Plate Tectonic - Igneous Genesis
1. Mid-ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental
Margins
5. Back-arc Basins
6. Ocean Island Basalts
7. Miscellaneous IntraContinental Activity

kimberlites, carbonatites,
anorthosites...
Igneous Minerals
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Quartz, Feldspars (plagioclase and alkaline),
Olivines, Pyroxenes, Amphiboles
Accessory Minerals – mostly in small quantities
or in ‘special’ rocks
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Magnetite (Fe3O4)
Ilmenite (FeTiO3)
Apatite (Ca5(PO4)3(OH,F,Cl)
Zircon (ZrSiO4)
Titanite (CaTiSiO5)
Pyrite (FeS2)
Fluorite (CaF2)
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Ca2+
O2Si4+
O2-
Mg2+
Na+
Fe2+
Liquid hot O2MAGMA
2O
O2- Si4+ O2- O2O2- O2- Si4+
Minerals which form are thus a
function of melt composition and
how fast it cools (re-equilibration?)
 governed by the stability of
those minerals and how quickly
they may or may not react with the
melt during crystallization
General Compositions  Silicic
(Si-rich), Sialic (Si and Al rich),
Intermediate, Mafic (Mg and Ferich), Ultramafic
Also ID’d based on alkalic (K and
Na) or alkaline (Ca-rich)
Mg2+
O2-
cooling
rock
Mg2+
Fe2+
Composition

From Magma we saw how a crystal’s
composition can change on crystallization 
different elemental composition from melt on
partial crystallization
Silica and Aluminum Content

Silica



Oversaturated if it contains Quartz
Undersaturated if it has silica-deficient minerals
(like the feldspathoids, ex: nepheline)
Aluminum


Peraluminous if it has a great excess of aluminum 
after feldspars form, more Al left over for Al-rich
phases like corumndum, garnet, kyanite, etc.
Peralkaline – So little Al left after feldspars form,
only Al-deficient minerals like aegerine (type of
pyroxene) and riebekite (sodic amphibole)
Classification of Igneous Rocks
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.
(1986) J. Petrol., 27, 745-750. Oxford University Press.
Igneous Rocks
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
Textures  aphanitic, phaneritic,
porphyritic, pegmatitic, vesicular, glass, and
pyroclastic
Compositions  Silicic, Intermediate,
Mafic, Ultramafic
Igneous Textures
Figure 3-1. Idealized rates of crystal
nucleation and growth as a function
of temperature below the melting
point. Slow cooling results in only
minor undercooling (Ta), so that
rapid growth and slow nucleation
produce fewer coarse-grained
crystals. Rapid cooling permits more
undercooling (Tb), so that slower
growth and rapid nucleation produce
many fine-grained crystals. Very
rapid cooling involves little if any
nucleation or growth (Tc) producing
a glass.
Figure 3-17. “Ostwald ripening” in a monomineralic material. Grain boundaries with significant negative
curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller grains and
establishing a uniformly coarse-grained equilibrium texture with 120o grain intersections (polygonal mosaic).
© John Winter and Prentice Hall
Textures I

Aphanitic - fine grain size (< 1 mm); result of
quick cooling
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Phaneritic - coarse grain size; visible grains (110 mm); result of slow cooling


Rhyolite, Basalt, Rhyolite, Andesite
Granite, Diorite, Gabbro
Pegmatitic - very large crystals (many over 2
cm)

Granite pegmatite or pegmatitic granite
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Porphyritic- Mixture of grain sizes caused by mixed cooling history;
slow cooling first, followed by a period of somewhat faster cooling.
Terms for the textural components:
 Phenocrysts - the large crystals
 Groundmass or matrix - the finer crystals surrounding the large
crystals. The groundmass may be either aphanitic or phaneritic.
Types of porphyritic textures:
 Porphyritic-aphanitic
 Porphyritic-phaneritic
Origin: mixed grain sizes and hence cooling rates, imply upward
movement of magma from a deeper (hotter) location of extremely slow
cooling, to either:
 a much shallower (cooler) location with fast cooling (porphyriticaphanitic), or
 a somewhat shallower (slightly cooler) location with continued
fairly slow cooling (porphyritic-phaneritic).
Typically Volcanic Textures
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Glassy - instantaneous cooling
 Obsidian = volcanic glass
Vesicular - contains tiny holes called vesicles which formed due to
gas bubbles in the lava or magma. Very porous. May resemble a
sponge. Commonly low density; may float on water.
 Vesicular Basalt, Pumice, Scoria
Pyroclastic or Fragmental - pieces of rock and ash come out of a
volcano and get welded together by heat. May resemble rhyolite or
andesite, but close examination shows pieces of fine-grained rock
fragments in it. May also resemble a sedimentary conglomerate or
breccia, except that rock fragments are all fine-grained igneous or
vesicular.
Tuff - made of volcanic ash
Volcanic breccia - contains fragments of fine-grained igneous rocks
that are larger than ash.
Classification based on Field Relations

Extrusive or volcanic rocks: typically aphanitic or glassy.
Many varieties are porphyritic and some have fragmental
(volcaniclastic) fabric. High-T disordered fsp is common
(e.g. sanadine). Also see leucite, tridymite, and
cristobalite.

