Download Igneous Petrology

Igneous and
metamorphic
Petrology
Mahrous M. Abu El-Enen
Faculty of Science, Mansoura University
E-mail: [email protected]
‫بسم هللا الرحمن الرحيم‬
‫ربنا ماخلقت هذا باطال‬
‫سبحانك فقنا عذاب‬
‫النار‬
Course objectives/ outcomes
The main objective of the coarse:
This introductory level course in Petrology is aimed to
help the students with the basic tools needed for
understanding igneous and metamorphic processes.
Specific student outcomes include:
1- identifying and describing igneous and
metamorphic rocks and their structures in the field and
hand specimen based on their mineralogical
composition and textures.
Cont. Course objectives/ outcomes
Specific student outcomes include:
2- Explaining (orally and in writing) the general
processes of formation of igneous and metamorphic
rocks.
3- Classifying igneous and metamorphic rocks properly
in hand specimen.
What is petrology?
Petrology is the branch of geology that
dealing with study of the rocks.
Igneous Petrology: is that branch of
petrology dealing with the study of rocks that
were originally molten (i.e. which formed from
magmas or lavas).
Metamorphic Petrology is the study of all
rocks that formed by recrystallization in the
solid state, beyond the field of diagenesis.
General divisions of Petrology
In general, all branches of petrology (whether
igneous, sedimentary or metamorphic) consist
of two main components:
A-
Petrography: which is the descriptive
component of petrology, and consists of describing
rocks either in the hand or under the microscope.
B- Petrogenesis: which is that part of
petrology which aims at understanding the origin of
rocks.
What is rock meaning?
Rocks are defined as a component of the Earth’s crust
that composed of one or more minerals with a
geologic extension. Rocks are defined as the main
unit of the Earth. Rocks could be:
1) Mono-mineralic rocks: such as quartizite and
marble.
1)Poly-mineralic rocks: such as granites, schists
and greaywackes.
This means that Meteorites are not belonged to
rocks
Exception for rock definition
Some rocks deviate from rock definitions as:
1) Coal: which originated from organic materials
1)Obsidian: which composes of non-crystallized
silicates.
Again, What is the Earth Crust
The Earth’s crust is the outer
solid materials of the Earth
(5-70 km) that composed of
rocks. It subdivided into:
-Oceanic crust: Thin (~ 10
km), dense, (Fe-Mg silicate
rocks, SIMA)
-Continental crust: Thick (2070 km, ~ 35 km average),
less dense (Na-K-Al-silicate
rocks, SIAL)
- sedimentary cover (attains
10 km thick), of sedimentary
rock composition
Upper Mantle
Depth km
Rock Cycle
The rocks are passed through a cycle during
their generation in the Earth, which known as:
Rock Cycle
The rocks are divided into:
- Primary rocks: Igneous rocks
- Secondary rocks:
metamorphic rocks.
Sedimentary and
Rock Cycle
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Igneous rocks
-Primary rocks
- Massive and hard
- Crystalline
- devoid fossils and pore
spaces
- Example: granite, basalt
Metamorphic rocks
-Secondary, hard, and deformed
- Foliated
- occur in tectonic active areas
Example: Schist, and gneiss
Sedimentary rocks
-Secondary, Soft, and
bedded
- Contain fossils, and porous
- Contain petroleum, coal
and phosphate
-Example: Sandstone,
Limestone and Claystone.
The source of igneous rocks is the molten materials,
i.e. magma. Therefore, these name of igneous rocks
came from the Latin “ignis”, which meaning fire.
The igneous rocks are formed by cooling and
crystallization of magma (molten silicate materials)
either below the earth as Intrusive rocks (plutonic)
or on the surface as extrusive rocks (Volcanic).
Then what is the magma and Lava
Magma and Lava
Magma: a mixture of a melt (predominantly silicate)
± crystals ± volatiles which occurs at depths and
has the ability to migrate to shallower levels
where it either crystallizes at depth giving rise to
igneous intrusions, or erupts at the surface to
form volcanic rocks. The magma occur in
equilibrium.
Lava: is erupted molten material that can flow on
the surface of the earth. therefore lava may be
considered a magma that has lost its gases (to
the atmosphere upon eruption).
Nature and composition of magma
Magma is a complex mixture of molten, little solid
and volatile phases that occur at high temperature
(700-1300° C) and high pressure and have a
different degrees of viscosity .
Chemical composition of magma
All chemical elements are represented in
magma, but with various proportions.
The most abundance elements are oxygen and
silicon; therefore magma could be described
as molten silicate materials.

Not all magmas have the same composition,
which proofed from diversity of igneous rocks
that occur at the surface of the earth or at
depth.
