Download GRANITOID ROCKS

GRANITOID ROCKS
The rock granite (or its volcanic equivalent rhyolite) has a limited range of composition that results in
crystallization of equivalent amounts of quartz, plagioclase, and K-feldspar. Granites (sensu stricto)
constitute only a limited portion of quartz and plagioclase-rich magma series that may include tonalites,
granodiorites, diorites, quartz monzonites, and granites. Therefore, the term granitoid is more useful. It
includes all rocks that are rich in quartz and plagioclase.
Granitoid rocks include:
 plutonic equivalents of subduction-related calc-alkaline volcanics that involve partial melting of
the mantle
 rocks that formed by partial melting of the mafic lower continental crust
 rocks that formed by partial melting of silicic crustal material (major)
The continental crust is involved somehow in the generation of the vast majority of granitoid magmas.
I. Occurrence
Orogenic associations


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mountain belts associated with Andean type subductions (e.g, the Andes, Sierra Nevada)
orogenic belts that formed by continent-continent collision (e.g., Himalayas, Trans-Hudson
orogen)
extensional “anorogenic” terranes (e.g., Basin and Range Province, St. Francois Mountains).
Style of occurrence

Large batholiths – The batholiths include numerous individual granitoid plutons. They primarily
occur in mountain belts associated with Andean type subductions.

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Individual distributed plutons – This style of occurrence is common for granites that formed
during extension. Volcanic rocks are also commonly associated.
Composite plutons formed from numerous dikes – This style is common for granites that formed
by partial melting of metasedimentary rocks during continental convergence. Pegmatites are also
associated with this style of granite plutonism.
II. Mineralogy
Major defining minerals: quartz, plagioclase, alkali feldspar
Other major minerals: Biotite, hornblende, muscovite and tourmaline are common, depending on the
chemical composition of the granitoid. Garnet and orthopyroxene are less common.
Ubiquitous trace minerals: zircon, apatite
Other trace minerals: sphene, oxides, allanite, monazite, xenotime
III. Classification
“Alphabet soup” classification There have been many classifications proposed for granitoids.
However, the one most often used is the S-I classification devised by two Australians Bruce Chappel
and Alan White. Their classification is based on the presumed source rocks – sedimentary (pelitic) and
igneous (mafic) - that impart characteristic composition on the granitoids. However, people found
additional granitoids with distinct characteristics - anorogenic (A-type) and those derived by fractional
crystallization of mafic magma (M-type). So now we have the S-I-A-M classification. The problem is
that this classification is hybrid, because it mixes composition types (S, I), with tectonic setting (A-type)
and petrogenetic process (M-type).
A much more useful classification is one based on tectonic setting. Fortunately, one can relate the two
classifications to have a sensible view of the meaning of different types of granitoids. Because the
“alphabet soup” classification is used as a means of communication, we will discuss it first.
Characteristic
S-type
I-type
A-type
M-type
(minor)
SiO2
65-79%
53-76%
60-80%
54-73%
Al2O3/(Na2O+K2O+CaO)
>1.1
<1.1
variable to <1
<1.2
Na20 in silicic rocks
variable
>3.2%
>2.8%
>3.2%
K2O/ Na2O
high
low
usually high
very low
Fe3+/Fe2++Fe3+
low
medium-high
medium
high
Eu
low?
low-high
low
?
Cr, Ni
high
low
low
?
normally
>0.708
normally
<0.708
0.703-0.712
usually <0.704
Nd
-4 to -17
-4 to -9
?
?
Main Enclaves
metasediments,
amphibolite,
diorite
enclaves rare
Hornblende
Key CIPW Minerals
>1% C
<1% C
di common
?
Key Modal Minerals
muscovite,
Hornblende,
biotite,
biotite, sphene,
sillimanite,
magnetite
ilmenite, garnet
biotite, alkali
Hornblende,
pyrox., alkali
Clinopyroxene
amph.,magnetite,
biotite, magnetite
fluorite
Common Range
of Rock types
leucogranite
to granodiorite
alkali granite to
anorthosite
Associated
Volcanic Rocks
siliceous
ash flows
87Sr/86Sr
initial
granite to
gabbro
rhyollite ash
flows, dacite,
andesite
alkalic
rhyolites
quartz diorite
to gabbro
Calc-alkalic Andesite,
dacite, Rhyolite;
tholeiite
Sources: Chappell and White (1974), White and Chappell (1977, 1983), Loiselle and Wones (1979), White (1979), Collins et
al. (1982), Didier, Duthon, and Larneyre (1982), W. S. Pitcher (1982), McHone and Butler (1984), A. J. R. White et al.
(1986), J. B. Whalen, Chappell, and Chappell (1987), Barbarin (1990a), Eby (1990).
