Transcript Rock Types - Perils of Classification

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
2:1
NaAlSiO4
nepheline
2:1
orthopyroxene
1:1
+
2SiO2
=
NaAlSi3O8
albite
6:1
Silica Saturation II
Silica-oversaturated: rocks contain Q (quartz or its polymorphscristobalite 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 (Mgolivine 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