Chemical classification of Magmas
Petrologists were able to classify magmas
into four main chemical groups:
1- Acidic magma: rich in SiO2 (66-77 wt./%), Na2O
and K2O. acidic magmas are broadly known as
"granitic"
2- Intermediate magma: rich in SiO2 (52-66 Wt.%),
Na2O, K2O as well as CaO and Al2O3.
3- Basic magma: rich in CaO, MgO and FeO and
moderate SiO2 (52-45 wt.%). Basic magmas are
broadly known as "basaltic".
Chemical composition of magmas,
cont.
4- Ultrabasic magma : Poor in SiO2 (< 45 ~ 38 wt.%)
but with large amounts of FeO and MgO.
Oxide
Acidic (Granite)
Intermediate (Andesite)
Basic (Basalt)
Ultrabasic (Peridotite)
SiO2
71.3
57.94
49.2
42.26
TiO2
0.31
0.87
1.84
0.63
Al2O3
14.32
17.02
15.74
4.23
Fe2O3
1.21
3.27
3.79
3.61
FeO
1.64
4.04
7.13
6.58
MnO
0.05
0.14
0.2
0.41
MgO
0.71
3.33
6.73
31.24
CaO
1.84
6.79
9.47
5.05
Na2O
3.68
3.48
2.91
0.49
K2O
4.07
1.62
1.1
0.34
H2O
0.77
1.17
0.95
3.91
CO2
0.05
0.05
0.11
0.30
P2O5
0.12
0.21
0.35
0.10
Generation of magma
Temperature and pressure rises with depth,
eventually reaching to a level wherein the strongest
rock is melted and turned to a liquid form which is
called magma.
magmas are generated by partial melting in the
upper mantle or lower crust, such a process occurs
over a range of depths.
Therefore rising of magma connected to the
weakness area (plate margins) within the Earth.
Plate boundaries
Plate Motions
Magma crystallization
Crystallization of magma means that
transformation of magma from molten phase
to solids phase.
During crystallization of magma, all minerals
not crystallized at the same time, but according
to rules (melting points and specific gravity of
minerals)
Generally, the minerals with high melting
point and higher specific gravity crystallized
early and followed by minerals with lower
melting point and specific gravity.
A series of minerals wherein any early-formed phase will react with the
melt later in the differentiation to yield a new mineral further in the
series .
Mineral association in igneous rock .
Differentiation of magmas
Differentiation is the process by which magmas evolve
to give rise to a variety of magmas and rock types
There are different processes of magmatic
differentiation:
1- Fractional crystallization and gravity crystallization:
Is a sequence of separation of crystal from melt
Differentiation of magmas
2- filter Pressing
If a mixture of crystals and liquid
is suddenly subjected to
compressional stress, the liquid
will be squeezed out of the
mixture, and will therefore be
separated from the crystals.
3- Magmatic assimilation
Is the reaction of the magma with
the country rocks, whereby these
country rocks are incorporated in
the magma and eventually melt.
Differentiation of magmas
4- Magma Mixing
Mixing two magmas that are compositionally different
will produce a magma of intermediate composition.
Characters of igneous rocks
The igneous rocks have the following characters:
Hard crystalline rocks: most of igneous
rocks are crystalline and very little (obsidian)
are hyaline
Massive: occur as large massive bodies,
dykes and sills
They are not porous and devoid any fossils
content
Classification of Igneous Rocks
The igneous rocks are classified according to the following
bases:
1- Mode of occurrence of igneous rocks
2- Textures of the Igneous rocks
3- Chemical composition and corresponding
Mineralogy
4- The colour of Igneous rocks
Classification of the Igneous rocks
Igneous rocks are classified according to the
depth of generation (Mode of occurrence) into:
- Extrusive (volcanic) rocks: That formed at the
Earth’s surface
-Intrusive rocks: That formed under the Earth’s
surface, and could be:
- Plutonic rocks: That formed at quit depth of
the Earth’s crust.
- Hypabyssal rocks: formed at a depth in
between the plutonic and volcanic rocks
Plutonic Igneous rocks: Characterize by the
following:
1- They are generated at quit depth, so their
magma cooled slowly
2- Their yielded rocks have coarse-grained
texture
3- They form a large masses (Batholith, Boss,
Stock)
4- Examples are granite and gabbros
Volcanic Igneous rocks: Characterize by the
following:
1- They are generated at Earth’s surface from
lava cooling, so their cooling is fast
2- Their yielded rocks have very fine to glassy
rocks
3- They form Lava flow and occasionally contain
vesicles (such as Scoria)
4- Examples are rhyolite and basalt
Hypabyssal Igneous rocks: Characterize by the
following:
1- They are generated formed at a depth in
between the plutonic and volcanic rocks, So the
their rate of cooling is intermediate
2- Their yielded rocks have medium-grained
texture, and porphyritic texture
3- They form dykes, sills, lacoliths, lopoliths and
phacoliths forms
4- Examples are porphyrites
Classification of the Igneous rocks
Texture of Igneous rocks:
Texture: grain-grain relationships, and refers to degree of
crystallinity, grain size, and geometrical relationships
between the constituents of a rock
1) degree of crystallinity,
2) Sizes of crystals, and
3) Mutual relationships between the crystals.
