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

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

Classification of Rocks

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.

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

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

Igneous rock textures

Mineral composition of igneous rocks

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

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

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

Viscosity of volcanic lavas

Basaltic lava: low viscosity & volatiles

Silicic lavas: explosive, viscous

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

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

Dunites and pyroxenites Dunite xenolith in basalt Pyroxenite

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)

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

Classification of Rocks

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)

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

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

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

Metamorphic textures & protoliths

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

Metamorphic facies Pressure-temperature conditions of the different metamorphic facies and environments https://physicalgeology.pressbooks.com/chapter/10-4-index-minerals-and-metamorphic-facies/

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.

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

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

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