Alumina Saturation Index
The ratio Al2O3/(Na2O + K2O + CaO) is in terms of molar proportion. The related terminology is:
peraluminous: Al2O3 > (Na2O + K2O + CaO) or Al2O3/(Na2O + K2O + CaO) > 1.1; Rock will probably
have muscovite and may have garnet; will be corundum-normative.
metaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 > (Na2O + K2O) or Al2O3/(Na2O + K2O +
CaO)  1.0; rock may have hornblende
subaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 = (Na2O + K2O); Al2O3/(Na2O + K2O + CaO)
< 1.0; rock will probably have hornblende; will have Di in the norm.
peralkaline: Al2O3 < (Na2O + K2O) or Al2O3/(Na2O + K2O + CaO) << 1.0; rock will probably have lot
of K-feldspar in the norm, probably a feldspathoid or very little, if any quartz.
Tectonic Classification
A tectonic classification relates chemical and intrusive style of granitoids to periods of orogenic cycles
or periods of extension.
Some notes on Table above:
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M-type granites of island arcs are rare. They form by fractional crystallization of calc-alkaline or
tholeiitic magmas.
M-type granites of ocean ridges are extremely rare. They form by fractional crystallization of
calc-alkaline or tholeiitic magmas.
The Transitional Granitoids are also called Post-orogenic. Note that some granitologists also
refer to them as Anorogenic, because they form during extension.
IV. Petrogenesis
The granite problem (or, not really): During the first half of the 20th century, most people believed,
following Bowen’s idea of fractional crystallization, that granitoids represent the residual melts
formed by fractional crystallization of mafic magmas. This remains true of the M-type granites.
However, many mountain belts contain huge volumes of granitoids. To get that volume by
fractional crystallization would require >10 times greater volume of the mafic magmas. The huge
volumes of mafic magmas are not observed. Therefore, the general belief now is that granitoids
are generated primarily by the partial melting of various source rocks.
Relationship between tectonic setting, source rocks, and granitoid types
1) Active continental margin - Most prevalent are calc-alkaline I-type granitoids, including tonalites
and granodiorites. These are thought to form by partial melting of amphibolitic lower continental
crust or gabbroic rocks crystallized below the Moho. The mineralogy of the parent rocks is
dominated by amphibole and plagioclase. Magma mixing between the granitoids and basalts
intruded into them is common.
2) Continental collision - The most prevalent granitoids are S-type. They form by partial melting of
shales and graywackes. Some magmas bring up large amount of restite, pieces of the parent
rocks. In some cases they do not. The melts without restite usually contain <1% MgO, FeO, and
CaO; they are called leucogranites. Pegmatites are also common.
3) Post-orogenic extension - The source rocks for the granitoids are usually granulitic silicic rocks in the
lower crust. Formation of the silicic magma involves very high temperatures (~1000° C). The
high temperatures are achieved by intrusion of mafic magma from the mantle. Chemically bimodal volcanism is common, as is magma mixing in magma chambers.
Melt generation (anatexis)
Anatexis is a name given to the process that generates partial melts in a parent rock.
1) tonalites and granodiorites of active continental margins
The generation of these granitoids involves the following anatectic reactions in amphibolites or gabbros
in the lower crust or below the Moho:
33 hornblende + 5 plagioclase + 9 qtz  6 opx + 20 cpx + 19 tonalite melt
or
cpx + opx + 13 plagioclase  18 tonalite melt
These reactions produce melts high in Ca, Al, and Na that upon crystallization lead to rocks rich in
plagioclase with some quartz.
2) true granites in continental collisional orogens
The compositions of true granites is expressed well by the ternary NaAlSi3O8-KAlSi3O8-SiO2 system:
Note from the diagram on the right, that formation of true granites near a eutectic at high pressure
requires the presence of plagioclase, alkali feldspar and quartz. Plagioclase and quartz are usually found
in pelitic schists or metagraywackes. However, the required KAlSi3O8 component is found in muscovite
or biotite. Thus, generation of true granites involves the following incongruent melting reactions:
KAl2AlSi3O10(OH)2 + SiO2 + NaAlSi3O8  granite melt with H2O + Al2SiO5
K(Mg,Fe)3AlSi3O10(OH)2 + NaAlSi3O8  granite melt with H2O + 3(Fe,Mg)SiO3
Water is an important component in granite systems, because as shown in the above reactions, the
hydrous minerals contribute it to the melt.
The above and similar reactions can be illustrated in a P-T diagram appropriate for the conditions in the
continental crust:
The diagram also shows the granite solidus when free H2O is present (ab + ksp + qtz = L) and a typical
pressure-temperature-time (P-T-t) path of a portion of a deep crust during crustal thickening associated
with continental collision. A pelitic rock following this and similar paths will undergo partial melting to
generate an S-type granite when the melting reactions are intersected.
