CHAPTER 6 - IGNEOUS ROCKS, INTRUSIVE ACTIVITY AND THE ORIGIN OF IGNEOUS
ROCKS
Overview
Granite is the most abundant intrusive rock on continents. Unlike basalt that
can be seen crystallizing on Hawaii, no one has ever seen an intrusion
crystallize. Its intrusive nature is established through seven observations: 1)
mineralogically and chemically intrusive and extrusive rocks are identical; 2)
intrusive rocks are coarse-grained, volcanic rocks are fine-grained; 3) minerals
of intrusive rocks are temperature and pressure implying deep burial; 4)
country rock has been broken by the intruding magma; 5) country rock has
"baked" (metamorphosed) contacts next to the intrusion; 6) xenoliths in the
intrusion match the country rock; and 7) intrusive rocks exhibit chill zones
next to the country rock. Laboratory studies support interpretations of
intrusive rocks, but no one has ever made granite because of the time
required for truly slow cooling. A related problem is the role played by water in
controlling melt temperatures.
Texture is the size, shape and arrangement of grains. Extrusive rocks are finegrained from fast cooling, while plutonic rocks are coarse-grained from slow
cooling. Porphyritic textures represent two stages of cooling. Igneous rock
names are based on texture and mineralogy. Table 6.1 summarizes the
classification of igneous rocks, and Figure 6.4 illustrates variations in
mineralogy within the eight main divisions of igneous rocks. Discussion
establishes the characteristics of the mineralogy of equivalent pairs: graniterhyolite, diorite-andesite, gabbro-basalt and peridotite-komatiite. Varieties of
granite are used to illustrate the arbitrary nature of classification systems.
The chemistry of igneous rocks is highly variable and controls what minerals
actually form. Mafic rocks (gabbro-basalt) have a silica content close to 50%
and, as a consequence, are silica-deficient with a high magnesium, iron, and
calcium content. Felsic (silicic) rocks (granite-rhyolite) have a silica content
greater than 65% and are silica-rich. They lack ferromagnesian minerals and
have a high aluminum, sodium, and potassium content. Intermediate rocks
(diorite-andesite) fall in between mafic and felsic. Ultramafic rocks, of which
peridotite is the most abundant, are composed entirely of pyroxene and olivine
and have less than 45% silica with no fine-grained equivalents.
Intrusive bodies are differentiated by size, shape, depth and relationship to
country rock. Shallow intrusions include volcanic necks, dikes (tabular and
discordant) and sills (tabular and concordant). Plutons that crystallize at depth
include stocks, batholiths and diapirs. Abundance and distribution of plutonic
rocks is related to continents and ocean basins. Magma formation is treated in
some detail. Topics include the effects of geothermal gradient, mantle plumes,
pressure and the effects of water under pressure, and mixed mineralogies, and
how magmas of different compositions evolve.
Bowen's Reaction Series is discussed in detail, supported by Figure 6.16. The
problem of magmatic compositions and differentiation is treated by discussions
of crystal settling, partial-melting, assimilation and mixing.
The chapter concludes with an explanation of igneous activity related to plate
tectonics. Rock type and mechanics are differentiated for diverging and
converging boundaries and intraplate regions.
Learning Objectives
1. Intrusive rocks crystallize from magmas emplaced into country rock. They
possess mineralogies identical to volcanic rocks, but coarse-grained (= slow
cooling) textures. Intrusions exhibit both "baked" and chill zone contacts, and
they may contain xenoliths.
2. Names of plutonic rocks are the counterparts of extrusive rocks, sharing
their mineralogy, but distinguished by their coarse-grained textures.
Mineralogical equivalenta are granite-rhyolite, diorite-andesite, gabbro-basalt,
peridotite-komatiite. The gabbro-basalt pair is dominated by ferromagnesian
minerals and plagioclase feldspar. The peridotie-komatiite pair consists entirely
of ferromagnesian minerals.The granite-rhyolite pair is dominated by feldspars
and quartz. The diorite-andesite pair is composed of feldspars and significant
ferromagnesian minerals (30%-50%).
3. Classification systems are arbitrary and there is considerable variation in
the composition of granite and rhyolite.
