Igneous Petrology-Classification of igneous rocks

Classification of igneous rock

1. Classifications of Igneous Rocks

2. Classification of Igneous Rocks : Bases texture field relations Chemical composition Mineralogical

3. Classification based on texture Aphanitic- crystals too small to see by eye Phaneritic- can see the constituent minerals Fine grained- < 1 mm diameter Medium grained- 1-5 mm diameter Coarse grained- 5-50 mm diameter Very coarse grained- > 50 mm diameter Porphyritic- bimodal grain size distribution Glassy- no crystals formed

4. Field Relations Extrusive or volcanic rocks: typically aphanitic or glassy. Many varieties are porphyritic and some have fragmental (volcaniclastic) fabric. Intrusive or plutonic rocks: typically phaneritic

5. Compositional Classes: Felsic v. Mafic

6. Chemical compositions Silica percentage (% SiO2) >66%, felsic, granite/rhyolite 52-66%, intermediate, diorite/andesite 45-52%, mafic, gabboro/basalt <45%, ultramafic, peridotite/dunite/komatiite

7. Chemical : Silica vs. alkaline elements SiO2 Vs. Na2O+K2O Calcic Calc-alkalic Alkalic-calcic Alkalic

8. Classification based on chemical comp Figure 2-4 . A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al . (1986) J. Petrol., 27 , 745-750. Oxford University Press.

9. Chemical : Aluminum saturation (Al2O3 Vs. Na2O+K2O+CaO) Peraluminous Metaaluminous Peralkaline

10. If there is an excess of Alumina over that required to form feldspars, the rock is peraluminous : This condition is expressed chemically on a molecular basis as: Al 2 O 3 > (CaO + Na 2 O + K 2 O) In peraluminous rocks we expect to find an Al 2 O 3 -rich mineral present as a modal mineral such as muscovite [KAl 3 Si 3 O 10 (OH) 2 ], corundum [Al 2 O 3 ], topaz [Al 2 SiO 4 (OH,F) 2 ], or an Al 2 SiO 5 - mineral like kyanite, andalusite, or sillimanite. Peraluminous rocks will have corundum [Al 2 O 3 ] in the CIPW norm and no diopside in the norm. Peraluminous

11. Metaluminous for which the molecular percentages are as follows: Al 2 O 3 < (CaO + Na 2 O + K 2 O) and Al 2 O 3 > (Na 2 O + K 2 O) These are the more common types of igneous rocks.  They are characterized by lack of an Al 2 O 3 -rich mineral and lack of sodic pyroxenes and amphiboles in the mode.

12. Peralkaline rocks are those that are oversaturated with alkalies (Na 2 O + K 2 O), and thus undersaturated with respect to Al 2 O 3 .   On a molecular basis, these rocks show: Al 2 O 3 < (Na 2 O + K 2 O) Peralkaline rocks are distinguished by the presence of Na-rich minerals

13. classification based on mineralogy Determine the modal mineralogy IUGS classification

14. Q = % quartz (or other polymorph) A = % alkali feldspar (An<5) P = % plagioclase feldspar F = % feldspathoids (abbrev. ‘foids’) classification and nomenclature of volcanic rocks. After IUGS.

15. IUGS Classification of Phaneritic/ plutonic Igneous Rocks The rock must contain a total of at least 10% of the minerals below. Renormalize to 100% (a) Quartz-rich Granitoid 90 90 60 60 20 20 Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite 5 10 35 65 (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs. Syenite 10 (Foid) Monzosyenite (Foid) Syenite (Foid) Monzodiorite (Foid) Gabbro Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Tonalite Alkali Feldspar Granite Q A P F 60 International Union of Geological Sciences

16. Classification of Mafic Intrusive Rocks

17. A classification of the phaneritic igneous rocks: Ultramafic rocks. After IUGS.