Classification of igneous rocks

Classification of igneous rocksClassification of igneous rocks

Phaneritic: Phaneritic: crystals visible with naked eyecrystals visible with naked eye

Plutonic Plutonic oror intrusive rocks intrusive rocks

Aphanitic: Aphanitic: crystal too small for naked eyecrystal too small for naked eye

Volcanic Volcanic oror extrusive rocks extrusive rocks

Phaneritic

Aphanitic

Porphyritic: Porphyritic: two different, two different, dominant grain sizesdominant grain sizes

Large Crystals = Large Crystals = phenocrysts; Small Crystals Small Crystals = groundmass

Fragmental: Fragmental: composed of disagregated igneous material Pyroclastic rocks

Porphyritic

Pyroclastic

Pegmatitic: Pegmatitic: very large xtals (cm to 10s of cm); i.e., very large xtals (cm to 10s of cm); i.e., slowlyslowly cooled cooledForms veins or layers within plutonic bodyForms veins or layers within plutonic body

Glassy: Glassy: non-crystalline; cools very fast (e.g., non-crystalline; cools very fast (e.g., obsidian)obsidian)Volcanic rocksVolcanic rocks

Vesicular:Vesicular: vesicles (holes, pores, cavities) form as vesicles (holes, pores, cavities) form as gases expandgases expandVolcanic rocksVolcanic rocks

Pegmatitic

Glassy

Vesicular

Classification of Igneous RocksClassification of Igneous Rocks

Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

Pyroclastic rocks

Compositional terms for igneous rocksCompositional terms for igneous rocks

Felsic: Felsic: felfeldspar + dspar + sisilica lica

~55-70% silica, K-feldspar > 1/3 of feldspars present~55-70% silica, K-feldspar > 1/3 of feldspars present

light-colored silicate minerals — Continental crustlight-colored silicate minerals — Continental crust

Intermediate:Intermediate: between felsic and mafic between felsic and mafic

~55-65% silica, plag > 2/3 of feldspars present~55-65% silica, plag > 2/3 of feldspars present

Na-rich plag predominates over Ca-rich plagNa-rich plag predominates over Ca-rich plag

Mafic: Mafic: mamagnesium + gnesium + feferric ironrric iron

~45-50% silica; Ca-rich plag dominant feldspar~45-50% silica; Ca-rich plag dominant feldspar

dark silicate minerals — Oceanic crustdark silicate minerals — Oceanic crust

Ultramafic: Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars>90% mafic minerals, silica < 45%, few or no feldspars

Mantle-derivedMantle-derived

Classification of common igneous rocksClassification of common igneous rocks

CompositionComposition PhaneriticPhaneritic AphaniticAphanitic Color indexColor index(% dark minerals)(% dark minerals)

FelsicFelsic GraniteSyeniteMonzonite

RhyoliteTrachyteLatite

101520

IntermediateIntermediate GranodioriteDiorite

Dacite Andesite

2025

MaficMafic Gabbro Basalt 50

UltramaficUltramafic Peridotite 95

Mineralogical terms of Igneous RocksMineralogical terms of Igneous Rocks

Mineralogical terms of Igneous RocksMineralogical terms of Igneous Rocks

Classification of Igneous RocksClassification of Igneous Rocks

. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.

Classification of Classification of Phaneritic Phaneritic

Igneous RocksIgneous Rocks

Figure. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.

The rock must contain a total ofat least 10% of the minerals below.Renormalize to 100%

(a)

Quartz-richGranitoid

9090

6060

2020Alkali Fs.Quartz Syenite Quartz

SyeniteQuartz

MonzoniteQuartz

Monzodiorite

Syenite Monzonite Monzodiorite

(Foid)-bearingSyenite

5

10 35 65

(Foid)-bearingMonzonite

(Foid)-bearingMonzodiorite

90

Alkali Fs.Syenite

(Foid)-bearingAlkali Fs. Syenite

10

(Foid)Monzosyenite

(Foid) Syenite

(Foid)Monzodiorite

(Foi

d) G

abbr

o

Qtz. Diorite/Qtz. Gabbro

5

10

Diorite/Gabbro/Anorthosite

(Foid)-bearingDiorite/Gabbro

60

(Foid)olites

Quartzolite

Granite Grano-diorite

Tonalite

Alka

li Feld

spar

Gra

nite

Q

A P

F

60

Plutonic rocks

Classification of Classification of AphaniticAphanitic

Igneous RocksIgneous Rocks

A classification and nomenclature of volcanic rocks. After IUGS.

(foid)-bearing Trachyte

(foid)-bearing Latite

(foid)-bearing Andesite/Basalt

(Foid)ites

10

60 60

35 65

10

20 20

60 60

F

A P

Q

Rhyolite Dacite

Trachyte Latite Andesite/Basalt

Phonolite Tephrite

Volcanic rocks

Classification of Igneous RocksClassification of Igneous Rocks

Figure. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.

Plagioclase

OlivinePyroxene

Olivine gabbro

Plagioclase-bearing ultramafic rocks

90

(b)

Anorthosite

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

te

Wehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

(c)

Gabbroicrocks

Ultramaficrocks