1 Intrusive Igneous Rocks, part 3 Syenite, Phonolite, Ijolite, Carbonatite, Ultramafic Rocks and Lamprophyre.

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Intrusive Igneous Rocks, part 3

Syenite, Phonolite, Ijolite, Carbonatite, Ultramafic Rocks and

Lamprophyre

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IUGS Intrusive Igneous Rock Chart

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General Intrusive Rocks Classification

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Position on IUGS Diagram• Syenites are on the border between the QAP and the

APF triangles• The remaining rocks are in the APF triangle• Phonolite is actually an extrusive or hypabyssal

equivalent of nepheline syenite• The ultramafic rocks and lamprophyre are very rich in

mafic minerals• Consequently they cannot be plotted on the main part of

the main IUGS diagram• They plot on Figure2-2c (p. 27, Winter) in a

separatetriangle

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Syenite

• Intrusive igneous, plutonic• Q = 0-5 • P/(A+P) is 10-35• Name is for Syene (near Aswan), Egypt - Pliny

the Elder named granite-like rocks from this area for the locality

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Syenite Mineralogy

• Major mineral is feldspar, with greater than 65% alkali feldspar (K-spar or albite) K-spar is typically orthoclase, microcline or perthite -

in dikes sanidine may be present

• The ferromagnesian minerals are usually < 20%• Biotite is often present and is usually brown

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Syenite Mineralogy

• Plagioclase, if present, is generally subhedral and is often zoned (normal or oscillatory)

• Small amounts of feldspathoids, like nepheline and sodalite, may be present - if more than 5% of these minerals are present, the rock is called nepheline (or sodalite) syenite

• Accessory minerals: Titanite, apatite, ilmenite, magnetite, zircon, and monzonite

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Syenite Photomicrograph• Process: Albite and K-spar are

mutually soluble in the crystalline state to a limited extent

• Mutual solubility is greater at high temperature

• As a high-temperature K-rich alkali feldspar cools past the solvus into the two-feldspar domain, the dissolved albite exsolves to form irregular lamellae of albite within the potassium-feldspar host (perthite)

Perthite texture in twinned alkali feldspar (perthite) crystals in Syenite [CN, medium power.]

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Syenite Hand Specimen Photo

• K-feldspar megacrysts• Pyroxene• Location: Brenner

Pluton, Yukon• Photo: James Lang

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Alkali Syenite

• Intrusive igneous, plutonic

• Q = 0-5

• P/(A+P) is 0-10

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Alkali Syenite Mineralogy

• A syenite rich in sodium, this rock has strongly perthitic K-spars or anorthoclase

• Plagioclase is albite to sodic oligoclase. • Mafic minerals include iron-rich biotite, iron or

sodic amphiboles including hastingsite, arfvedsonite, or riebeckite, and pyroxene is either aegirine-augite or aegirine.

• Accessory feldspathoids may be present

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Nepheline Syenite

• Intrusive igneous, plutonic• P/(A+P) is 10-35• F = 0-10• Foid syenite composed of granular aggregates,

typically orthoclase, microcline, microperthite, cryptoperthite, or albite.

• Other essential felsics: nepheline • Accessory feldspathoids: cancrinite, sodalite,

hauyne, or nosean

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Mafic Minerals in Nepheline Syenite

• Mafics are soda-rich • Amphiboles arfvedsonite, or hastingsite• Pyroxenes aegirine-augite, aegirine, or titanaugite

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Nepheline Syenite Photomicrograph• Large, simple-twinned crystal

of perthite (alkali feldspar exhibiting wispy exsolution lamellae)

• Equant, colorless crystals of nepheline. The turbid areas of the nepheline crystals are where it has been altered to sericite (fine-grained muscovite).