1- Degree of crystallinity: is the quantity of crystal opposite to
glass in a given igneous rocks.
It depends on: 1)Rate of magma cooling
2)Viscosity of the magma
Degree of crystallinity
Generally the igneous rocks are described as:
1- Holocrystalline: composed wholly of crystals
2- Hypocrystalline: contains both glass and crystals
3- Holohayline: consists entirely of glass
Holocrystalline Texture
Holohyaline texture
Grain-size
As well grain size depends on:
1) rate of cooling,
2)Viscosity of magma, and
3) volatile content in magma
Igneous rocks are classified according to their grain size
into:
1-Cryptocrystalline: crystals cannot be distinguished even
with a microscope
2- Aphanitic: crystals not visible to the unaided eye
3- Phaneritic: grains readily distinguished with the unaided
eye. If the grains of the rock are roughly the same size:
Fine < 1 mm
Medium 1-5 mm
Coarse 5 mm-3 cm
Very coarse > 3 cm
Grain size
Aphanitic Texture
Coarse-grained tex
Fine-grained tex
Medium-grained tex
Very coarse-grained tex
Mutual relationships between the crystals
There are different types of textures,
1- Porphyritic texture: Large
(Phenocrysts) in Fine-grained
(matrix). It forms due to two cycles
of cooling, initially very slowly
underground, then rapidly at
Earth's surface.
1-Flow Texture: directive large
phenocrysts in matrix. It forms due
to magma flowing
3- Vesicular texture: refers to
vesicles (holes, pores, or cavities)
within the igneous rock. It forms as
the result of gas expansion
(bubbles), which often occurs
during volcanic eruptions.
Grain size
Porphyritic Texture
Vesicular texture
Mineralogical composition
As we discussed, minerals separated from magma
according to Bown’s reaction Series.
Most of the common essential minerals are:
1- Quartz: occur in the high-silica igneous rocks (SiO2>66
Wt.%). It is glassy in appearance.
2- K-Feldspar: include Orthoclase (buff color). It is also
occur in rocks of high-silica content, and Plagioclase (milky
white in color), which occur in all rocks and include (Naplagioclase to Ca-plagioclase).
3- Mica: include white mica (muscovite) and dark mica
(biotite). All are flakey.
Mineralogical composition
4- Amphiboles: their common mineral is hornblende. It
occur in rocks with moderate-silica content. It is dark green
in color.
5- Pyroxene: include complex silicate minerals (e.g.
Augite, enstatite, diopside, and Aegerine). They occur in
rock that poor in silica. They are dark green colour.
6- Olivine: include fyalite and forsterite varieties. They are
green in colour, prismatic in habit. They occur only in rocks
that are deficient in silica.
Common Minerals
Fig. 5.4
Granite
Quartz
Orthoclase
Granite
Biotite
Plagioclase
Compositional
Classification
Granite
Granodiorite
Granite
Compositiona
l
Classification
Quartz
Amphibole
Plagioclase
Granite
Compositiona
Granodiorite
l
Classification
Diorite
Granite
Plagioclase
Amphibole
Granite
Compositiona
Granodiorite
l
Classification
Diorite
Granite
Gabbro
Plagioclase
Pyroxene
Granite
Compositiona
Granodiorite
l
Classification
Diorite
Granite
Gabbro
Pyroxene
Olivine
Peridotite
Chemical composition
Chemically, the igneous rocks are classified into:
1- Acidic (felsic) rocks: rich in SiO2 (66-77 wt./%), and
little FeO and MgO. They contain minerals such as: quartz,
Na-plagioclase, K-feldspars (orthoclase, microcline), white
mica. Their rocks are leucocratic. E.g. granite, rhyolite,
granodiorite
2- Intermediate rocks: Contain SiO2 (52-66 wt./%) and a
moderate FeO and MgO. They contain minerals such as: CaNa-plagioclase, K-feldspars (orthoclase), amphibole. Their
rocks are mesochratic. E.g. Diorite, andesite
Chemical composition
3- Basic (mafic) rocks: rich in CaO, MgO and FeO and
moderate SiO2 (52-45 wt.%). They contain minerals such as:
Na-Ca-plagioclase, Pyroxene. Their rocks are
melanochratic. E.g. basalt and gabbro.
