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Rock Types - Perils of Classification. In principle, a Rock Type has a narrowly defined composition and particular fabric. In practice, only a few major names are unambiguous and used uniformly by petrologists.

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Rock types perils of classification l.jpg
Rock Types - Perils of Classification

  • In principle, a Rock Type has a narrowly defined composition and particular fabric.

  • In practice, only a few major names are unambiguous and used uniformly by petrologists.

    Option 1: Adopt a flexible strategy for naming and classification because of the continuous chemical spectrum observed for igneous rocks on Earth.

    Option 2: Use IUGS approach of fixed, well-defined limits and well established and agreed upon names. This method results in several different classification schemes and diagrams for broadly different rock suites.

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Granitic Rocks

Quartz-rich felsic rocks collectively referred to as granitoids

3 special fabric categories:

PORPHYRY: Phorphyritic aphanitic to finely

phaneritic w/ abundant phenocrysts and

occurring in a pluton

APLITE: Fine grained phaneritic, leucocratic

(all fsp and qtz), typically found in thin dikes

PEGMATITE: Phaneritic rocks w/ highly

variable grain size. Individual xtals range

in size from cm’s to m’s.

Barker, 1979

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Gabbros and Ultramafic Rocks

GABBROS:Phaneritic rocks composed of plagioclase, pyroxene,

and olivine - compositionally similar to basalts

ULTRAMAFICS: Phaneritic rocks w/ <10 modal % felsic minerals

Le Maitre, 1989

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Whole Rock Chemistry Classification

  • Aphanitic and Glassy rocks - very old classification system developed prior to the advent of modern chemical analyses.

  • Example: Overlap in chemical compositions of Dacite and Andesite, but global average composition of each is distinct.

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Global Averages for Felsic Rocks

Shaded areas correspond to those of the IUGS diamond

Asterisks represent global average.

2864 analyses for andesite

and 727 analyses for dacite

Le Bas et al., 1992

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Mafic Rock Types

  • Diabase or Dolorite: rock of basaltic composition with a transitional grain size between phaneritic and aphanitic. Commonly occurs as dikes and sills.

  • Picrite: olivine-rich basalt or picrobasalt with MgO >18 wt.% and Na2O+K2O between 1 to 3 wt.%

  • Komatiite: similar to picrite, but low total alkalies (Na2O+K2O) and TiO2. Both are less than 1 wt.%

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CIPW Norm Calculations

  • Developed by Cross, Iddings, Pirsson, and Washington to determine a hypothetical mineral assemblage from whole-rock chemical analyses.

  • Useful to facilitate comparisons between basaltic rocks in which complex solid solutions in mineral phases tend to conceal whole-rock chemical variations.

  • Allows easy comparison between aphanitic and glassy rocks.

  • Allows comparison between mica and amphibole bearing rocks and those that do not contain hydrous phases, but are similar chemically.

NB that “norms” or normative abundance

refers to the calculated wt.% of a specific mineral

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IUGS Classification of Aphanitic and Glassy Rocks

Distinction between

Trachyte (Q <20%) and

Trachydacite (Q > 20%)

based on normative qtz

from a recalculation


The amount of normative

olivine distinguishes

Tephrite (<10%) from

Basanite (>10%)

Dotted line encloses 53%

of all rocks from the global


Le Maitre, 1989

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Silica Saturation I

  • CIPW norm emphasizes the concentration of silica in relation to other oxides -> assign SiO2 first to feldspars, then, pyroxenes, and finally to quartz.

  • Calculations done based on moles not weight percentages. Related to variations in the the SiO2 to MgO+FeO ratio and the SiO2 to Na2O ratios as shown below. This serves as a model for a crystallizing magma and illustrates the degree of silica saturation.

    (Mg,Fe)2SiO4 + SiO2 = 2(Mg,Fe)SiO3

    olivine orthopyroxene

    2:1 1:1

    NaAlSiO4 + 2SiO2 = NaAlSi3O8

    nepheline albite

    2:1 6:1

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Silica Saturation II

Silica-oversaturated: rocks contain Q (quartz or its polymorphs-

cristobalite and tridymite), such as granite

Silica-saturated: rocks contain Hy, but no Q, Ne, or Ol (no quartz,

feldspathoids, or olivine), such as diorite and andesite

Silica-undersaturated: rocks contain Ol and possibly Ne (Mg-

olivine and possibly feldspathoids, analcime, perovskite, melanite

garnet, and melilite), such as nepheline syenite

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Alumina Saturation I

Index based on Al2O3/(K2O + Na2O + CaO)

Ratio equals 1 for feldspars and feldspathoids

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Alumina Saturation II

  • Inherent weakness of either silica or alumina saturation classifications is the mobility of Na and K. These elements are easily mobilized and transferred out of a magma by a separate fluid phase. Preferential alkali loss may be inferred from the presence of metaluminous minerals as phenocryts (formed prior to extrusion) in a glassy matrix.

  • Si can also be mobilized in escaping steam.

  • Al tends to be less mobile.

    • Peralkaline rhyolites can be subdivided into:

      • Comendites: Al2O3 > 1.33 FeO + 4.4 (wt. %)

      • Pantellerites: Al2O3 < 1.33 FeO + 4.4 (wt. %)

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Alkaline and Subalkaline Rock Suites

15,164 samples

  • NaAlSiO4 + 2SiO2 = NaAlSi3O8

Irregular solid line defines the boundary between Ne-norm rocks

Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988

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Tholeiitic vs. Calc-alkaline Trends

Terms emerged from tangled history

spanning many decades. CA label

proposed by Peacock in 1931.

Tholeiite originated in mid-1800’s

from Tholey, western Germany.

Rocks show stronger Fe/Mg

enrichment than CA trend.

Tholeiites are commonly found

island arcs, while CA rocks

are more commonly found

in continental arcs.

Cole, 1982

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K2O content of subalkaline rocks

K2O content

may broadly

correlate with

crustal thickness.

Low-K 12 km

Med-K 35 km

High-K 45 km

Ewart, 1982

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Classification of Basalts

  • Three basalt types recognized based on their degree of silica saturation:

    • Quartz-hypersthene normative (Q + Hy)

      quartz tholeiite

    • Olivine-hypersthene normative (Ol + Hy)

      olivine tholeiite

    • Nepheline normative (Ne)

      alkaline basalt

  • Tholeiitic basalts make up the oceanic crust, continental flood basalt provinces, and some large intrusions.

  • Alkaline basalts are found in oceanic islands and some continental rift environments.

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Yoder & Tilley Basalt Tetrahedron

Yoder & Tilley, 1962; Le Maitre