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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.
                     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
       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
   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.
    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
                  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.%
                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
IUGS Classification of Aphanitic and Glassy Rocks
                                 Distinction between
                                 Trachyte (Q <20%) and
                                 Trachydacite (Q > 20%)
                                 based on normative qtz
                                 from a recalculation
                                 Q+An+Ab+Or=100%

                                 The amount of normative
                                 olivine distinguishes
                                 Tephrite (<10%) from
                                 Basanite (>10%)

                               Dotted line encloses 53%
                               of all rocks from the global
                               database


                                               Le Maitre, 1989
                  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
                  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
       Alumina Saturation I
 Index based on Al2O3/(K2O + Na2O + CaO)
Ratio equals 1 for feldspars and feldspathoids
                 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. %)
         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
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
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
            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.
              Yoder & Tilley Basalt Tetrahedron




Yoder & Tilley, 1962; Le Maitre

				
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