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 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 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 will be visited on SFC 2012

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 BC Photo: 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