Intrusive rock

1. Igneous Chemistry
Felsic/acid = > 65% SiO
2

–lots of Na, K, Al but little Mg, Fe, Ca
–e.g. continental crust; granite
Intermediate = 53 - 65% SiO
2
–e.g. subduction zone volcanic rocks; andesite
Mafic/basic = 45 - 52% SiO
2

–lots of Mg, Fe, Ca but silica poor
–e.g. oceanic crust; basalt
Ultramafic = < 45% SiO
2
–e.g. mantle

2. Igneous Compositions
The type of rock that melts and/or the
composition of the parent magma
determines the chemistry of the rock
that crystallizes.
Other processes affect this too…
–Crystallization sequence of minerals
–Crystal settling
–Assimilation
–Magma mixing

3. Within the field of geology, Within the field of geology, Bowen's reaction seriesBowen's reaction series
is the work of the petrologist, Norman L. Bowen who is the work of the petrologist, Norman L. Bowen who
was able to explain why certain types of minerals was able to explain why certain types of minerals
tend to be found together while others are almost tend to be found together while others are almost
never associated with one another. He experimented never associated with one another. He experimented
in the early 1900s with powdered rock material that in the early 1900s with powdered rock material that
was heated until it melted and then allowed to cool to was heated until it melted and then allowed to cool to
a target temperature whereupon he observed the a target temperature whereupon he observed the
types of minerals that formed in the rocks produced. types of minerals that formed in the rocks produced.
Bowen’s Reaction Series

4. He repeated this process with He repeated this process with
progressively cooler temperatures and progressively cooler temperatures and
the results he obtained led him to the results he obtained led him to
formulate his reaction series which is formulate his reaction series which is
still accepted today as the idealized still accepted today as the idealized
progression of minerals produced by progression of minerals produced by
cooling magma.cooling magma.
Bowen’s Reaction Series

5. Bowen’s Reaction Series
Helps understand how you can get intermediate and
mafic rocks from mafic magmas.
When a melt cools, different minerals crystallize at
different temperatures!
Minerals that cool at nearly the same temperatures
tend to occur together in rocks. Only certain minerals
can occur together.
If you remove the minerals that have crystallized at a
certain temperature, you change the overall chemistry
of the system…

6. Bowen’s Reaction Series
C
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C
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D
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s
c
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t
i
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D
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7. Bowen’s Reaction Series
Discontinuous Series
Only ferromagnesian.
One mineral changes to
next as temperature
slowly drops.
Each change is a
chemical reaction
between solids
(crystals) and fluids
(melt, water, gas)
present at the time.
Reactions not always
complete.

8. Bowen’s Reaction Series
Continuous Series
Nonferromagnesian
plagioclase feldspar (Ca
to Na)
As magma cools, Ca-
rich plagioclase reacts
with melt and
proportionally more Na-
rich plagioclase
crystallizes
Continues until all Ca
and Na is used up.
Plagioclase is often,
therefore, zoned with Ca
rich core and Na rich
rims.

9. Bowen’s Reaction Series
Felsic Minerals
Not really part of
series.
As Mg, Fe, Ca, and
Na are used up, left
over magma has
more and more
SiO2, K, Al, water,
and other exotic
stuff (U).
Form K feldspars,
muscovite, quartz,
accessory
minerals.
Increasing
Si
content

10. Minerals of Low SilicationMinerals of Low Silication
If there is a considerably deficiency of silica in magma, elements If there is a considerably deficiency of silica in magma, elements
may not be able to combine with sufficient silica to form may not be able to combine with sufficient silica to form
minerals. This type of minerals are called minerals of low minerals. This type of minerals are called minerals of low
silication. Example: K and Na form Leucite and Nepheline silication. Example: K and Na form Leucite and Nepheline
instead of orthoclase and albite.instead of orthoclase and albite.
Minerals of High SilicationMinerals of High Silication
In magma, when there is an adequate amount of silica, elements In magma, when there is an adequate amount of silica, elements
form the minerals which are rich in silica contents. These form the minerals which are rich in silica contents. These
minerals are termed as minerals of high silication.minerals are termed as minerals of high silication.
Mineral of Low silication+silicaMineral of Low silication+silica Mineral of High silicationMineral of High silication

11. Minerals of Low Silication Minerals of High Silication
Leucite Orthoclase
Nepheline Albite
Analcite Anorthoclase
Olivine Orthorhombic pyroxene
Biotite Augite
Hornblende

12. Quartz(SiOQuartz(SiO
22) doesn’t exist with minerlas of low silication) doesn’t exist with minerlas of low silication
Any excess of silica which may be left over after the bases are Any excess of silica which may be left over after the bases are
fully satisfied crystallizes out as quartz. The inter relationship of fully satisfied crystallizes out as quartz. The inter relationship of
minerals of low and high silication shows that minerals of low minerals of low and high silication shows that minerals of low
silication and SiOsilication and SiO
22 combinedly forms minerals of high silication. combinedly forms minerals of high silication.
Hence, quartz can’t co-exists with minerals of low silication.Hence, quartz can’t co-exists with minerals of low silication.
NaAl(SiONaAl(SiO
44) + 2(SiO) + 2(SiO
22) = NaAl.Si) = NaAl.Si
33OO
88
Nepheline Quartz AlbiteNepheline Quartz Albite

