Geol 2312 Igneous and Metamorphic Petrology. Lecture 2 Classification of Igneous Rock or How many ways can you skin a rock?. Jan. 23, 2009. Goals of a Rock Classification Scheme. Practical – especially for field use Descriptive – minimal genetic terms
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Geol 2312 Igneous and Metamorphic Petrology
Classification of Igneous Rock
How many ways can you skin a rock?
Jan. 23, 2009
Most common schemes based on mode (vol. %) of “essential” primary minerals ...
(When observation of primary minerals is not possible (e.g., too fine, altered) chemistry is commonly used)
...with modification by texture and structural features and occurrence of “accessory” and minor minerals
=% Dark Minerals
*International Union of Geological Sciences
Mineral X – 59.5%
Mineral Y – 17%
Mineral Z – 8.5%
Mineral A – 12%
Mineral B – 3%
Normalization to XYZ only
X+Y+Z = 85%
Xn = 59.5/85 = 70%
Yn = 17/85 = 20%
Zn = 8.5/85 =10%
Severson and Hauck (1990)
- uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox)
- defines modal boundaries that bracket natural modal populations and cotectic proportions determined from experimental data
- uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field.
From Miller (1986)
Absolute Grain Size
Bulk Rock Texture –based on pyroxene habit
Serpentinized, modally layered, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITE
Why PYROXENE-BASED BULK-ROCK TEXTURES?
From Streckeisen (1976)
Attach mafic mineral prefix
to root rock name for intermediate rocks...
or “ferro” if uncertain of
mafic mineral phase
Can’t “see” AN content in hand samples
Even DC felsic rocks are dominated by pyx over hb and bio
Suggested Field Criteria for DC
Dark Mineral Habitprismatic, subprismaticgranular to poikilitic
Felsic Mesostasis >5 % <5 %
SiO2< 52% > 52%
CI > 35% < 35%
Calculated from whole rock geochemical analyses;
distributes major elements among rock-forming minerals