Francis, Intro Petrol. 186-212, 2014
PETROLOGY LAB 3: Volcanic Rocks
The identification and classification of volcanic rocks is a two fold process that requires
the:
1.
determination the type of cooling unit
2.
estimation of the chemical composition
eg. basaltic lava flow, rhyolite lapilli tuff
1. Determination of the type of cooling unit, eg.; lava flow, tuff, pillow lava, etc.
This is best done in the field where a given rock can be seen in the context of
its surroundings. The diagram below presents a brief summary of the
common types of volcanic units. You will see a variety of these in a
Powerpoint presentation at the beginning of the lab.
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2. Classification according to chemical composition.
Because of the fine grain-size of volcanic rocks, a full classification requires a chemical analysis.
The official nomenclature of volcanic rocks is defined in a plot to total alkalis versus silica:
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In the field, one must rely simply on the look of hand specimens, using critieria such as colour and
hardness of the matrix, phenocryst mineral assemblage, etc. This is a somewhat qualitative
process and in some cases it is best to just use the terms mafic for darkest coloured volcanic rock,
felsic for lightest coloured volcanic rocks, and intermediate for those in between.
Hi temperature
Hi Mg/(Mg+Fe)
Hi Ca/ (Ca + Na)
Low temperature
Low Mg/(Mg+Fe)
Low Ca/ (Ca +
Na)
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Francis, Intro Petrol. 186-212, 2014
Hand Specimen Criteria for Estimating Composition:
Mafic
Phenocrysts:
Crystalline Matrix Colour:
Weathered Colour:
Matrix Hardness:
Matrix Fracture:
Flow Banding:
Felsic
Oliv, Cpx, Opx Ca-Plag
dark grey to black
red to brown
scratches, gritty
irregular
no
K-Spar, Qtz, Feldspathoid
light
white
harder than knife
concoidal
yes
In mafic rocks, the presence of olivine-rich xenoliths, or mica and/or amphibole phenocrysts indicates an
alkaline composition.
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Station A: Ultramafic Volcanic Rocks
Ultramafic volcanic rocks are relatively rare and essentially restricted to Archean greenstone belts.
Komatiites are ultramafic lava flows characterised by the presence of quench-textured olivine
known as ’spinifex’ that has a platy habit. Spinifex texture is best developed in the upper chilled
margin of komatiite lava flows. Komatiites are typically somewhat lighter green in colour than
associated mafic volcanic rocks such as basalts because of the formers high Mg content, and they
were often confused with andesites by early field workers because of their light green colour.
They commonly weather to a brown colour because of the high abundance of olivine.
Olivine
Spinifex
Olivine
Spinifex
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Station B: Mafic to Intermediate Volcanic Rocks
Mafic or basaltic volcanic rocks are characterised by their dark coloured matrix and the
common presence of mafic phenocrysts. The most mafic or primitive samples are characterised
by abundant olivine (picritic basalt) or olivine and clinopyroxene phenocrysts (ankaramitic
basalt), while more evolved basalts tend to have larger proportions of plagioclase than mafic
phenocrysts or be aphyric. Alkaline basalts are typically darker in colour than associated
subalkaline basalts and, unlike the later, commonly crystallize clinopyroxene phenocyrsts before
plagioclase. The presence of olivine-rich spinel lherzolite xenoliths is diagnostic of alkaline
basalts.
Olivine-phyric basalt or picrite
Olivine-pyroxene-plagioclase phyric basalt
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Intermediate volcanic rocks such as andesites, hawaiites, and dacites are commonly, but not
always, characterised by lighter coloured matrices than basalts and a dominance of feldspar as a
phenocryst phase. Olivine is commonly absent. In the most evolved examples, amphibole may
appear as a mafic phenocryst phase.
Plagioclase-phyric basaltic
andesite to andesite
Plagioclase – hornblende
-phyric andesite
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Hornblende-plagioclase-oxide
- phyric dacite
Hornblende-plagioclaseoxide- phyric dacite
Identify the phenocrysts in each of the specimens at this station and classify the rock as basalt,
andesite, dacite etc, eg. olivine-phyric basalt or amphibole plagioclase-phyric andesite. Think
about the probable type of cooling unit the sample represents, and in the case of flows, where in
the flow the sample was taken (eg. flow top, flow interior, flow base, remember last week’s lab on
flow textures).
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Station C: Felsic Volcanics – Lava Flows
Felsic volcanic rocks commonly have the lightest coloured matrices, if they are holocrystalline.
