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 extrusive igneous 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. Pegmatite Photo Haapaluoma, Finland. Width of sample 15 cm.
Tourmaline pegmatite. Tourmaline is black, white is plagioclase, gray is quartz. Haapaluoma, Finland. Width of sample 15 cm.
Source:
Haapaluoma, Finland. Width of sample 15 cm.

17. 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

18. 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

19. Aplite- Pegmatite
Collected by Richard Shaw, who described it as “a gorgeous slab of granite from Megiliggar Rocks in south-west England, displaying fantastic aplite-pegmatite layering.”
Source:
Shaw is an economic geologist

20. 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.

21. 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

22. K-spar, Quartz in Granite
Two of the essential minerals of granite

23. 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
Upper photo# min30m.jpg
Lower photo# min31m.jpg

24. 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

25. 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
Photo #mlb85m.jpg

26. 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

27. 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

28. Other Accessory Minerals in Granite
Muscovite may be present, often as patches around the biotite
Apatite may be present

29. Foliation in Granite
Many granites show primary flow foliation, due to movement of magma before and during the crystallization process

30. Granite Location : Antinouri Lake, New Brunswick.
K-fieldspar (pink), plagioclase (white), quartz(grey) and biotite (black).
Texture: phaneritic
Photo: M.L. Bevier
Photo #mlb88m.jpg

31. 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
Photo #a21.gif

32. 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

33. 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

34. Granodiorite Intrusive igneous, plutonic. Q = 20-60% quartz
P/(A+P) is between 65-90%
Accessory minerals hornblende or biotite

35. 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

36. Granodiorite photo Location : Yosemite National Park California USA
Photo: M.L. Bevier
Mafic schlieren in the Cathedral Peak granodiorite, Cretaceous Tuolumne Intrusive Complex.
Photo #mlb11m.jpg

37. 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

38. Granodiorite Hand Specimen
Location : Tombstone Plutonic Complex, Pukelman Stock, Clear Creek District, Yukon Territories
Photo: James Lang
K-feldspar megacrystic granodiorite
Photo # jl_pos9m.jpg

39. 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.
Photo #a22.gif

40. 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

41. 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

42. Tyrolean Alps
Monzoni is actually in Italy (South Tyrol), north of Trento

43. 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)

44. 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
Photo #rw9m.jpg

45. 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

46. Quartz Monzonite Mineralogy
Plagioclase is sodic
The mafic minerals are generally biotite, hornblende, and/or pyroxene (generally augite)

47. 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
Upper photo, mlb78m.jgp
Lower photo, mlb77m.jgg

48. 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.
Photo #a27.gif

49. 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

50. 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

51. 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

52. 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

53. 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

54. Pegmatite and Aplite Photo
Location : Phuket, Thailand
Photo: Fletcher and Baylis
Pegmatite and aplite dikes in granite
Photo # wkf26m.jpg

55. Pegmatite in Gneiss
Location : Painted Wall, Black Canyon of the Gunnison River, Colorado
Photo: M.L. Bevier
Pegmatites intruding Precambrian gneiss
Photo #mlb12m.jpg
This location was visited on SFC 2013

56. 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.

57. 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

58. 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

59. Aplite Photo Location : Pender Harbour, Southwest BC
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 BC
Photo: C.A. Giovanella

60. 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