Transcript Granite, Alkali Feldspar Granite, Granodiorite

Intrusive Igneous Rocks, part 2
Granite, Alkali Feldspar Granite,
Granodiorite, Quartz Monzonite, and
Monzonite
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IUGS Intrusive Igneous Rock
Chart
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“Granite”
• Rocks in this lab are plutonic
 They formed by very slow cooling and show
corresponding large crystals
 Most of the rocks are medium to coarse grained
• These rocks are often collectively called “granite”
• When used in this sense, granite means any lightcolored, medium to coarse grained intrusive rock
containing quartz
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Granite Batholiths
• Granite batholiths are common and are often huge
• Examples include:
 Sierra Nevada batholith
 Idaho batholith (which extends into western Montana)
 Coast Range Plutonic Complex of western British
Columbia
 Alaska-Aleutian Range batholith of Alaska, among many
others
• All major batholith complexes are composite,
consisting of up to 100 or more discrete smaller
batholiths or stocks
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Geologic Environment and
Composition
• Major granitoid batholiths are found in a tectonic
environment similar to arc volcanic rocks
• However, they are restricted to continental crust
• When the underlying crust is basaltic (oceanic), the
plutons are more mafic, often dioritic
• When the underlying crust is thick and felsic
(continental), plutons are large and dominantly tonalite
to granodiorite to granite in composition
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Back Arc and Non-Arc
Environments
• Granitoid plutons also form in back-arc
environments such as the ash-flow-calderaring-dike complexes seen in the
southwestern United States
• Non-arc environments, such as the White
Mountain Magma Series of New
Hampshire, are also possible
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Time of emplacement
• Major batholith emplacement takes place over a
few tens of millions of years and is generally
temporally associated with folding (synorogenic)
• Regional metamorphism is commonly associated
with granitoid plutons, especially the major deepseated plutons
• Temporally, the more mafic batholiths are
generally earlier than the more felsic batholiths
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East vs. West
• Several major batholiths, including the SierraNevada and Idaho, are characteristically quartz
diorite or tonalite along their western boundaries,
with a rapid transition to granodiorite or granite in
the major part of the batholith and the eastern
boundary
• The western boundary rocks are also more mafic
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East vs. West Continued
• Western boundary rocks are emplaced in basaltic
to andesitic and related sedimentary rocks, or
rocks associated with oceanic to island-arc
environments
• The bulk of the batholith is more felsic, and is
emplaced in nonvolcanogenic rocks, primarily
metamorphosed shales and sandstones and
quartzofeldspathic basement rocks of old
continental crustal environments
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Quartz-Diorite Line
• J.G. Moore has suggested that a “quartz-diorite
line” exists in the batholiths of the Western U. S.
and Canada
 Divides rocks of dominantly quartz diorite
composition on the oceanic side of the batholith from
the granodiorite to granite composition on the eastern
side of the batholith
 The division is nearly complete in most
batholiths
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Granitic Magmas
• Granitic magmas often are formed by the
melting of sediments, and are quite felsic
• Granitic rock:
 Essential minerals: quartz, K-spar, and
plagioclase feldspar, in varying proportions
 Accessory minerals: Small amounts of biotite,
hornblende or other silicates may be present
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Pegmatite-Aplite Formation
• Granitic magmas are generally wet
• As crystallization proceeds, most of the water
remains in the magma
• Early formed minerals in the mafic branch of
Bowens reaction series are anhydrous
• The feldspar branches are also anhydrous
• Thus, much of the water in the original magma will
be concentrated in the “residual liquid”, the less than
10% or so of the liquid remaining after
crystallization nears completion
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Pegmatite Formation
• These very water-rich residual liquids are
much less viscous than the original magma
• Crystal size increases, often in the coarse to
very coarse range
• Resulting rock is a pegmatite
