GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 2

Classification of Igneous Rock

or

How many ways can you skin a rock?

Jan. 23, 2009

GOALS OF A ROCK CLASSIFICATION SCHEME

Practical – especially for field use Descriptive – minimal genetic terms Systematic – allows for easy information retrieval Hierarchical – allows for greater levels of detail Natural – defines boundaries that agree with nature

Most common schemes based on mode (vol. %) of “essential” primary minerals ...(When observation of primary minerals is not possible (e.g.,

too fine, altered) chemistry is commonly used)

...with modification by texture and structural features and occurrence of “accessory” and minor minerals

“ESSENTIAL” MINERALS

DARK MINERALS•Melanocratic•Mafic Minerals

LIGHT MINERALS•Leucocratic•Felsic Minerals

Color Index-CI=% Dark Minerals

Synonymous –Terms

Synonym

ousT

erms

Acid

Basic

Spinel

A SIMPLE (BUT LIMITING) CLASSIFICATION SCHEME

CLASSIFICATION OF PHANERITIC MAFIC IGNEOUS ROCKS RECOMMENDED BY THE

IUGS*

*International Union of Geological Sciences

troctolite

Ol gabbro

gabbro

PLOTTING ON TERNARY DIAGRAMS

Rock AMineral X – 59.5%Mineral Y – 17%Mineral Z – 8.5%Mineral A – 12%Mineral B – 3%

Normalization to XYZ onlyX+Y+Z = 85%

Xn = 59.5/85 = 70%Yn = 17/85 = 20%Zn = 8.5/85 =10%

CLASSIFICATION OF ULTRAMAFIC IGNEOUS ROCKS RECOMMENDED BY

IUGS

Olivine

ClinopyroxeneOrthopyroxene

Lherzolite

Har

zbur

gite

Wehrlite

Websterite

Orthopyroxenite

Clinopyroxenite

Olivine Websterite

Peridotites

Pyroxenites

90

40

10

10

Dunite

MAFIC ROCK CLASSIFICATION SCHEMES

APPLIED TO THE DULUTH COMPLEX

Phinney (1972a)Severson and Hauck (1990)

Davidson (1969a)

LeMaitre (1989)Streckeisen (1976)

ELEMENTS OF A PRACTICAL CLASSIFICATION SCHEME FOR DULUTH COMPLEX MAFIC ROCKS- uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and

Feox)- defines modal boundaries that bracket natural modal populations

and cotectic proportions determined from experimental data

- uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field.

From Miller (1986)

PREFERRED MODAL CLASSIFICATION

MODIFIERS – FOR A COMPLETE ROCK DESCRIPTION

Alteration

Foliation/Layering

Absolute Grain Size

Bulk Rock Texture –based on pyroxene habit

Accessory/Minor Minerals

For example:

Serpentinized, modally layered, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITE

Ophitic Ophitic-Subophitic

Subophitic Intergranular

Why PYROXENE-BASED BULK-ROCK TEXTURES?

Pl

Ol

Cpx

CLASSIFICATION OF INTERMEDIATE AND FELSIC ROCKS

Root RockName

Attach mafic mineral prefixto root rock name for intermediate rocks...

or “ferro” if uncertain of mafic mineral phase

From Streckeisen (1976)

Felsic

Intermediate(“ferro-”)

THE MAFIC TO INTERMEDIATE TRANSITION Typically based on average An content (> or < An50) Hyndman (1972) suggests also considering:

rock associations (diorite with more granitoid rocks, gabbro with more mafic rocks)

mafic mineral assemblage (diorite = hornblende or biotite pyroxene; gabbro = pyroxene olivine hornblende)

plagioclase color (diorite whitish or nearly so; gabbro - greenish gray to gray)

Blatt and Tracy (1995) suggest using diorite for rock that contains hornblende over pyroxene and that contain less than 35 modal percent high-temperature mafic minerals (olivine, pyroxene).

ProblemsCan’t “see” AN content in hand samplesEven DC felsic rocks are dominated by pyx over hb and bio

Suggested Field Criteria for DC

Intermediate MaficDark Mineral Habit prismatic, subprismatic granular to poikilitic Felsic Mesostasis >5 % <5 %

CLASSIFICATION OF VOLCANIC ROCKS RECOMMENDED BY THE 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

Basalt/Andesite Distinctions

Basalt AndesiteSiO2 < 52% > 52%

CI > 35% < 35%

Foid MineralsNephelineLeuciteKalsiliteAnalcimeSodalite

CHEMISTRY-BASED TAS CLASSIFICATION OF APHANITIC OR METAMORPHOSED IGNEOUS

ROCKS

CIPW NORMATIVE

CALCULATIONS

PSEUDO-MINERALOGY

Calculated from whole rock geochemical analyses;distributes major elements among rock-forming minerals

CLASSIFICATION OF PYROCLASTIC ROCKS