Intrusive Igneous Rocks part 2 Granite Alkali Feldspar

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

IUGS Intrusive Igneous Rock Chart 2

“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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

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 13

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 14

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 Kspar host • The presence of these unusual ions may make the pegmatites ores 15

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 16

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 17

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. 18

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 19

K-spar, Quartz in Granite • Two of the essential minerals of granite 20

Orthoclase Photomicrographs • (Upper) Photomicrograph of orthoclase in x-nicols showing the interference colors (firstorder gray) - width of the field of view is 5. 5 mm • Photomicrograph of orthoclase in pp • This example is very clouded (clay alteration) rather than colorless - the width of the field of view is 5. 5 mm 21

Microcline photomicrographs • (Upper) Microcline in CN showing the characteristic tartan (gridiron)twinning, and low interference colors - width of the field of view is 5. 5 mm • (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. 5 mm 22

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. 85 mm rocks, but in small amounts • Look closely at feldspar crystals on medium to high power to find apatite 23

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 24

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 25

Other Accessory Minerals in Granite • Muscovite may be present, often as patches around the biotite • Apatite may be present 26

Foliation in Granite • Many granites show primary flow foliation, due to movement of magma before and during the crystallization process 27

Granite • Location : Antinouri Lake, New Brunswick. • K-fieldspar (pink), plagioclase (white), quartz(grey) and biotite (black). • Texture: phaneritic • Photo: M. L. Bevier 28

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 29

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 30

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 31

Granodiorite • • Intrusive igneous, plutonic. Q = 20 -60% quartz P/(A+P) is between 65 -90% Accessory minerals hornblende or biotite 32

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 33

Granodiorite photo • Location : Yosemite National Park California USA • Photo: M. L. Bevier • Mafic schlieren in the Cathedral Peak granodiorite, Cretaceous Tuolumne Intrusive Complex. 34

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 35

Granodiorite Hand Specimen • Location : Tombstone Plutonic Complex, Pukelman Stock, Clear Creek District, Yukon Territories • Photo: James Lang • K-feldspar megacrystic granodiorite 36

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. 37

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 38

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 39

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) 40

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 41

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 42

Quartz Monzonite Mineralogy • Plagioclase is sodic • The mafic minerals are generally biotite, hornblende, and/or pyroxene (generally augite) 43

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. 2 mm • Texture: porphyro - aphanitic. • Upper photo, CN; lower, PP 44

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. 45

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 46

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 47

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 48

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 49

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 50

Pegmatite and Aplite Photo • Location : Phuket, Thailand • Photo: Fletcher and Baylis • Pegmatite and aplite dikes in granite 51

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 2011 52

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. 53

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 54

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 55

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. 56

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 57