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Enrichment in Pegmatites
• Residual liquids are always enriched in
water, and often in other substances
 Volatile substances, including carbon dioxide,
sodium, and potassium may be enriched
 Very large cations, such as uranium, barium,
lead, etc. may be enriched
 Very small cations with low charge, such as
lithium (+1) or beryllium (+2) may also be
enriched
 Rare earth elements may also be enriched
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Pegmatite Enrichment Continued
• These elements are usually difficult to
accommodate in the major minerals of granitic
rocks
• They may form their own, generally rare, minerals
in the last stages of crystallization
• Sometimes the large cations will be present in a
K-spar host
• The presence of these unusual ions may make the
pegmatites ores
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Aplite Formation
• As residual-liquids approach the surface,
they often experience a rapid loss of
volatiles
• This results from an encounter with a
fracture or fault directly connected to the
surface
• Volatiles, especially water, separate and
may form a gas phase
• Gas phase pushes the liquid rapidly upward
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Aplite Formation, Continued
• Loss of volatiles:
 Greatly increases viscosity
 Lowers the temperature of the liquid due to the heat of
vaporization of the gas phase
 Results in rapid crystallization of a very fine-grained,
felsic rock known as aplite
• Aplites are usually associated with the margins of
pegmatite intrusions, especially pegmatite dikes
which intersect the surface
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Granite
• Intrusive igneous, plutonic.
• Medium to coarse-grained
• Essential Minerals quartz, alkali feldspar,
plagioclase feldspar
• Q = 20% to 60% quartz,
• P/A+P = 10 to 65%
• Accessory minerals biotite, hornblende or
other silicates
• Name: From the Latin granum, meaning
grain.
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Granite Mineralogy
• K-spar may be orthoclase and/or microcline
• Plagioclase is sodic, either oligoclase or
andesine
• Quartz is almost always anhedral in granites
• Quartz grains often contain inclusions
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K-spar, Quartz in Granite
• Two of the essential minerals of granite
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Orthoclase Photomicrographs
• (Upper) Photomicrograph of
orthoclase in x-nicols showing
the interference colors (firstorder gray) - width of the field
of view is 5.5mm
• Photomicrograph of orthoclase
in pp
• This example is very clouded
(clay alteration) rather than
colorless - the width of the field
of view is 5.5mm
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Microcline photomicrographs
• (Upper) Microcline in CN showing
the characteristic
tartan (gridiron)twinning, and low
interference colors - width of the
field of view is 5.5mm
• (Lower) Microcline in PP showing
its lack of color, although this
sample is extremely clouded (clay
alteration) - width of the field of
view is 5.5mm
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Apatite Photomicrograph
• Photomicrograph in PP
showing large apatite end
section (indicated by
arrows)
• Note: hexagonal shape
• Greenish-brown
•Apatite is almost always present in phenocryst is hornblende
most igneous and metamorphic • Width of view is 0.85mm
rocks, but in small amounts
•Look closely at feldspar crystals
on medium to high power to find
apatite
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Perthites in Granite
• Alkali feldspar is commonly microcline
perthite (mixture of microcline and
plagioclase)
• In some perthites, the albite and K-feldspar
are completely separated, possibly due to
recrystallization
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Mafic Minerals in Granite
• Biotite is usually brown or brownish green
and often contains inclusions
• Hornblende is dark green and pleochroitic
• Biotite may form a reaction rim around the
hornblende
• If pyroxene (diopside) is present,
hornblende may form a rim around the
pyroxene
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Other Accessory Minerals
in Granite
• Muscovite may be present, often as patches
around the biotite
• Apatite may be present
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Foliation in Granite
• Many granites show primary flow foliation,
due to movement of magma before and
during the crystallization process
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Granite
• Location : Antinouri
Lake, New Brunswick.
• K-fieldspar (pink),
plagioclase (white),
quartz(grey) and biotite
(black).
