Download LECTURE 09 -

Introduction to Igneous Rocks
IN THIS LECTURE
– The basis of classification
– Major versus trace versus REE
– Classification of Igneous Rocks
• Plutonic
• Volcanic
– Other Classification Schemes
Classification of Igneous Rocks
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Why is it necessary to have a system for the classification of igneous rocks?
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Lots of different names used lots of difference people
A classification scheme is therefore useful so that everyone knows if they are
talking about the same rock.
The Basis of Classification
Major versus Trace versus REE
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Mineral compositions normally only specify the wt % oxide of the major
elements, these being, Si, Ti, Al, Fe, Mg, Ca, Na, K, P and sometimes Cr,
Ni, and S.
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For a whole rock composition, major elements (same of minerals) as well
as trace elements and REE’s are often reported and can be used in the
classification of igneous rocks and discrimination diagrams.
Classification of Igneous Rocks
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The primary classification of igneous rocks should be based on their
mineral content or mode using the QAPF diagram
– Q = modal volume proportion of quartz, tridymite, cristobalite
– A = modal volume proportion of alkali feldspar, including orthoclase,
microcline, perthite, anorthoclase, sanidine, and albitic plagioclase
(An0 to An5)
– P = Plagioclase (An5 to An100) and scapolite
– F = feldspathoids or foids including nepheline, leucite, kalsilite,
analcime, sodalite, nosean, haüyne, cancrinite, and pseudoleucite.
– This diagram should not be used in rocks that have M > 90% where
M = mafic and related minerals e.g. mica, amphibole, pyroxene,
olivine, opaque minerals, accessory minerals (e.g. zircon, apatite,
titanite, monazite), epidote, allanite, garnet, melilite, monticellite,
and primary carbonate.
Classification of Igneous Rocks
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Plutonic Rock – an igneous rock with a phaneritic texture, i.e. a
relatively coarse-grained (>3mm) rock in which individual minerals can be
distinguished with the naked eye.
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Volcanic Rock – an igneous rock with an aphanitic texture, i.e. a
relatively fine-grained (<1mm) rock in which most of the individual
minerals cannot be distinguished with the naked eye.
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Therefore QAPF diagrams are useful for classifying plutonic rocks but
not so much for volcanic rocks
Plutonic rocks M<90%
Volcanic Rocks M<90%
IUGS Classification of Gabbroic Rocks
IUGS Classification of Ultramafic Rocks
Geochemical Classification Volcanic Rocks
Chemical Clasification of Volcanic Rocks
using TAS (total Alkali-Silica Diagram)
after le Bas et al., 1986
Other Classification Groups
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Other classification groups are:
1. Pyroclastic rocks and tephra
• Formed by fragmentation as a result of explosive volcanic
eruptions or processes
2. Carbonatites
• Rocks that contain more than 50 % modal carbonate and have a
plutonic or volcanic origin
3. Melilite-bearing rocks
• Rocks that contain more than 10% modal melilite and if
feldspathoids are present melilite > feldspathoid
4. Kimberlites
• Group I and Group II
5. Lamproites
– Based on mineralogical and geochemical criteria
Other Classification Groups
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Other classification groups are:
1. Leucite-bearing rocks
• Rocks containing little or no feldspar
2. Lamprophyres
• Several characteristic criteria
3. Charnockitic rocks
• Characterised by the presence of orthopyroxene (or fayalite +
quartz) and in many rocks perthite, mesoperthite or
antiperthite.