Classification of igneous rocks

1. Classification of igneous rocks Chapter 2

2. How to classify • Composition • Chemistry • Mineralogy • Texture/fabric

3. Classification based on texture/fabric • Phaneritic • Minerals large to see with naked eye • Cooling history? • Aphanitic • Minerals need microscope to see • Porphyritic: 2 different grains sizes • Cooling history? • Phenocrysts: larger grains • Groundmass or matrix: smaller grains

4. Classification based on texture/fabric • Glassy • Rocks with glass • Volcaniclastic • Minerals and rock fragments that are broken in pieces

5. Classification based on field relations • Intrusive • Usually phaneritic • Extrusive • Aphanitic and glassy • Many are porphyritic • Some are volcaniclastic

6. Classification based on mineralogy • Felsic • Feldspar and silica • Light colored rocks • Mafic • Magnesium and iron • Dark colored rocks • Ultramafic • Especially rich in Mg and Fe

7. Classification based on mineralogy • Mode: % of each mineral in rock • Must add up to 100% • Quick ID of major minerals • Choose which diagram to name your rock

8. Q Phaneritic rock with >10% felsic minerals . Quartzolite 90 90 Quartz-rich Granitoid 60 60 • QAPF diagram • Anorthosite: >90% plag • Gabbro >35% mafic mineras • Diorite <35% mafic minerals Grano- Tonalite Granite Alkali Feldspar Granite diorite Alkali Fs. Qtz. Diorite/ 20 20 Quartz Syenite Qtz. Gabbro Quartz Quartz Quartz Monzonite Syenite Monzodiorite Alkali Fs. 5 Diorite/Gabbro/ 5 Syenite Syenite Monzodiorite Monzonite Anorthosite 90 35 10 65 A P (Foid)-bearing (Foid)-bearing (Foid)-bearing Syenite Monzonite Monzodiorite (Foid)-bearing 10 10 Diorite/Gabbro (Foid)-bearing Alkali Fs. Syenite (Foid) Syenite (Foid) (Foid) Monzosyenite Monzodiorite (Foid) Gabbro 60 60 (Foid)olites F

9. Q Phaneritic rock with >10% felsic minerals . Quartzolite 90 90 Quartz-rich Granitoid 60 60 • Other names: • Porphyry: porphyritic rock • Aplite: uniformly fine-granite phaneritic light colored granite (sugary) • Pegmatite: phaneritic with highly variable grain size (can be large) Grano- Tonalite Granite Alkali Feldspar Granite diorite Alkali Fs. Qtz. Diorite/ 20 20 Quartz Syenite Qtz. Gabbro Quartz Quartz Quartz Monzonite Syenite Monzodiorite Alkali Fs. 5 Diorite/Gabbro/ 5 Syenite Syenite Monzodiorite Monzonite Anorthosite 90 35 10 65 A P (Foid)-bearing (Foid)-bearing (Foid)-bearing Syenite Monzonite Monzodiorite (Foid)-bearing 10 10 Diorite/Gabbro (Foid)-bearing Alkali Fs. Syenite (Foid) Syenite (Foid) (Foid) Monzosyenite Monzodiorite (Foid) Gabbro 60 60 (Foid)olites F

10. Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Phaneritic mafic and ultramafic rocks (c)

11. Classification of Igneous Rocks • Mafic rocks: plag, pyroxene, olivine • Norite: gabbro with more opx than cpx • Top and bottom fields at 10% tick marks • Side fields at 5% tick marks (c)

12. Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Classification of igneous rocks • Ultramafic rocks: opx, cpx, olivine • If can’t tell opx vs. cpx use general names of pyroxenite and peridotite

13. Classification of Igneous Rocks • Aphanitic rocks • Based % from phenocrysts • Need to normalize A and P • Not the most definite way to name rock Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.

14. Classification of Pyroclastic rocks Figure 2-5. Classification of the pyroclastic rocks. a.Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b.Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

15. Classification of Igneous Rocks • Rock with 70% X, 20% Y, 10% Z • Plot mineral composition of triangle diagram Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

16. Classification of Igneous Rocks • Figure out % of each mineral • Reduce to 3 minerals of choice and normalize • In rock: • X=9% • Y=2.6% • Z=1.3% Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

17. Classification of Igneous Rocks • In rock: • X=9% • Y=2.6% • Z=1.3% • Of these 3: • X=70% • Y=20% • Z=10% • X+Y+Z • X/(X+Y+Z) Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

18. Classification of Igneous Rocks • Of these 3: • X=70% • Y=20% • Z=10% • X/(X+Y+Z) =70 • Now normalize %Y and Z • Z/(Z+Y) and Y/(Z+Y) • 20/30 (67%) for Y • 10/30 (33%) for Z Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

19. Naming aphanitic rocks based on chemistry 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.

20. Rock classification based on chemistry • Silica concentration principal oxide • Silicic rocks with SiO2 • >66 wt% silica is felsic • 52-66 wt% silica intermediate • 45-52 wt% silica is mafic • >45 wt% silica is ultramafic • Also: • Magnesian, aluminous, alkaline (Na2O and K2O)

21. Naming rocks • Use Fig. 2.8 and 2.10 to name: • Example: 45% quartz, 25% k-spar, 10% plag, 15% biotite, 5% hornblende • 56% quartz, 31% k-spar, 13% plag • 30% plag, 70% k-spar • Granite • Hornblende, biotite granite • Modifying minerals listed in increasing abundance