• Texture: phaneritic
• Photo: M.L. Bevier
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Granite Photomicrograph
• Field of view 4 mm., CN
• Microcline shows gridiron
twinning
• Clear (white), anhedral
mineral is quartz
• Partially altered, smaller
grains are plagioclase
• Bright mineral in the upper
right is biotite
• Typical mineral assemblage
in granites
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Alkali Feldspar Granite
•
•
•
•
•
Intrusive igneous, plutonic
Medium to coarse-grained
Q = 20% to 60% quartz,
P/(A+P) ratio < 10
Small amounts of biotite, hornblende or
other silicates
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Alkali Feldspar Granite
Mineralogy
• The plagioclase is often albite or sodic
oligoclase
• K-spars are strongly perthitic or anorthoclase
• Biotite, if present, is iron-rich
• Amphiboles include hastingsite, arfvedsonite,
or riebeckite
• Pyroxenes include aegirine-augite or aegirine
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Granodiorite
•
•
•
•
Intrusive igneous, plutonic.
Q = 20-60% quartz
P/(A+P) is between 65-90%
Accessory minerals hornblende or biotite
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Granodiorite Mineralogy
• The plagioclase is typically andesine or oligoclase,
often zoned
• Zoning may be oscillatory - thin shells with abrupt
borders, alternating in composition
• Plagioclase is euhedral to subhedral, rectangular in
form, with borders corroded by quartz and alkali
feldspar
• The alkali feldspar is often orthoclase or
orthoclase perthite, but may be microcline
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Granodiorite photo
• Location : Yosemite
National Park California
USA
• Photo: M.L. Bevier
• Mafic schlieren in the
Cathedral Peak
granodiorite, Cretaceous
Tuolumne Intrusive
Complex.
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Schlieren
• Tabular bodies, generally a few inches to tens
of feet long, that occur in plutonic rocks
• Same general mineralogy as the plutonic rocks
 Because of differences in mineral ratios they are
darker or lighter
 Boundaries with the rock tend to be transitional
• Some schlieren are modified inclusions, others
may be segregations of minerals
• Etymol: German for a flaw in glass due to a
zone of abnormal composition
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Granodiorite Hand Specimen
• Location : Tombstone
Plutonic Complex, Pukelman
Stock, Clear Creek District,
Yukon Territories
• Photo: James Lang
• K-feldspar megacrystic
granodiorite
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Granodiorite Photomicrograph
• Field of view 4 mm across.
• Highly twinned mineral is
plagioclase
• Clear, untwinned, white to
pale yellow mineral is quartz
• Orthoclase is the grain in the
upper left corner
• Hornblende grain (with high
relief) is in the upper right
corner.
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Mafic Mineralogy of
Granodiorite
• Hornblende is the most common mafic
mineral, followed by biotite
• Hornblende may occur in prisms or in
ragged plates
• Pyroxene is rare
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Monzonite
• Intrusive igneous, plutonic
• The feldspar in these rocks is between ⅓
and ⅔ microcline or orthoclase, with the
remainder being plagioclase
• P/(A+P) is 35-65
• Q = 0-5
• The name is for Monzoni in the Tyrolean
Alps
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Monzonite Mineralogy
• Plagioclase is sodic
• Zoning of the plagioclase is fairly common best developed when monzonite occurs in
stocks or on the borders of batholiths.
• Mafic minerals usually comprise 10-40% of the
rock and are generally biotite, hornblende,
and/or pyroxene (generally augite)
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Monzonite Photomicrograph
• Subhedral clinopyroxene,
anhedral biotite and
moderately sericitized
plagioclase form a nearly
equigranular texture
containing an interstitial
Location: Ann Property,
groundmass of K-Feldspar
near Lac La Hache, B.C.
and minor albite
• Opaques are magnetite
Photo: Robin Whiteaker • Field of view is 5 mm
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Quartz Monzonite
• Intrusive igneous, plutonic.
• The feldspar in these rocks is between ⅓
and ⅔ microcline or orthoclase, with the
remainder being plagioclase
• P/(A+P) is 35-65
• Q = 5-20 quartz.
