Petrol. 186-212B; Francis, 2014
PETROLOGY LAB 4: Plutonic Igneous Rocks
Plutonic rocks occur in intrusions that have crystallized within the Earth’s crust or at the crustmantle boundary. They are commonly recognized by their coarser grain-size in comparison to
volcanic rocks, indicating crystallization at slower cooling rates. Plutonic rocks are classified
according to their modal mineralogy:
Figure 4.1
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Petrol. 186-212B; Francis, 2014
Station A - Ultramafic Rocks:
The samples at this station are ultramafic rocks, that is they contain over 85 % mafic minerals, and
feldspar is either absent or so low in abundance that it can not be seen in hand specimen. If present,
feldspar is always an interstitial mineral.
Most ultramafic plutonic rocks are at least in part cumulates. They do not represent silicate melts,
but rather the accumulation of crystals that have crystallized from a silicate melt in a magma
chamber, along the walls of a dyke, etc.
Cumulate rocks are generally characterized by their low number of prominent mineral species (1-3)
and the common presence of layering in terms of variations in modal mineralogy and sometimes
grain-size.
Layering in an olivine and chromite cumulate
The cumulus minerals represent early crystals that accumulated from a magma to form the
framework of the rock and are typically sub-equant in habit.
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Petrol. 186-212B; Francis, 2014
Inter-cumulus minerals crystallize from the residual
trapped liquid between the cumulus crystals. They
are typically anhedral and occur as either small
crystals interstitial to the larger cumulus minerals, or
as large oikocrysts that poikilitically enclose smaller
chadocrysts of earlier cumulus minerals. Some
cumulate rocks can be recognized in hand specimen
by the "flash" of large oikocryst cleavage surfaces
containing numerous chadocryst inclusions when
the sample is rotated in the light.
Olivine chadocrysts in an orthopyroxene oikocryst:
Some ultramafic rocks, however, are restites; representing the residual solid left behind after the
extraction of partial melts. They commonly display recrystallized granular textures, and lack
oikocrysts. They commonly display tectonic textures such as mineral lineations
For each rock, identify as many minerals as you can and classify it according to Fig. 4-2. In
addition, try to distinguish cumulus minerals from inter-cumulus minerals, and thus the order in
which the minerals in the rock crystallised.

Orthopyroxene can be recognised by its brownish green to brownish-black colour and
prominent woody parting (100).

Clinopyroxene is typically green to greenish black, and characterised by a prominent pearly
smooth parting (001).

