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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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.
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.
GABBROS:Phaneritic rocks composed of plagioclase, pyroxene,
and olivine - compositionally similar to basalts
ULTRAMAFICS: Phaneritic rocks w/ <10 modal % felsic minerals
Le Maitre, 1989
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
NB that “norms” or normative abundance
refers to the calculated wt.% of a specific mineral
Trachyte (Q <20%) and
Trachydacite (Q > 20%)
based on normative qtz
from a recalculation
The amount of normative
Tephrite (<10%) from
Dotted line encloses 53%
of all rocks from the global
Le Maitre, 1989
(Mg,Fe)2SiO4 + SiO2 = 2(Mg,Fe)SiO3
NaAlSiO4 + 2SiO2 = NaAlSi3O8
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
Index based on Al2O3/(K2O + Na2O + CaO)
Ratio equals 1 for feldspars and feldspathoids
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
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.
Low-K 12 km
Med-K 35 km
High-K 45 km
Yoder & Tilley, 1962; Le Maitre