Migmatites
The process of granite formation is evident in migmatites. Migmatites are rocks from which granitoid
melts were not removed. The light parts of migmatites contain the solidified granitoid melts. The dark
parts of migmatites contain the residual minerals.
For partial melting of amphibolites or gabbros to produce the tonalites-diorite-granodiorite series (I-type
granites), the residual minerals are usually hornblende, cpx, opx, (± garnet).
For partial melting of metapelites or metagraywackes to produce S-type granites, the residual minerals
are usually biotite, aluminosilicate, garnet, Ca-rich plagioclase.
V. Magma Ascent
Style of Ascent
Up to approximately the late 1970’s, most people believed that granitic magmas ascended through the
crust as diapirs. Now, probably the majority of people believe that granitic magmas ascent through
dikes, as the magmas fracture the overlying rocks. Another possible mechanism is stoping, in which the
magmas ascent by breaking-up the overlying rocks.
Level of Emplacement in the Crust
The level of magma emplacement in the crust is to a large extent controlled by the amount of water in
the magma. Consider the diagram below that shows the amount of water that a granite melt must have
as a function of P and T:
The diagram shows the liquidi of a granite melt as a function of the amount of water dissolved in the
melt (thin lines) and the granite solidus. The liquidi essentially show how much water must be in the
melt at the P-T conditions defined by a given liquidus. If the magma has less water than indicated, it will
become solid. The diagram shows that a magma that is generated at a higher temperature will have
lower H2O content than a magma that is generated at a lower temperature. Upon adiabatic ascent
through the crust, the high-temperature, dry magma can reach the surface, whereas the lowertemperature magma with more H2O will hit the water-saturated solidus before reaching the surface.
Therefore, only relatively dry, high-temperature granitic magmas can lead to volcanism. Magmas that
initially contained a high concentration of H2O will be deep-seated. Note that S-type magmas, which
often form by relatively low-T, muscovite melting of metapelites are almost never volcanic. On the
other hand, high-T magmas that form during extension very often have volcanic centers.
VI. Magma Chamber Process
Granitoid magmas are generally too viscous to undergo significant accumulation of crystals, as often
happens in mafic magmas. However, two important mechanisms by which compositions of granitoid
magmas change occur in magma chambers – magma mixing, and side-wall fractional crystallization.
This diagram shows processes that typically occur in calc-alkaline granitoid magma chambers.
Magma mixing
In convergent orogens and during extension, mafic magmas from the mantle may go up the same
conduits as the granitoid magmas. Very often, the two types of melts end-up in the same magma
chamber. Usually, the mafic melt pool at the bottom of the magma chamber, because they are denser
than the silicic melts. However, because of convection in magma chambers, two contrasting melts may
mix, sometimes completely. It is possible that many diorites and granodiorites may be the products of
mixing mafic and silicic melts. A good example of the magma mixing process is the Tuolumne Meadows
magma series in the Yosemite National Park.
Side-wall (boundary layer) crystallization
This mechanism was proposed by Alexander McBirney in the 1980’s to explain the commonly inferred
chemical stratification in silicic magma chambers. For example, in the Bishop tuff, a thick pile of
extrusive rocks from the Long Valley volcanic complex, more evolved (silicic) compositions occur at
the bottom, whereas the more mafic varieties occur at the top of the erupted pile. The rocks lowest in the
pile crystallized from magma from the top of the underlying magma chamber and the top rocks represent
deeper parts of the magma chamber.
The chemical stratification that is thought to occur in magma chambers develops when magma begins to
crystallize from the margins inward. There, the local interstitial residual liquid becomes silica- and H2Orich. Because it is more buoyant than the crystals and melt further inside the chamber, it migrates along
the sidewall to the top of the magma chamber. The high H2O content of the magma at the top of the
magma chamber promotes explosive eruption, sometimes large enough to partially empty the chamber
and create a caldera.
VII. Pegmatites
Pegmatites are very low-viscosity hydrous melts. They can be residual melts resulting from fractional
crystallization of granites or partial melts from metapelites. They typically contain extremely large
crystals and are rich in rare elements such as Li, B, P, Be, Ta, Cs. These elements permit pegmatites to
crystallize at extremely low temperatures, down to ~350° C. Some minerals that occur in pegmatites
(aside from quartz, microcline, and albite, include:
spodumene: LiAlSi2O6 (pyroxene)
lepidolite: K(Li,Al)2-3AlSi3O10(OH)2 (mica)
beryl: Be3Al2Si6O18
tourmaline: (Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)6(BO3)3Si6O18(OH)4