Intrusive or plutonic rocks: typically phaneritic.
Monomineralic rocks of plagioclase, olivine, or pyroxene
are well known but rare. Amphiboles and biotites are
commonly altered to chlorite. Muscovite found in some
granites, but rarely in volcanic rocks. Perthitic fsp,
reflecting slow cooling and exsolution is widespread.
Names of Igneous Rocks
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Texture + Composition = name
Set up diagrams (many ternary ones again, you
remember how these work?) to represent
composition changes for rocks of a certain
texture
Composition can be related to specific
minerals, or even physical characteristics of
mineral grains
Modal Composition - % of minerals
comprising a rock
Visual Estimation of Modal Abundance
Classification based on Modal Mineralogy
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Felsic rocks: mnemonic based on feldspar and
silica. Also applies to rocks containing abundant
feldspathoids, such as nepheline. GRANITE
Mafic rocks: mnemonic based on magnesium and
ferrous/ferric. Synonymous with ferromagnesian,
which refers to biotite, amphibole, pyroxene,
olivine, and Fe-Ti oxides. BASALT
Ultramafic rocks: very rich in Mg and Fe.
Generally have little feldspar. PERIDOTITE
Silicic rocks: dominated by quartz and alkali fsp.
Sometimes refered to as sialic (Si + Al).
Classification of
Igneous Rocks
granite
granodiorite
Q
Classification of Phaneritic
Igneous Rocks
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q=quartz
A=Alkali fledspars
(An0-An5)
P=Plagioclase feldspars
(An5-An100)
F=Feldspathoid
Granodiorite
Granite
Alkali Fs.
Quartz Syenite
Alkali Fs.
Syenite
20
20
Quartz
Monzonite
Quartz
Syenite
5
10
A
Syenite
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
Figure 2-2. A classification of the phaneritic igneous
rocks. a. Phaneritic rocks with more than 10% (quartz +
feldspar + feldspathoids). After IUGS.
(Foid)
Monzosyenite
(Foid)
Monzodiorite
60
60
(Foid)olites
F
Qtz. Diorite/
Qtz. Gabbro
5 Diorite/Gabbro/
90
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
Aphanitic rocks
basalt
rhyolite
Q
Classification of
Igneous Rocks
60
60
Rhyolite
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
Figure 2-3. A classification and nomenclature
of volcanic rocks. After IUGS.
60
60
(Foid)ites
F
P
Classification of Igneous Rocks
Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn
(1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the
size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Classification of Igneous Rocks
Plagioclase
Plagioclase
Feldspar
Anorthosite
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
lite
cto
Tro
Ga
bb
ro
90
Olivine
gabbro
Olivine
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Pyroxene
Lherzolite
Olivine
Olivine
(b)
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Igneous Textures
Figure 3-2. Backscattered electron image of
quenched “blue glassy pahoehoe,” 1996
Kalapana flow, Hawaii. Black minerals are
felsic plagioclase and gray ones are mafics.
a. Large embayed olivine phenocryst with
smaller plagioclase laths and clusters of
feathery augite nucleating on plagioclase.
Magnification ca. 400X. b. ca. 2000X
magnification of feathery quenched augite
crystals nucleating on plagioclase (black) and
growing in a dendritic form outward. Augite
nucleates on plagioclase rather than preexisting augite phenocrysts, perhaps due to
local enrichment in mafic components as
plagioclase depletes the adjacent liquid in Ca,
Al, and Si. © John Winter and Prentice Hall.
Igneous Textures
Figure 3-4. a. Skeletal olivine phenocryst with rapid growth at edges enveloping melt
at ends. Taupo, N.Z. b. “Swallow-tail” plagioclase in trachyte, Remarkable Dike, N.Z.
Length of both fields ca. 0.2 mm. From Shelley (1993). Igneous and Metamorphic
Rocks Under the Microscope. © Chapman and Hall. London.
Igneous Textures
Figure 3-5. a. Compositionally
zoned hornblende phenocryst with
pronounced color variation visible
in plane-polarized light. Field
width 1 mm. b. Zoned plagioclase
twinned on the carlsbad law.
Andesite, Crater Lake, OR. Field
width 0.3 mm. © John Winter and
Prentice Hall.
Figure 3-6. Examples of plagioclase zoning profiles determined by microprobe point traverses.
a. Repeated
sharp reversals attributed to magma mixing, followed by normal cooling increments. b. Smaller and irregular
oscillations caused by local disequilibrium crystallization. c. Complex oscillations due to combinations of
magma mixing and local disequilibrium. From Shelley (1993). Igneous and Metamorphic Rocks Under the