4- Ultrabasic (ultramafic) rocks: Poor in SiO2 (< 45 ~ 38
wt.%) but with large amounts of FeO and MgO. They contain
minerals such as: Ca-plagioclase, Pyroxenes, Olivine. Their
rocks are melanochratic. E.g. dunite, pyroxenite,
anorthosite, peridotite
5- rocks Color
The color of the
rock depends on
the ratios of
mafic to felsic
mineral
constituents.
5- rocks Color
The color of the rock depends on the ratios of
mafic to felsic mineral constituents.
Generally, rock are classified, depends on estimating the
volume % of the dark (mafic) minerals. Igneous rocks are
classified into 4 groups based on C.I.:
1- leucocratic rocks: < 30%
2- Mesocratic: C.I. = 30 – 60
= Felsic rocks
= Intermediate rocks
3- Melanocratic: C.I. = 60 – 90
4- Hypermelanic: C.I. > 90.
= Mafic rocks
= Ultramafic rocks
General classification of Igneous rocks
Based on:
1- Mode of occurrences
2- Textures
3- Chemical composition and silica saturation
4- Mineralogical composition
5- Rock colour
Igneous rocks are classified into:
1- Acidic-felsic-oversaturated igneous rocks
2- Intermediate-saturated igneous rocks
3- Basic-saturated igneous rocks
4- Ultrabasic-undersaturated igneous rocks
Classification of igneous rocks
Classification of igneous rocks
Classification of igneous rocks
Classification of igneous rocks
Classification of igneous rocks
General classification of Igneous rocks
Based on:
1- Mode of occurrences
2- Textures
3- Chemical composition and silica saturation
4- Mineralogical composition
5- Rock color
Igneous rocks are classified into:
1- Acidic-felsic-oversaturated igneous rocks
2- Intermediate-saturated igneous rocks
3- Basic-saturated igneous rocks
4- Ultrabasic-undersaturated igneous rocks
Classification of igneous rocks
Chemical
Composition &
Silica
Saturation
Mineral
composition
Acidic
Oversaturated
Qtz + Or <
Na-rich Pl +
mica
Intermediate
saturated
Qtz + Na-rich Pl Or>Pl +
< Or + mica
mica +
amphibole
Pumice
Vesicular
Glassy
Volcanic
(Fine grained
Hypapyssal
(mediumgrained)
Plutonic
(Coarsegrained)
Color
Pl<Or +
mica +
amphibole
BasicSaturated
Pl +
Pyroxene
Ultrabasic –
undersaturated
Pyroxene +
Olivine + Carich Pl
Scoria
Obsidian
Rhyolite
Dacite
Trachyte
Porphritic- Porphyriticdacite
rhyolite
Pegmatite
Pgmatite
Granite
Granodiorite
Leucocratic
(felsic)
Syenite
Andesite
Basalt
Porphyritic
andesite
dolerite
Diorite
Mesocratic
(Intermediate)
Gabbro
Dunite
Pyroxenite
Predotite
Anorthosite
Melanocratic
(Mafic)
Hypermelanic
(Ultramafic)
Granite
Acidic Igneous rocks
Acidic Igneous rocks
Rhyolite
Acidic Igneous rocks
Granodiorite
Acidic Igneous rocks
Dacite
Acidic Igneous rocks
Pegmatite and Rhyolite porphyry
Acidic Igneous rocks
Pumice
Intermediate Igneous rocks
Syenite
Intermediate Igneous rocks
Trachyte
Intermediate Igneous rocks
Diorite
Intermediate Igneous rocks
Andesite
Intermediate Igneous rocks
Andesite porphyry
Basic Igneous rocks
Gabbro
Basic Igneous rocks
Basalt
Basic Igneous rocks
Dolerite
Basic Igneous rocks
Scoria
Dunite
Ultrabasic Igneous rocks
Ultrabasic Igneous rocks
Pyroxenite
Ultrabasic Igneous rocks
Predotite
Ultrabasic Igneous rocks
Anorthosite
Ultrabasic Igneous rocks
Obsidian
Syllabus of the course
Igneous rocks:
1- Introduction, magmas and lavas, magma source,
chemical composition of magma, crystallization of
magma, Occurrence and abundance of igneous rocks
2- Characters of igneous rocks
3- Classification of igneous rocks
4- Common metamorphic rocks
General classification of Igneous rocks
Based on:
1- Mode of occurrences
2- Textures
3- Chemical composition and silica saturation
4- Mineralogical composition
5- Rock colour
Igneous rocks are classified into:
1- Acidic-felsic-oversaturated igneous rocks
2- Intermediate-saturated igneous rocks
3- Basic-saturated igneous rocks
4- Ultrabasic-undersaturated igneous rocks
Text books
1- Abu El-Enen: Lecture notes on Igneous and
Metamorphic Petrology.
2- Turner, Francis J.: Igneous and metamorphic
petrology
3- Best, Myron G. : Igneous and metamorphic
petrology
4- The available “Physical Geology” at Literary.