Olivine is typically present as equant grains, but its colour can be tricky because it is very
susceptible to alteration. When fresh it is light glassy green in colour. If partially altered to
serpentine, it is typically black because of the presence of fine-grained magnetite. If oxidised,
especially on weathered surfaces, it is commonly reddish brown in colour.
Don’t be discouraged if you can’t always identify the minerals. Olivine-rich rocks in which
clinopyroxene and/or orthopyroxene cannot be distinguished are best called by the more general
name peridotite. In the field, especially if the rock is altered, often all you can say is that the rock is
ultramafic.
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Petrol. 186-212B; Francis, 2014
Station B - Mafic Rocks:
All the samples at this station are mafic or ‘gabbroic’ rocks. They contain more than 15%, but less
than 60% feldspar. Determine the mineralogy of each of these rocks and classify them according to
Figure 4-3. Where possible, distinguish cumulate minerals from intercumulus minerals and try to
estimate the order in which the minerals crystallised.
If hornblende is present in moderate amounts, then the prefix hormblende is used: hornblende
gabbro, hornblende peridotite, etc. Mafic igneous rocks with over 60% hornblende are termed
hornblendites The term amphibolite is reserved for amphibole-rich metamorphic rocks.
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Petrol. 186-212B; Francis, 2014
Station C - Intermediate to Felsic Plutonic Rocks:
At this station you will see plutonic rocks with intermediate to felsic whole-rock compositions:
diorite, granodiorite, tonalite, monzonite, granite, syenite, and nepheline syenite. Determine
their mineral assemblages and use Figure 4-4 to classify the specimens. In which rock types are
hornblende or biotite the predominant mafic minerals? Can you see differences in grain size?
What can you say about the cooling rate of the samples?
The mineralogy of felsic igneous rocks is strongly dependent on silica activity. In silica-saturated
rocks, quartz is relatively abundant, whereas in silica-undersaturated rocks quartz does not
occur, but feldspathoids are present. Near the silica saturation boundary, syenites typically contain
either small quantities of quartz or feldspathoids (but never both), which commonly cannot be
detected in hand specimen. The rocks at this station are either silica-saturated granites, silicaundersaturated nepheline syenites or syenites. Classify the specimens according to Figure 4-4 by
identify the mineral assemblage present in each specimen.
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Petrol. 186-212B; Francis, 2014
Station D – Hypersolvus versus Subsolvus Granites
Water pressure (PH2O) is an important
variable in the crystallization of plutonic
rocks and two types of granites with
texturally different feldspars can be
distinguished depending on the water
pressure at which the rock crystallized. In
subsolvus granites (water pressures greater 
5 kb) two distinct types of feldspar are
present, which may have undergone
subsequent exsolution.
In hypersolvus
granites crystallization at relatively low
water pressures ( 4kb) results in the
formation of a single feldspar that may
become perthitic at subsolidus temperatures.
Note that both subsolvus and hypersolvus
granites require high Na/Ca ratios. Identify
the feldspars in the specimens at this station
and determine whether these rocks are
subsolvus or hypersolvus granites.
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Petrol. 186-212B; Francis, 2014
Station E – Granites by Source Type:
The tectonic environment of granite formation can be determined by analyzing the
composition of the granitic rocks.
S-type granites are generated during partial melting processes (anatexis) in the continental
crust. Their precursor rocks are aluminous metasediments and therefore S-type granites are
typically rich in muscovite and may contain alumino-silicates (kyanite, sillimanite), and/or
garnet. Biotite may also be present. S-type granites are typically peraluminous in
composition.
I-type granites originate from mantle-derived magmas or due to partial melting of mafic
igneous rocks. They typically contain hornblende and/or biotite as mafic minerals.
A-type granites (anorogenic) are relatively rare and not associated with regional
metamorphism or convergent plate tectonics. Their protolith is granulitic lower crust and
they are generated in an extensional within-plate tectonic regime. A-type granites are
typically peralkaline in composition, containing sodic amphibole or pyroxene as their
accessory mafic mineral, and typically do not contain mica.
The samples at this station represent a mix of S and I type granites. See if you can
distinguish them on the basis of the mineralogy.
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Petrol. 186-212B; Francis, 2014
Characterisitics of Granitoid Suites
Feature
Rock Types :
Mafic
Minerals:
A -Type
I - Type
S - Type
granite
tonalite to
granite
granodiorite
to granite
hornblende
biotite
muscovite
biotite
Na-amphibole
Na-pyroxene
Accessories:
fluorite
topaz
tourmaline
Oxide:
ilmenite
magnetite
ilmenite
Feldspar :
perthite
hypersolvus
K-spar
and plag
subsolvus
K-spar
and plag
subsolvus
(Na+K)/Al:
garnet
Al-silicates
cordierite
corundum
>1
per-alkaline
<1
<< 1
(Na+K+Ca×2)/Al:
>> 1
>1
meta-aluminous
<1
per-aluminous
Restite/
Enclaves:
few
amphibolite
biotite-cordierite
sillimanite-garnet
gneiss
Source
Regions:
granulitic
lower crust
with halogen
volatile flux
mafic igneous
rocks or
mantle-derived
magmas (M)
sedimentary
rocks
Tectonic
Setting:
extensional
within-plate
compressional
volcanic arcs,
extensional
interpolate
compressional
continental
collisions
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Station F: Porphyry Dykes
High level dykes and sills are typically finegrained and commonly difficult to distinguish
from the massive portions of thick flows without
their field context. If contacts are exposed, sills
can be distinguished from flows by the presence
of symmetric finer-grained quenched margins,
as opposed to the thick upper quench and thin
lower quench zones of flows. Cross cutting
field relationships are indicative of dykes.
High-level dykes are commonly porphyritic with
larger phenocrysts of crystals that grew before
emplacement enclosed in a finer grained matrix
that reflects rapid cooling at the shallow site of
emplacement.
Felsic Dykes:
Quartz feldspar porphyry rhyolite dykes (QFP dykes) are especially important as they are
commonly associated with porphyry copper and gold mineralization in volcanic terranes.
Without field context, it may be difficult to decide if a quartz-feldspar-phyric felsic rock is a
dyke or the massive portion of a lava flow. Often it is best to indicate the phenocryst
assemblage in a prefix, and simply call the rock a felsite, eg feldspar-phyric felsite. The
presence of quartz phenocrysts is indicative of a rhyolitic composition.
Mafic to Intermediate Dykes:
Fine-grained mafic dykes that are aphyric or contain phenocrysts of feldspar are commonly
simply referred to as basaltic or diabase dykes, or even more generally as mafic or intermediate
dykes. If the phenocyrst assemblage does not contain plagioclase, but does contain amphibole,
mica, and/or clinopyroxene and olivine, then mafic dykes are likely to be lamprophyres.
Lamprophyre dykes commonly have compositions similar to alkaline basalts. If feldspar is
absent in the matrix, then the term ultramafic lamprophyre is used. In the field, lamprophyre
dykes are best named using a prefix indicating the types of phenocrysts present, eg. hornblendephyric lamprophyre dyke. The presence of hydrous phenocrysts such as hornblende or
phlogopite is diagnostic of lamprophyres and the fact that the rock was a high level intrusion
rather than a volcanic flow. Water escapes in mafic lavas at the surface, and thus amphibole is
typically not stable.
The phenocrysts of thin porphyritic mafic dykes are commonly concentrated towards the centre
of the dyke by flow differentiation.
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Petrol. 186-212B; Francis, 2014
Kimberlite:
Kimberlite is an ultramafic rock (no feldspar) that is commonly rich in olivine megacrysts.
There are many types of kimberlite, ranging from olivine-rich dykes with dark fine-grained
matices (hypabyssal facies), which resemble ultramafic lamprophyres in hand specimen, to
polymictic beccias that contain fragments of both the mantle (olivine-rich xenoliths) and the
crustal rocks they intrude (diatreme facies). Kimberlite dykes are best distinguished from
lamprophrye dykes by the presence of purple-red garnet and brown phlogopite megacrysts.
Kimberlite dykes are rare, but their importance comes from the fact that they are the host
rocks for diamonds.
pyropic garnet
& phlogopite
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