Microscope. © Chapman and Hall. London.
Figure 3-7. Euhedral early pyroxene with late interstitial plagioclase (horizontal twins). Stillwater
complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.
Figure 3-8. Ophitic texture. A single pyroxene envelops several well-developed
plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland. © John Winter and
Prentice Hall.
Figure 3-9. a. Granophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm dike.
Golden Horn granite, WA. Width 1mm. b. Graphic texture: a single crystal of cuneiform
quartz (darker) intergrown with alkali feldspar (lighter). Laramie Range, WY. © John
Winter and Prentice Hall.
Figure 3-10. Olivine mantled by orthopyroxene in (a) plane-polarized light and (b)
crossed nicols, in which olivine is extinct and the pyroxenes stand out clearly. Basaltic
andesite, Mt. McLaughlin, Oregon. Width ~ 5 mm. © John Winter and Prentice Hall.
Figure 3-11. a. Sieve texture in a cumulophyric cluster of plagioclase phenocrysts. Note
the later non-sieve rim on the cluster. Andesite, Mt. McLoughlin, OR. Width 1 mm. ©
John Winter and Prentice Hall.
Figure 3-11. b. Resorbed and embayed olivine phenocryst. Width 0.3 mm.
Winter and Prentice Hall.
© John
Figure 3-11. c. Hornblende phenocryst dehydrating to Fe-oxides plus pyroxene due to
pressure release upon eruption, andesite. Crater Lake, OR. Width 1 mm. © John Winter
and Prentice Hall.
Figure 3-12. a. Trachytic texture in which
microphenocrysts of plagioclase are aligned
due to flow. Note flow around phenocryst
(P). Trachyte, Germany. Width 1 mm. From
MacKenzie et al. (1982). © John Winter
and Prentice Hall.
Figure 3-12. b. Felty or pilotaxitic texture
in which the microphenocrysts are
randomly oriented. Basaltic andesite, Mt.
McLaughlin, OR. Width 7 mm. © John
Winter and Prentice Hall.
Figure 3-13. Flow banding in andesite. Mt.
Rainier, WA. © John Winter and Prentice
Hall.
Figure 3-15. Intergranular texture in basalt.
Columbia River Basalt Group, Washington.
Width 1 mm. © John Winter and Prentice
Hall.
Figure 3-14. Development of cumulate textures. a. Crystals accumulate by crystal settling or simply form in
place near the margins of the magma chamber. In this case plagioclase crystals (white) accumulate in mutual
contact, and an intercumulus liquid (pink) fills the interstices. b. Orthocumulate: intercumulus liquid
crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (colored). There is
little or no exchange between the intercumulus liquid and the main chamber. After Wager and Brown (1967),
Layered Igneous Rocks. © Freeman. San Francisco.
Figure 3-14. Development of cumulate textures. c. Adcumulates: open-system exchange between the
intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allows components that
would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the
available space. d. Heteradcumulate: intercumulus liquid crystallizes to additional plagioclase rims, plus
other large minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the plagioclases. .
After Wager and Brown (1967), Layered Igneous Rocks. © Freeman. San Francisco.
Figure 3-16. a. The interstitial liquid (red) between bubbles in pumice (left) become 3-pointed-star-shaped
glass shards in ash containing pulverized pumice. If they are sufficiently warm (when pulverized or after
accumulation of the ash) the shards may deform and fold to contorted shapes, as seen on the right and b. in
the photomicrograph of the Rattlesnake ignimbrite, SE Oregon. Width 1 mm. © John Winter.
Figure 3-18. a. Carlsbad twin in
orthoclase. Wispy perthitic exsolution is
also evident. Granite, St. Cloud MN.
Field widths ~1 mm. © John Winter
and Prentice Hall.
Figure 3-18. b. Very straight multiple albite
twins in plagioclase, set in felsitic
groundmass. Rhyolite, Chaffee, CO. Field
widths ~1 mm. © John Winter and Prentice
Hall.
Figure 3-18. (c-d) Tartan twins in
microcline. Field widths ~1 mm. ©
John Winter and Prentice Hall.
Figure 3-19. Polysynthetic deformation twins in plagioclase. Note how they concentrate in
areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away
toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and
Prentice Hall.
Figure 3-20. a. Pyroxene largely
replaced by hornblende. Some
pyroxene remains as light areas (Pyx)
in the hornblende core. Width 1 mm. b.
Chlorite (green) replaces biotite (dark
brown) at the rim and along cleavages.
Tonalite. San Diego, CA. Width 0.3
mm. © John Winter and Prentice Hall.
Pyx
Hbl
Chl
Bt
Figure 3-21. Myrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L.
Collins. http://www.csun.edu/~vcgeo005