13. Essential Igneous rock forming mineralsEssential Igneous rock forming minerals
Felsic/Silicic/Light colored Felsic/Silicic/Light colored
mineralsminerals
 QuartzQuartz
 Potash FeldsperPotash Feldsper
 Mica(muscovite)Mica(muscovite)
Mafic/Ultramafic/DMafic/Ultramafic/D
ark colored mineralsark colored minerals
 OlivineOlivine
 PyroxenePyroxene
 AmphibolesAmphiboles
 BiotiteBiotite
Plagioclase feldsparPlagioclase feldspar

14. Intrusive Igneous Rock Bodies
Magmas crystallized beneath the Earth's
surface form intrusive bodies of igneous rock
known as plutons.
The term pluton (after the Greek god Pluto)
refers to any igneous intrusion regardless of
size, shape or composition of the magma.
Classification of plutons is based on:
1.   Geometry of intrusion: size and shape
2.   Relationship to surrounding rocks:
–concordant or boundaries parallel to layering in surrounding rocks
–discordant or boundaries cut across layering in surrounding rocks

15. Intrusive Igneous Rock Bodies

16. Intrusive Igneous Rock Bodies

17. Concordant Igneous Bodies
Sill: A sill is a
concordant body, few
cm to >1 km thick,
produced when
magma is injected
between layers of
older sedimentary or
volcanic rock, and
are generally
composed of
intermediate to basic
composition magma.

19. Concordant Igneous Bodies
Laccolith: A laccolith
represents magma
that pushes overlying
rock layers upward to
form a condordant,
mushroom-shaped,
sill-like body,
typically comprising
magma of
intermediate to
granitic composition.
It causes folding of the overlying It causes folding of the overlying
rock layersrock layers
Flat bottom and convex upward Flat bottom and convex upward
dome shaped igneous bodydome shaped igneous body

20. Concordant Igneous Bodies
Lopolith: A lopolith is a
spoon-like shaped
concordant body
similar to a sill except
the floor and roof sag
downward. The
intrusions are generally
magma of intermediate
to basic composition.
These are basin or saucer shaped concordant bodies with top These are basin or saucer shaped concordant bodies with top
nearly flat and convex bottom.nearly flat and convex bottom.

21. PhacolithsPhacoliths
These are concordant These are concordant
bodies that occurs along the bodies that occurs along the
crests and troughs of the crests and troughs of the
folded sedimentary strata.folded sedimentary strata.
In folding, the crest and In folding, the crest and
trough are regions of trough are regions of
weakness and tension, weakness and tension,
magma often intruded into magma often intruded into
these trough and crest.these trough and crest.
Southern Patagonia Phacoliths, ChileSouthern Patagonia Phacoliths, Chile

22. Discordant Igneous Bodies
Dike:Dike: A dike is a discordant body, few A dike is a discordant body, few
cm to >100 m thick, produced when cm to >100 m thick, produced when
magma is injected along fractures in magma is injected along fractures in
surrounding rock layers. Dikes ftypically surrounding rock layers. Dikes ftypically
form from magmas of basic to granitic form from magmas of basic to granitic
composition. Ring and Radial dikes are composition. Ring and Radial dikes are
discordant bodies having either a discordant bodies having either a
concentric (circular) or radial concentric (circular) or radial
distribution; develop above a large distribution; develop above a large
subsurface intrusive body (batholith or subsurface intrusive body (batholith or
stock) or adjacent to volcanic pipes or stock) or adjacent to volcanic pipes or
necks. necks.

23. Discordant Igneous Bodies
Batholith:Batholith: A batholith is a discordant magma body with A batholith is a discordant magma body with
exposed surface area of more than 100 square kilometers; exposed surface area of more than 100 square kilometers;
typically consists of multiple intrusions. Batholith are typically consists of multiple intrusions. Batholith are
usually magma of granitic composition with minor usually magma of granitic composition with minor
intermediate varieties. intermediate varieties.
BatholithsBatholiths
are the largest bodies of are the largest bodies of
Igneous rock, irregular in Igneous rock, irregular in
shape and occupies large shape and occupies large
area. Their occurrence is area. Their occurrence is
commonly associated with commonly associated with
the mountain-building the mountain-building
process. process.

24. Discordant Igneous Bodies
Stock:Stock: A stock is a discordant magma body with exposed A stock is a discordant magma body with exposed
surface area of less than 100 square kilometers; may surface area of less than 100 square kilometers; may
represent exposed portion of a much larger intrusion. It is represent exposed portion of a much larger intrusion. It is
usually magma of granitic composition with minor intermediate usually magma of granitic composition with minor intermediate
varieties. varieties.
StocksStocks
Are irregular Are irregular
masses of masses of
batholiths habit of batholiths habit of
smaller dimension.smaller dimension.

25. Discordant Igneous Bodies
Volcanic  pipes  and Volcanic  pipes  and 
necks:necks: are discordant are discordant
bodies that represent bodies that represent
the upper part of the the upper part of the
conduit that connects conduit that connects
the volcanic vent the volcanic vent
(crater) with an (crater) with an
underlying magma underlying magma
source (magma source (magma
chamber or reservoir). chamber or reservoir).
Volcanic necks are Volcanic necks are
erosional remnants of erosional remnants of
magma that solidified in magma that solidified in
the pipe or conduit.the pipe or conduit.

26. Intrusive Igneous Rock Bodies