Primer on Igneous Rocks
Geol 4110
IGNEOUS ROCK - Composed of aggregates of minerals crystallized from molten rock ( magma). Major
distinctions in rock type are based on the chemical composition of the magma and the environment of
magma emplacement.
COMPOSITIONS: Magmas are complex mixtures of elemental oxides. Typically more than half of all
magmas are composed of SiO2 with the remainder being varible proportions of TiO2, Al2O3, FeO,
MnO, MgO, CaO, Na2O, K2O, P2O5 and other elements measured in the parts per million (called
trace elements). Although spanning a continuum of compositions, three main groups of composition
are generally recognized:
MAFIC – Remember “(partially) Melting the Mantle Makes Mafic Magma!” - Enriched in Mg,
Fe, Al, Ca; Crystallizes dark minerals - Ca-rich plagioclase, olivine, pyroxene, and iron oxide.
INTERMEDIATE – Formed by magma mixing in subduction zones - Enriched in Fe, Ti, P;
moderately in Si, Na, and K; Crystallizes medium colored minerals: Na-rich plagioclase,
pyroxene, amphibole, biotite, iron-titanium oxide, K feldspar, and some quartz.
FELSIC – Magma formed from partially melting crustal rocks - Enriched in Si, Na, K; Crystallizes
light colored minerals: Na plagioclase, K-feldspar, quartz, biotite, muscovite, iron oxide and
some amphibole
Intermediate to felsic magmas are commonly created by partially melting crustal rocks as well as by
magma differentiation (see below)
MAGMATIC ENVIRONMENTS: Two general types.
VOLCANIC (Extrusive) - Where magma (lava) is erupted on the earth's surface and cools quickly to
form finer grained minerals.
PLUTONIC (Intrusive) - Where magma is intruded at some depth into the earth's crust and cools
slowly to produce coarser grained minerals.
These two criteria define six general igneous rock types.
MAFIC
VOLCANIC |
PLUTONIC |
BASALT
DIABASE*
GABBRO
INTERMEDIATE
FELSIC
ANDESITE
RHYOLITE
DIORITE
GRANITE
*diabase is a term used to describe finer-grained gabbro that commonly occurs in small, shallow intrusions
IGNEOUS ROCK TEXTURES - The coarseness of an igneous rock generally relates to the rate at which it
cooled. Cooling rate controls the rate at which crystals nucleate (i.e., make "seeds") relative to the rate
at which they grow. This can be graphically explained in the figure below. At slow rates of cooling
(plutonic conditions), nucleation rate is slow relative to growth rates, thus resulting in fewer but larger
crystals. At fast rates of cooling (volcanic conditions), nucleation rate is fast relative to growth rates,
thus resulting in many small crystals. Some igneous rocks display two distinct grain sizes, called
pophyritic texture, which implies two different rates of cooling.
Porphyritic
Texture
Plutonic - - Volcanic
~5mm
Rate of Cooling
MAGMA TO ROCK - As magmas cool, the soup of elements that comprise them are distributed into
specific types of igneous minerals, each having distinct compositions. A particular magma
composition will thus crystallize a predictable mineral assemblage. The table below shows how
various elements in typical mafic and felsic magmas will crystallize mineral assemblages that
correspond to the rock types gabbro and granite, respectively.
MAFIC
MAGMA
Elements
Magma
=
Major Minerals
Composition Mineral
Silicon (SiO2)
48
40%
55%
Groups- Mg-Fe Silicate Feldspar
X
X
Aluminum (Al2O3) 14
(X)
14
X
Magnesium (MgO)
9
X
Calcium (CaO)
9
X
Sodium (Na2O)
2.5
X
Potassium (K2O)
0.5
(X)
Titanium (TiO2)
2.0
FELSIC
Olivine
Pyroxene
5%
Oxide
X
X
(X)
Mineral
Names-
Rock
X
Iron (FeO)
Volatiles(H2O,CO2) <1.0
TOTAL
100.0
=
X
Plagioclase
Magnetite
Ilmenite
=
GABBRO/
BASALT
MAGMA
Elements
Magma
=
Composition
10%
Silicon (SiO2)
68
Aluminum (Al2O3) 11
Iron (FeO)
5
Major Minerals
Mg-Fe Sil.
X
65%
Feldspar
X
X
X
X
(X)
Calcium (CaO)
2
(X)
X
Sodium (Na2O)
6
X
X
Potassium (K2O)
4
X
X
Titanium (TiO2)
0.5
(X)
Amphibole Plagioclase
Biotite
K-Feldspar
(X) - occurs in minor amounts (< 2%)
Rock
20%
Silica Mineral
X
X
Magnesium (MgO) 0.5
Volatiles(H2O,CO2) 3.0
TOTAL
100.0
<5%
Oxide
=
(X)
Magnetite
Quartz
=
GRANITE/
RHYOLITE
Magmatic processes creating compositional diversity in igneous rocks
• Partial melting-ƒ(composition of source, degree of melting)
• Magma differentiation - Development of more evolved magmas (i.e., Fe, Na, K, Sirich) by the crystallization and accumulation of minerals along the margins of a
magma chamber, a process called fractional crystallization. This occurs because the
crystals formed in the magma are more Mg- and Ca-rich than the magma itself (see
diagram above). This typically occurs in large mafic to intermediate magma
chambers and results in the formation of cumulate rocks within mafic layered
intrusions. Extremely efficient magma differentiation can produce a complete range
of derivative magma compositions from mafic to felsic.
Fractional Crystallization in
Mafic Layered Intrusions
Cooling
Crystallization
M agma Convection
Accumulation
Cumulate Igneous Rocks
Crust
Mafic
Magma
Chamber
The bulk effect of fractional crystallization of a mafic magma can be demonstrated by
mass balance:
Parent Magma= (Derivative magma x % Liquid) + (Bulk solid x % Solid),
or Derivative Magma = (Parent magma - (Bulk solid x % Solid)) ÷ % Liquid
High-T Mineral Assemblage
Elements
SiO2
Al2O3
FeO
MgO
CaO
Na2O
K2 O
TiO2
OH
Parent
Magma
48
15
14
9.0
9.0
2.3
0.5
2.0
0.2
35%
Olivine
35
20
45
1
65%
Plagioclase
52
30
15
2.7
0.3
Bulk
Solid
46
19
7
16
10
1.8
0.2
0
0
Derivative Magma
30% Fractional
Crystallization
48.9
13.3
17.0
6.0
8.6
2.5
0.6
2.9
0.3
• Magma mixing- Nearly all ranges of magma composition are mixable, although in
extreme cases of felsic and mafic magma mixing, the contrast in viscosity and
temperature makes mixing difficult. Magma mixing can occur in all tectonic
environments but is most significant in convergent plate boundaries where a range of
magma are produced by partial melting and differentiation.
M AGM A SOURCES
IN CONVERGENT
PLATE BOUNDARIES
Partial M elting of
Lower Crust
Partial M elting of
M antle above
Downgoing Slab
Partial M elting of
Subducted Oceanic Crust
M agma Differentiation
in Layered Intrusions
M AGM A
COM POSITIONS
Felsic
Intermediate
M afic