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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

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Geol 2312 Igneous and Metamorphic Petrology

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Geol 2312 igneous and metamorphic petrology l.jpg

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 l.jpg

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 l.jpg

Color Index-CI

=% Dark Minerals

“Essential” Minerals

  • DARK MINERALS

  • Melanocratic

  • Mafic Minerals

  • LIGHT MINERALS

  • Leucocratic

  • Felsic Minerals

Synonymous –

Terms

Synonymous

Terms

Spinel

Basic

Acid


A simple but limiting classification scheme l.jpg

A SIMPLE (but limiting) CLASSIFICATION SCHEME


Classification of phaneritic mafic igneous rocks recommended by the iugs l.jpg

Classification of Phaneritic Mafic Igneous Rocks Recommended by the IUGS*

troctolite

gabbro

Ol gabbro

*International Union of Geological Sciences


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Plotting on Ternary Diagrams

Rock A

Mineral X – 59.5%

Mineral Y – 17%

Mineral Z – 8.5%

Mineral A – 12%

Mineral B – 3%

Normalization to XYZ only

X+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 l.jpg

Olivine

Dunite

90

Peridotites

Wehrlite

Lherzolite

Harzburgite

40

Pyroxenites

Olivine Websterite

Orthopyroxenite

10

Websterite

10

Orthopyroxene

Clinopyroxene

Clinopyroxenite

Classification of Ultramafic Igneous Rocks Recommended by IUGS


Mafic rock classification schemes applied to the duluth complex l.jpg

Mafic Rock Classification Schemes Applied to the Duluth Complex

Streckeisen (1976)

LeMaitre (1989)

Phinney (1972a)

Davidson (1969a)

Severson and Hauck (1990)


Elements of a practical classification scheme for duluth complex mafic rocks l.jpg

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 l.jpg

Preferred Modal Classification


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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


Slide12 l.jpg

Why PYROXENE-BASED BULK-ROCK TEXTURES?

Ophitic-Subophitic

Ophitic

Subophitic

Intergranular

Pl

Ol

Cpx


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Classification of Intermediate and Felsic Rocks

Intermediate

(“ferro-”)

Felsic

From Streckeisen (1976)

Root

Rock

Name

Attach mafic mineral prefix

to root rock name for intermediate rocks...

or “ferro” if uncertain of

mafic mineral phase


The mafic to intermediate transition l.jpg

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).

    Problems

    Can’t “see” AN content in hand samples

    Even DC felsic rocks are dominated by pyx over hb and bio

    Suggested Field Criteria for DC

    IntermediateMafic

    Dark Mineral Habitprismatic, subprismaticgranular to poikilitic

    Felsic Mesostasis >5 % <5 %


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Classification of Volcanic Rocks Recommended by the IUGS

Basalt/Andesite Distinctions

BasaltAndesite

SiO2< 52% > 52%

CI > 35% < 35%

Foid Minerals

Nepheline

Leucite

Kalsilite

Analcime

Sodalite


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Chemistry-Based TAS Classification of Aphanitic or Metamorphosed Igneous Rocks


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CIPW Normative CalculationsPseudo-mineralogy

Calculated from whole rock geochemical analyses;

distributes major elements among rock-forming minerals


Classification of pyroclastic rocks l.jpg

Classification of Pyroclastic Rocks


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