Slide also shows green alkali pyroxene (bottom left, interference color distorted by green body color) and altered biotite [CN, low power]

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Nephelinite • Nephelinite (alkaline mafic

lava) consisting of equant, low-birefringence crystals of nepheline and euhedral crystals of alkali pyroxene, often showing zoning, set in a glassy groundmass

• Presence of the feldspathoid nepheline indicates that the magma was silica-undersaturated

• CN, medium power

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Nephelinite• Nephelinite (alkaline

mafic lava) consisting of equant, sometimes 6-sided colorless crystals of nepheline and euhedral to subhedral greeny-brown crystals of alkali pyroxene

• PP, medium power

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Larvikite

• Intrusive igneous, plutonic• Pearl gray alkali syenite, sometimes opalescent,

which grades into monzonite• Typically coarse textured• The name is for the locality, Larvik, Norway

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Larvikite Mineralogy

• Prominent crystals are rhombs of very finely twinned sodic-plagioclase and alkali feldspar

• Mafics are diopsidic augite and titanaugite • Accessory apatite, ilmenite, titaniferous magnetite,

nepheline, olivine, or biotite

• Often used as a decorative stone because of a beautiful opalescence when polished

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Larvikite Polished Section Photo• Polished surface on coarse-

grained larvikite rock, a type of syenite from Norway - widely marketed under the trade name 'Blue Pearl'

• Composed mainly of large crystals of alkali feldspar (K + Na), with lesser amounts of dark minerals

• The large crystal in middle of photo is about 2 centimeters long, and shows a hint of concentric growth layers

• The rock is particularly attractive because the feldspar cleavage surfaces (visible as bright areas in photo) show as pearly pale blue iridescent flashes• Location: Larvik district, southeast coast of Norway

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Ijolite

• Intrusive igneous, plutonic to hypabyssal• F = 60-100• M = 30-70• Sodium > potassium• The name is for the locality parish Ijo, Finland

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Ijolite Mineralogy• A feldspathoid-rich rock, containing essential

nepheline (50-70%) • Essential: pyroxene, generally aegirine • Ijolites, carbonatites, and syenites are often

associated, and are often rich in alkali elements • The silicate and carbonatite minerals have similar

trace and minor element chemistries, including high values of Ba, Sr, Ce, La, Zr, P, and Cl

• Frequent host rocks for rare-earth and apatite minerals

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Carbonatite

• Intrusive igneous, hypabyssal• Igneous rock in which the carbonate minerals are

primary• Apatite and pyrochlore are often considered

essential• Accessory: Siderite, calcite, dolomite, or ankerite• Less common accessory minerals: Mg-rich

magnetite, phlogopite, and pyrite

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Carbonatite Occurrence

• Carbonatites occur as plugs with surface areas of up to 8 km2.

• They are enclosed in mafic alkaline rings.

• Mafic rocks include biotite pyroxenite, ijolite, nephelinite (35% nepheline, 65% mafic on average), and jacupirangite (rocks consisting of pure magnetite, magnetite with accessory pyroxene, pyroxene with accessory magnetite, or pyroxene and nepheline with biotite and olivine).

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Carbonatite Origin

• The similarity between silicate and carbonate phase trace and minor elements has convinced most petrologists that the carbonatites must originate together with the silicates, possibly from the same source, and not from limestones, as early workers had suggested

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Extrusive Rock Classification

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Phonolite

• Igneous extrusive, hypabyssal?

• The rock is usually a lava, but may occur in shallow dikes

• Name, from the Gr. phone, meaning sound Supposedly the rock rings when struck with a

hammer

• Other names are clinkstone and echodolite

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Phonolite Mineralogy

• An aphanitic rock composed of alkali feldspar, often anorthoclase or sanidine, nepheline, and mafics

• Other feldspathoids may replace nepheline

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Ultramafic Rocks

• Igneous intrusive, plutonic

• M 90

• Examples: dunite, peridotite, and pyroxenite

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Ultramafic Classification Diagram

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Mineralogy of Ultramafic Rocks

• Ultramafic rocks are composed of minerals near the top of Bowen’s Reaction series

• These minerals, especially iron-rich varieties, are prone to weathering.

