1 Intrusive Igneous Rocks, part 2 Granite, Alkali Feldspar Granite, Granodiorite, Quartz Monzonite, and Monzonite.

1

Intrusive Igneous Rocks, part 2

Granite, Alkali Feldspar Granite, Granodiorite, Quartz Monzonite, and

Monzonite

2

IUGS Intrusive Igneous Rock Chart

3

“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 light-

colored, medium to coarse grained intrusive rock containing quartz

4

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

5

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

6

Back Arc and Non-Arc Environments

• Granitoid plutons also form in back-arc environments such as the ash-flow-caldera-ring-dike complexes seen in the southwestern United States

• Non-arc environments, such as the White Mountain Magma Series of New Hampshire, are also possible

7

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 deep-seated plutons

• Temporally, the more mafic batholiths are generally earlier than the more felsic batholiths

8

East vs. West

• Several major batholiths, including the Sierra-Nevada 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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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.

19

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

20

K-spar, Quartz in Granite

• Two of the essential minerals of granite

21

Orthoclase Photomicrographs• (Upper) Photomicrograph of

orthoclase in x-nicols showing the interference colors (first-order 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

22

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

23

Apatite Photomicrograph• Photomicrograph in PP

showing large apatite end section (indicated by arrows)

• Note: hexagonal shape• Greenish-brown

phenocryst is hornblende• Width of view is 0.85mm

•Apatite is almost always present in most igneous and metamorphic rocks, but in small amounts•Look closely at feldspar crystals on medium to high power to find apatite

24

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

25

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

26

Other Accessory Minerals in Granite

• Muscovite may be present, often as patches around the biotite

• Apatite may be present

27

Foliation in Granite

• Many granites show primary flow foliation, due to movement of magma before and during the crystallization process

28

Granite

• Location : Antinouri Lake, New Brunswick.

• K-fieldspar (pink), plagioclase (white), quartz(grey) and biotite (black).

• Texture: phaneritic• Photo: M.L. Bevier

29

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

30

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

31

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

32

Granodiorite

• Intrusive igneous, plutonic.

• Q = 20-60% quartz

• P/(A+P) is between 65-90%

• Accessory minerals hornblende or biotite

33

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

34

Granodiorite photo• Location : Yosemite

National Park California USA

• Photo: M.L. Bevier• Mafic schlieren in the

Cathedral Peak granodiorite, Cretaceous Tuolumne Intrusive Complex.

35

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

36

Granodiorite Hand Specimen

• Location : Tombstone Plutonic Complex, Pukelman Stock, Clear Creek District, Yukon Territories

• Photo: James Lang• K-feldspar megacrystic

granodiorite

37

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.

38

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

39

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

40

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)

41

Monzonite Photomicrograph• Subhedral clinopyroxene,

anhedral biotite and moderately sericitized plagioclase form a nearly equigranular texture containing an interstitial groundmass of K-Feldspar and minor albite

• Opaques are magnetite • Field of view is 5 mm

Location: Ann Property, near Lac La Hache, B.C.

Photo: Robin Whiteaker

42

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

43

Quartz Monzonite Mineralogy

• Plagioclase is sodic

• The mafic minerals are generally biotite, hornblende, and/or pyroxene (generally augite)

44

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

45

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.

46

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

47

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

48

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

49

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

50

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

51

Pegmatite and Aplite Photo

• Location : Phuket, Thailand

• Photo: Fletcher and Baylis

• Pegmatite and aplite dikes in granite

52

Pegmatite in Gneiss

• Location : Painted Wall, Black Canyon of the Gunnison River, Colorado

• Photo: M.L. Bevier• Pegmatites intruding

Precambrian gneiss

• This location was visited on SFC 2013

53

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.

54

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

55

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

56

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

• The evidence for this is given by a small inclusion of the granite enclosed in the basalt, visible just below the cord at left portion of photo.

Location : Pender Harbour, Southwest BCPhoto: C.A. Giovanella

57

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