• An alternative name is adamellite
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Quartz Monzonite Mineralogy
• Plagioclase is sodic
• The mafic minerals are generally biotite,
hornblende, and/or pyroxene (generally
augite)
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Quartz Monzonite
Photomicrograph
• Location : Crested Butte,
Colorado (Crested Butte
quartz monzonite porphyry. )
• Photo M.L. Bevier
• Biotite phenocrysts
• Resorbed quartz crystals.
• Width of view is 3.2mm
• Texture: porphyro - aphanitic.
• Upper photo, CN; lower, PP
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Quartz Monzonite
Photomicrograph
• Quartz monzonite is a rather
abundant rock type in many
orogenic areas
• Albite twinned plagiocase
• Bright white perthite
• Clear quartz
• Minor biotite.
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Pegmatite
• Intrusive igneous, hypabyssal and
diaschistic
• An exceptionally coarse grained rock,
generally of granitic composition
• Grain size is often very uneven
• The name is from the Greek pegma,
meaning framework
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Diaschistic
• Said of the rock of a minor intrusion that
consists of a differentiate, i.e. its
composition is not the same as that of the
parent magma
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Pegmatite Mineralogy
• Composition is quite variable
• May consist exclusively of quartz and K-spar, or
may include many accessory minerals
• Possible accessory minerals include rare minerals
with the following ions: beryllium, boron,
chlorine, fluorine, lithium, molybdenum, niobium,
phosphorous, sulfur, tantalum, tin, tungsten,
uranium, zirconium, and the rare earth elements
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Alternative Pegmatite
Composition
• Although generally of granitic composition,
pegmatite facies of many other plutonic
rocks are known
• The name of the other rock type is than used
as an adjective, i.e. ijolite pegmatite
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Pegmatite Occurrence
• Found in tabular dikes, lenses, or veins
• Pegmatites commonly form at the margins
of batholiths and represent the last, most
hydrous portions of the magma to
crystallize
• Pegmatites may grade rapidly into aplites if
the volatiles are lost
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Pegmatite and Aplite Photo
• Location : Phuket,
Thailand
• Photo: Fletcher and
Baylis
• Pegmatite and aplite
dikes in granite
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Pegmatite in Gneiss
• Location : Painted
Wall, Black Canyon of
the Gunnison River,
Colorado
• Photo: M.L. Bevier
• Pegmatites intruding
Precambrian gneiss
• This location will be visited on SFC 2012
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Aplite
• Intrusive igneous, hypabyssal and
diaschistic
• Light-colored hypabyssal igneous rock
characterized by fine, anhedral grains
• Colors include white, cream, yellow,
reddish, or gray
• Term is derived from a Greek haploos,
simple, referring to the simple composition.
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Aplite Composition
• The term "aplite' used without modifier,
generally means a rock similar in
composition to granite, with the essential
minerals being quartz, K-feldspar, and sodic
plagioclase
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Aplite Composition, Continued
• However, the term "aplite" is sometimes
used to represent fine-grained igneous rocks
phases whose composition ranges from
granitic to gabbroic
• Rock names are usually used as adjectives,
i.e. gabbroic aplite
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Aplite Photo
• The thin pink (aplite) dike cuts
across or intrudes the other two
rocks, hence it is the youngest of
the three units present
• The black (basaltic) rocks are
also dikes, intruding the granite,
which must then be the oldest
unit
Location : Pender
• The evidence for this is given by
Harbour, Southwest BC
a small inclusion of the granite
Photo: C.A. Giovanella
enclosed in the basalt, visible
just below the cord at left portion
of photo.
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Association with Pegmatite
• Aplites are often associated with
pegmatites, either as border zones around
the pegmatite, or as aplitic dikes.
• Aplites form by the rapid loss of volatiles
from a late-stage water-rich magma
• Loss of volatiles causes a rapid
crystallization into many small crystals
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