Rock Types Perils of Classification

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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.
• 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 cms to ms.
Barker, 1979
3
Gabbros and Ultramafic Rocks
GABBROSPhaneritic rocks composed of plagioclase,
pyroxene, and olivine - compositionally similar
to basalts ULTRAMAFICS Phaneritic rocks w/ 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 Na2OK2O between 1 to 3 wt.
• Komatiite similar to picrite, but low total
  alkalies (Na2OK2O) 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 Trachydacite (Q 20) based on normative
qtz from a recalculation QAnAbOr100
The amount of normative olivine distinguishes
Tephrite (10)
Dotted line encloses 53 of all rocks from the
global database
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 MgOFeO 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
• 21 11
• NaAlSiO4 2SiO2
  NaAlSi3O8
• nepheline
  albite
• 21 61

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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-unde
rsaturated 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

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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-1800s 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