• Serpentine minerals are common weathering products of olivine or pyroxene

• Serpentine may be accompanied by talc, another weathering product, or by magnesite, formed by the precipitation of MgCO3 from sea-water after the release of Mg ion

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Mineralogical Changes in Ultramafic Rocks

• Magnetite is a very common accessory mineral in felsic to mafic rocks

• In ultramafic rocks, almost all of the iron is associated with various mafic silicates minerals, so there is no iron oxide phase present

• Opaque accessory mineral is often chromite• If enough chromite is present, the rock may become an

ore of chromium• In hand specimen, chromite is mostly easily seen in

fresh specimens of dunite

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Mineralogical Changes in Ultramafic Rocks

• Titanium is often present, and titanoaugites or other titanium rich minerals may be present

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Pyroxenite

• Intrusive igneous, plutonic

• ol/(ol + cpx +opx) < 40%

• M 90

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Pyroxenite Mineralogy• Some pyroxenites contain more iron and are

predominantly composed of opx - either hypersthene or bronzite

• Bronzite is named for the noticeable bronze reflection (Schiller luster) that may be seen in hand specimen.

• If clinopyroxene is present it is often the variety diallage which shows well-developed parting parallel to {100} in the augite-diopside composition range.

• Ti-bearing augite or aegirine is also possible

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Pyroxenite Mineralogy

• Accessory minerals include: Hornblende, generally brown, may be present Chromite - Crystals may be diamond or square

shaped - crystals in hand specimen are small, seldom showing octahedral habit

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Clinopyroxenite

• This rock is dominated by the clinopyroxene augite

• Location: West Point, GA

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Bronzitite

• Intrusive igneous, plutonic • A type of pyroxenite composed almost exclusively

of bronzite • Accessory minerals may include olivine, picotite

(chromian spinel), chromite, hornblende. • Secondary serpentine may be present • Also called bronzite

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Bronzitite Photomicrograph• Almost all of the grains in

this photomicrograph are hypersthene

• This rock also contains some interstitial plagioclase, better seen in plane light.

Location: Stillwater Complex, Montana

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Hypersthene Photomicrograph• Orthopyroxenes are noted

for having low, first-order interference colors

• Also note the cleavages that intersect at about 90 degrees.

• Another identifying characteristic of orthopyroxene is its parallel extinction.

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Peridotite

• Intrusive igneous, plutonic • ol/(ol + cpx +opx) > 40% • M 90% • The mineralogy is similar to the pyroxenites,

except that a mica, phlogopite, is often present• The rock is feldspar free • Name - from the French name for olivine, péridot

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Dunite

• Dunite is olivine peridotite • Ol > 90%

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Dunite Photomicrograph

• Almost all of the grains in this rock are olivine - note the high order interference colors of olivine

• Secondary calcite occurs as veinlets through the sample

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Harzburgite• Intrusive igneous, plutonic• Harzburgite is a type of peridotite in which the

major minerals are orthopyroxene (generally near enstatite) and Mg-rich olivine.

• M 90% • ol/(ol+opx+cpx) = 40-90• cpx/(ol+opx+cpx) < 5%.

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Harzburgite

• Harzburgite is believed to represent depleted mantle material after the removal of a basaltic component

• Name - for the locality, Harzburg, Germany

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Chromitite

• Intrusive igneous, plutonic• Igneous rock composed of 95%+ of chromite • Similar rocks with larger percentages of

ferromagnesian minerals may be named for the mineral, e.g. olivine chromitite

• Rock is denser than most other ultramafic rocks

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Hornblendite

• Intrusive igneous, plutonic

• M 90%

• hb/(hb+px+ol) > 90%

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Kimberlite• Potassium-rich peridotites which usually occur as

“pipes” which originate in the mantle • Usually have an abundant volatile content, especially

CO2 and H2O

• Found in old (>2500 my), stable continental areas but are themselves much younger, often Mesozoic in age

