deformation
As metamorphism is commonly closely related to deformation,
many features in metamorphic rocks can be examined for clues
to the relative timing of metamorphism and strain. The
following sections highlight common examples of pre-, syn-
and post-kinematic features that can be observed in the fi eld.
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