Introduction to Igneous Rocks 



A. History

-Description was the first emphasis > groupings

-Classification > genetic interpretation > refined classification

-Rocks defined on basis of texture and composition

B. Terms

-Textural : phaneritic (Coarse (>5 mm)/Medium (1-5 mm)/Aphanitic = fine (<1 mm)/Fragmental = made of pieces of igneous material (crystals, glass, rocks)

-Porphyritic = 2 distinct grain sizes - 2 stages of cooling: phenocrysts = larger grains; groundmass = smaller grains


II. International Union of Geological Sciences (IUGS) classification

A. Determine mode = volume % of minerals present

-Method: point counting

-Find the following: Q'' = % quartz;/P'' = % plagioclase > An5/A'' = % alkali feldspar;/F'' = % feldspathoids/M = % mafics.

-Many igneous rocks can be described with Q'' OR F'', + A'' and P'' (Q and F don''t coexist) : if sum > 10, then recalculate QAP or FAP to 100% (phaneritic rocks and volcanic) : if sum < 10, recalculate O-Py-P or O-Opx-Cpx to 100% .

B. Felsic lutonic rocks

-2 parts, depending on amount of silica (Q vs. F) Note this ~Ne-Q double binary eutectic system

-QAP: most rocks have <60% Q, so granitoids and quartzolite are rare ; left side: add qualifier specifying alkali feldspar. Ex: orthoclase syenite or microcline granite ; below 20%Q, triangle is divided by A/P ratio into A-syenite, syenite, monzonite, monzodiorite; right side: problems near P. Diorite vs. gabbro distinguished by An content of P or by %SiO2 - neither shows on QAP diagram => need other diagram

-FAP: less common, so fewer divisions. Rock names are modified with specific feldspathoid.

C. Mafic and ultramafic plutonic rocks

- pyroxene is not distinguished and rock contains plag (gabbroic rocks)

- pyroxene separated into Opx and Cpx with no plag (ultramafic rocks)

-Peridotite is a general name for ultramafics with >40% O; pyroxenite for Ol<40%

D. Volcanic rocks

- equivalents of plutonic rocks in QAP and FAP diagrams

- trouble near P => (chemical classification)

E. Other rock groups with separate classification

-Fragmental rock

-Hypabyssal rocks (shallow intrusives)

-Carbonatites (calcite, dolomite - Oldoinyo Lengai, Tanzania)

-Lamprophyres (alkali-rich, mafic)

-Spilites and keratophyres (sodic basalts and andesites with hydrothermal alteration - ocean floor)

III. Normative classification

A. Def: calculated mineralogical composition based on a standard conversion of a whole-rock chemical analysis into the formulas of common minerals.

-Oxides are allocated to simple anhydrous end-member formulas of the rock-forming and common accessory minerals.

-These are the minerals that would form under low pressure, so mode ~ norm for volcanic rocks.

-Acts as a "common denominator" to compare rocks.

-Particularly useful for rocks that are too fine-grained for point-counting, or are glassy.

-Originally devised by 4 petrologists: CIPW norm. Typically expressed as wt%

B. Method

-How to distribute 13 oxides among many common minerals?: Ca could go to anorthite, pyroxene, wollastonite; Si goes into many at various proportions.

-Process is simplified by petrographic constraints - only certain combinations work.

-CIPW''s process has worked!

-Norm calculation can now be justified thermodynamically.

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