• Kimberlite magmas intrude along deep fractures, sometimes extensions of ocean fracture zones

• Name - for the locality Kimberly, South Africa

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Kimberlite Xenoliths

• Kimberlites are most interesting scientifically because they carry a rich variety of xenoliths and xenocrysts

• Many are these xenoliths are believed to be part of the mantle or deep crust They are pieces that fell into the kimberlite magma during

its passage to the surface

• Xenoliths include garnet peridotites, lherzolites, and eclogites, which are believed to be samples of the mantle lithosphere and even the upper asthenosphere

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Kimberlite Mineralogical Alteration

• Kimberlites often show signs of alteration

• Olivine is transformed to serpentine, usually antigorite

• Phlogopite is transformed to chlorite, or vermiculite, with accompanying magnetite and/or calcite

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Kimberlite breccia

• Kimberlites may also occur as breccia This probably represents turbulent

emplacement, where the magma is mixed with many different kinds of xenoliths

• Xenoliths from depth are rounded, probably as the result of wear during the ascent, and the upper xenoliths are more angular

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Kimberlite Composition• Intrusive igneous, dike or pipe• Kimberlite is a porphyritic peridotite or peridotite

breccia• Essential: Olivine, usually altered to serpentine, and

phlogopite (commonly chloritized)• Accessory: carbonates• Groundmass is usually calcite, serpentine, chlorite,

phlogopite and accessory minerals such as chromian pyrope, ilmenite, magnetite, and perovskite

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Lherzolite Photomicrograph

• The picture shows olivine (right side of picture), orthopyroxene (lower left) and clinopyroxene (upper left)

• Lherzolites contain over 60% olivine, and some clinopyroxene and orthopyroxene

• Lherzolite is a major constituent of the mantle and the sourceof most basaltic magmas.

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Lamprophyre

• Lamprophyre chemical composition is very similar to that of alkaline olivine basalt

• Lamprophyres are widespread but volumetrically insignificant rocks

• Occurrence: As dikes, often cutting granitic batholiths or stocks - dikes may be radiating or may form a dike swarm of sub-parallel dikes

• Dikes are small - a few meters, at most, thick, but may be miles long

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Lamprophyre Composition

• Fine-grained, dark-colored, often containing mafic phenocrysts, but never felsic phenocrysts

• Groundmass is predominantly felsic and may include plagioclase, nepheline, sanidine, melalilite ([(Na,Ca)2(Mg,Al)(Al,Si)2O7], and analcite (NaAlSi2O6.H2O), a zeolite

• Xenoliths of country rock are common

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Lamprophyre Parent Rock

• Parent material of lamprophyres is probably alkaline olivine basalt

• Movement of alkaline olivine basalt magma toward the surface might induce decompressive melting, and the resulting magma could be injected into fractures resulting from compressive failure

• Mafic phenocrysts would have formed early, before injection

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Lamprophyre Composition

• Intrusive igneous, diaschist• Lamprophyre rocks are melanocratic or mesotype

hypabyssal rocks • Mafic phenocrysts occur in a fine-grained

groundmass• Name, from the Greek lampros, meaning bright, in

allusion to the bright reflection from biotite phenocrysts found in some lamprophyres

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Lamprophyre Mineralogy

• Mafic minerals are olivine (or serpentine), biotite, hornblende (usually green), and pyroxene (green diopside or Fe-Ti augite, which is purplish)

• Felsic minerals are often alkaline, such as alkali feldspar (K-spar or albite) and Na-rich nepheline

• Plagioclase is common, while quartz may be present

• Often associated with carbonatites

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Nepheline Diorite Lamprophyre

• Intrusive igneous, diaschist

• A dioritic lamprophyre is one in which the major feldspar is plagioclase

• In addition this rock contains the feldspathoid nepheline

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Lamprophyre Photomicrograph

• Lamprophyre (alkaline mafic dike-rock) with microphenocrysts of biotite and pyroxene

• CN