However, silica-rich magmas will often quench to a glass that is dark in colour and similar in
appearance to basaltic glass, unless examined through a thin chip, in which case basaltic glass has a
darker brown colour. It is unusual, however, for an entire sample of basalt to be glassy because it is
harder to quench silica-poor mafic magmas. In addition, glassy felsic lavas frequently exhibit
spherulites, small spherical bodies composed of radiating felsic crystals that are though to have grown
by the devitrification of glass.
spherulites
Extremely fine-grained varieties of felsic lavas will also be relatively dark in colour and easily confused
with more mafic lavas in the field. The presence of concoidal fracture in a extremely fine-grained dark
matrix that cannot be scratched with a knife is typically diagnostic of a felsic composition. No matter
how dark the fresh surface, the weathering surface of felsic rocks will typically be light coloured
compared to that of mafic to intermediate rocks.
obsidian
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Felsic volcanic flows are commonly flow banded. Flow banding is defined by alternating layers
of glass and finely crystalline material, sometimes coalescing spherulites, which define the slip
planes along which the viscous felsic magma flowed. The crystalline layers are thought to be those
along which volatile bubbles have concentrated during lamellar flow. These volatiles aided the
crystallisation of the magma in these layers, which tend to be lighter coloured than the darker
layers that are depleted in such bubbles and quenched to glass. This banding is frequently
preserved even when the felsic volcanic rock has been completely recrystallized. This type of
banding is not observed in mafic to intermediate volcanic rocks. Furthermore, the margins of
felsic flows are always fragmented, and their brecciated margins constitute a larger proportion
(thicker) of the flow than in the case of mafic aa flows. Ropy or pahoehoe tops are not observed
in felsic lavas.
Flow-banded rhyolite
Although glassy and crypto-crystalline felsic volcanic rocks are relatively dark in colour, most
old felsic lavas have recrystallized, and thus are light in colour reflecting the fact that their
compositions are rich in felsic minerals (E8733, E8969, 2.7 Ga).
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Station D: Felsic Fragmental Volcanics – Pyroclastic rocks
The majority of felsic volcanic rocks are not lava flows, but pyroclastic rocks produced by the
accumulation of clasts or fragments produced in an explosive volcanic eruption. Felsic magmas
commonly erupt explosively because their Si-rich compositions make them so viscous that
exsolving volatiles can not escape. Because of the combination of high melt viscosity and high
gas contents, the majority of felsic magmas erupt as pyroclastic deposits than rather than
effusive lava flows. Mafic magmas can also produce fragmental volcanic rocks during
explosive eruptions, but typically erupt as more effusive lava flows. Some petrologists view
pyroclastic rocks as sedimentary rather than igneous rocks.
This station contains a wide variety of fragmental volcanic rocks. The classification of
fragmental rocks requires the consideration of a number of features: grain size, matrix versus
clast support, degree of sorting, presence of layering or grading, monomictic versus polymictic
character, signs of aerial or hot emplacement. Examine each specimen in terms of these
features and classify them according to the first figure in this lab.
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Air-fall tuffs are deposited relatively cold and may
or may not be layered. Water-lain tuffs are more
typically layered and exhibit better sorting than air
fall tuffs. Air fall tuffs are commonly reworked by
wind and water, showing structures such as finescale cross-bedding. Such rocks are technically
sedimentary, rather than volcanic, and they are
sometimes referred to as epiclastic as opposed to
pyroclastic rocks. If you don’t know which a rock
is, it is safer to use the term fragmental volcanic or
volcanoclastic rock can be used – a clastic rock
made up of fragments of volcanic rock and/or
broken crystals.
Some of the samples in this section are welded tuffs or ignimbrites. They are tuffs that have between
deposited by pyroclastic flows. They are characteristically poorly sorted and lack well defined layering,
although they commonly exhibit
grading. In some cases, they are hot
enough that the fragments are plastic
and welded together after emplacement.
Signs of hot deposition include
flattened pumice or glass fragments
known as fiamme, columnar joints,
alteration rims around larger fragments,
and the presence of a hard glassy matrix.
Strongly welded rocks are typically
physically hard to break and when they
do, they tend to break across fragments.
Many ash flow tuffs are not welded,
however, and are recognized on the
fiamme
basis of lack of layering and/or poor
sorting. In some cases, the interior of ash flows are welded, while the margins are not, or ash flows are
welded near their source, but not welded far from their source.
Pyroclastic Surge deposits are distinguished from pyroclastic flow deposits by the presence of well
developed planar to wavy layering and low-angle, large-scale antidune cross-bedding. They are
typically thinner than pyroclastic flow deposits.
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Station E: 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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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 garnet and phlogopite megacrysts. Kimberlite dykes are
rare, but their importance lies in the fact that they are the host rocks for diamonds.
Pyropic garnet
& phlogopite
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