1. Refresher Lecture 3 Igneous and metamorphic basics
ESS 400A
Summer 2017

2. Outline Rock classification Igneous rock characteristics
Sedimentary vs igneous vs metamorphic
Global distribution of major rock types
Igneous rock characteristics
Textures and composition
Intrusive rocks
Extrusive (volcanic) rocks
Ultramafic rocks
Metamorphic rock characteristics
Textures
Metamorphic facies and key minerals

3. Classification of Rocks

4. Geologic provinces & common rock types
This map shows the distribution of major rock types on the surface of the earth
Among the notable aspects of this distribution is the fact that continental cores (shields) are underlain by very old rocks, compared to continental margins, which are typically underlain by young rocks.
Note the distribution of volcanoes, and by inference batholiths, around the Pacific Ocean, where oceanic crust is subducting beneath shallower plates.

5. Igneous rocks are generally categorized by how much SiO2 they contain
Plutonic rocks have coarse textures; volcanic rocks have fine-grained textures

6. Igneous rock textures
Note the grain size differences between these igneous rocks
Silicic volcanic ash is compositionally similar to pumice; the explosiveness of the volcanism can cause the pumice to break into fine-grained shards

7. Igneous rock textures

8. Mineral composition of igneous rocks

9. Intrusive igneous rock classification
In the field, try to quantify the relative percentages of quartz and the two feldspars
Most rocks in typical batholiths are granites, granodiorites, and tonalites

10. Quick key for naming igneous rocks
Grain size, color, texture and mineralogy are the primary ways we distinguish different igneous rocks from one another

11. Where do igneous rocks form?
The composition of an igneous rock tells us about where in the crust or mantle the melt may have originated
The texture of the rock tells us where it was finally crystallized in the crust as intrusive material or atop the crust as extrusive material

12. Viscosity of volcanic lavas

13. Basaltic lava: low viscosity & volatiles

14. Silicic lavas: explosive, viscous

15. Morphology of volcanoes
Generally the more mafic the magma, the broader and lower relief the volcanic edifice
Calc-alkaline strato-volcanoes, which make up many of the peaks in the world’s volcanic arcs, are commonly steep-sided and andesitic to dacitic in composition
Shield volcanoes, like Hawaii, tend to have broader, more subdued topography

16. Ultramafic rock types
Ultramafic rocks are commonly subdivided by the amount of Olivine vs Orthopyroxene vs Clinopyroxene
Ultramafic rocks are relatively rare at the earth’s surface. The can be found at the base of ophiolites and also at locations where the crust has been highly attenuated

17. Dunites and pyroxenites
Dunite xenolith in basalt
Pyroxenite

18. Cumulate textures
Cumulate rocks look like pebble conglomerates but crystals are olivine and pyroxene
Ultramafic cumulate rocks form by fractional crystallization in the bottom of a magma chamber
Olivine precipitates first, followed by pyroxene and then plagioclase
(Olivine)
(Pyroxene)
(Plag)

19. Mg rich ultramafic rock types
Ultramafic rocks become hydrated in some near-seafloor environments where they can undergo retrograde metamorphism to become serpentinite

20. Classification of Rocks

21. Textures of metamorphic rocks
Metamorphism involves changes to a rock’s mineralogy and texture due to changes in pressure and temperature conditions
Metamorphism can take place by burial to hotter and higher-pressure conditions (prograde) or during uplift to cooler and lower-pressure conditions (retrograde)

22. Metamorphic rock types (pelites)
A shale protolith will become a slate, phyllite, schist and gneiss with increasing burial to higher P-T conditions

23. Metamorphic rock types (meta-basites)
A basalt protolith will become a greenstone and greenschist or blueschist upon burial metamorphism

24. Some common metamorphic rocks
Marble
Some common metamorphic rocks
Greenschist with garnet
Augen gneiss
As with other rock types, describe as many characteristics as you can about a metamorphic rock – color, grain-size, mineralogy, fabric, etc

25. Metamorphic textures & protoliths

26. Types of metamorphism
Contact metamorphism takes place in local “aureoles” around hot plutons or stocks. It is commonly high-temperature but low-pressure
Barrovian metamorphism is commonly called ‘’regional” metamorphism, which happens in actively deforming mountain belts where the crust is tectonically thickening
Blueschist metamorphism occurs in high-pressure, low-temperature environments, commonly in forearc regions and in the deeper levels of subduction zones

27. Metamorphic facies
Pressure-temperature conditions of the different metamorphic facies and environments

28. Common metamorphic minerals
The mineralogy and petrology of a metamorphic rock are used to place it in its pressure-temperature environment of formation
Because the mineral composition of a metamorphic rock can change several times during its evolution through P-T-t space, it is always desirable to determine whether all mineral phases appear stable in a thin-section or whether some phases have been or are in the process of being replaced by more stable minerals.

29. Plate tectonics & metamorphism
High P, Low T
Low-mod P, High T
This simple cartoon shows the main environments of metamorphism in a convergent margin setting

30. Metamorphic basement rocks in Montana (Sims et al, 2004, USGS SCIENTIFIC INVESTIGATIONS MAP 2829)

31. Summary Igneous rock characteristics Metamorphic rock characteristics
Use grain size to determine intrusive vs. extrusive, emplacement depth
Use color to determine how mafic or silicic the composition is
Describe mineralogy and texture
Use map relations and radiometric dates to determine age
Place in regional context by comparing to similar-aged rocks nearby
Metamorphic rock characteristics
Determine grade by looking at grain size and mineralogy
Describe texture – foliation, schistosity, cleavage
Determine protolith by considering bulk composition
Use facies, map relations and radiometric dates to determine age and environment of metamorphism
Place in regional context by studying regional distribution of similar rocks. Be sure to consider how and when the metamorphic rocks became uplifted to the surface