4. Silica content varies significantly among rock types and influences the
minerals comprising various rock types. Mafic rocks contain 50% or less silica
by weight. They are silica-deficient and have high magnesium, iron, and
calcium content. Silicic (Felsic) rocks are silica-rich (greater than 65%), and
have significant content of aluminium, sodium, and potassium. Intermediate
rocks fall between mafic and silicic (felsic), and Ultramafic rocks, of which
peridotite is the most abundant, are composed of pyroxene and olivine and
have less than 45% silica. They have no fine-grained counterparts.
5. Intrusive bodies are defined by size, shape and relationship to country rock.
Volcanic necks are the solidified throats of volcanoes, dikes are discordant,
tabular intrusions, while sills, are concordant, tabular intrusions. Plutons
crystallize at great depth, and most are granite. Batholiths are large and
discordant, while stocks are small and discordant. Detached bodies of magma
that moved to shallow depths are called diapirs.
6. Granite comprises the bulk of continents. Basalt and to a lesser extent
gabbro underlay the oceans, while andesite forms most volcanoes along
continental margins. Ultramafic rocks are thought to form the mantle.
7. Magmas are melted by a combination of the effects of the geothermal
gradient, mantle plumes, water under pressure, pressure release, and mixed
mineralogies.
8. Bowen's Reaction Series (Fig. 6.16) explains the variation in rock
composition that can be produced from a single magma. Crystallization
proceeds simultaneously along two branches: a discontinuous branch for
ferromagnesian minerals that remain reactive with the magma, and a
continuous branch for plagioclase feldspars that exhibit zoning from changes in
calcium and sodium content. These minerals are formed by silicon-oxygen
tetrahedral that control their silica content. Any magma left after the
discontinuous and continuous branches are complete is enriched in silica, and
the last minerals to form are potassium feldspar, muscovite and quartz.
Differentiation, crystal-settling, partial melting, assimilation, and magma
mixing also account for compositional differences in magmas.
9. Basaltic magmas are produced at diverging plate boundaries from partial
melting of the asthenosphere and build oceanic crust. Mantle plumes produce
intraplate volcanism that is basaltic under oceanic crust, and rhyolitic under
continental crust. Converging plate boundaries produce andesite by partial
melting, and magmatic underplating that promotes melting of the lower
continental crust for granite production.
Boxes
6.1 - IN GREATER DEPTH - PEGMATITE - A ROCK MADE OF GIANT
CRYSTALS: Extremely coarse textured igneous bodies are associated with
some granitic intrusions, although they can form in rocks of any composition.
They represent slow cooling of a low viscosity fluid, probably water under high
pressure. The water contains ions that crystallize into potassium and sodium
feldspars, micas and quartz, but unusual and rare minerals, such as lithium
micas, uranium-ores and gemstones, can also form. Pegmatites occur as podlike bodies, if trapped within the magma chamber, or hydrothermal veins
above the intrusion, if cracks in the country rock allow the fluid to escape.
Short Discussion/Essay
1. How are igneous rocks classified?
2. If Bowen's Reaction Series is accurate, why don't all magmas end-up as
granite?
3. If all the minerals on Bowen's Reaction Series are silicates, why are mafic
rocks silica-poor, while felsic rocks are silica-rich?
4. How are intrusive rocks classified?
5. How can basalt be both intrusive and extrusive, yet granite is only
intrusive?
Longer Discussion/Essay
1. How does the igneous rock classification reflect Bowen's Reaction Series?
2. How do magmas form and why do they vary in composition?
3. How can you prove that an igneous rock exposure is intrusive?
4. Why are continents formed of granite and other felsic rocks, while ocean
floors are formed from basalt, a mafic rock?
5. Why are most plutons granitic and not mafic, even though they occur deep
within the crust?
Selected Readings
There are a number of textbooks on igneous petrology available that cover the material
presented in this chapter. Some other references include:
LeMaitre, R.W., (ed.), 1989. The classification of igneous rocks and glossary of
terms. Oxford: Blackwell Scientific Publications
Brown, V.M. and Harrell, J.A. 1991. "Megascopic classification of Rocks,"
Journal of Geological Education 39: 379-387
Coffin, M.F., and Eldholm, O., 1993. "Large Igneous Provinces." Scientific
American, 264:42-49.
Shelly, David, 1993. Igneous and metamorphic rocks under the microscope.
New York: Chapman and Hall.