Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks Chapter 3 Photo credit: G. Mattioli

The Rock Cycle A Plate Tectonic Example Igneous Rocks Igneous Rock Textures Identification of Igneous Rocks Varieties of Granite Chemistry of Igneous Rocks

Fundamental Questions How are rocks sampled in the field and analyzed in the lab to determine their chemical, modal, and mineralogical composition? What do these analyses tell us about the composition of magmatic rocks? How can the data be presented to elucidate compositional patterns and contrasts? How do we classify magmatic rocks to convey meaningful petrogenetic information on the origin and evolution of the magma from which they solidified?

The Rock Cycle

Intrusive Rocks in Northern Victoria Land, Antartica

Plate Tectonics and the Rock Cycle

Igneous Rocks: Terminology Igneous rocks are formed as a result of cooling and crystallization from a magma. Magma is molten rock (fluid), rich in silica (SiO 2 ), which contains dissolved volatiles (e.g. CO 2 and H 2 O). Lava is magma extruded on or very near the Earth’s surface. Most lavas have been significantly degassed en route to the surface.

Classification of Igneous and Volcanic Rocks Based on hand specimen fabric Based on field relationships and textures Based on mineralogy and chemistry –Color Index

Classification Schemes I Based on Fabric –Phaneritic: rocks with mineral grains that are large enough to be identified by eye. Texture is typical of slowly cooled intrusive rocks. –Aphanitic: rocks with grain too small to be identified by eye. Texture is most common in rapidly solidified extruded magma and marginal facies of shallow intrusions.

Classification based on Field Relations and Textures Extrusive or volcanic rocks: typically aphanitic or glassy. This means that they are generally fine grained in texture. Grains are typically 0.5 to 1 mm. Common example is basalt. –Many varieties are porphyritic. This means that the grain size is bimodal, with a fine grained matrix surrounding larger grains that are called phenocrysts. Common example is andesite. Intrusive or plutonic rocks: typically phaneritic. This means that they are generally coarse grained and this texture is often quite uniform. Grains can range in size but are often clearly visible to the naked eye (>2-3 mm). Common example is granite. –Amphiboles and biotites are commonly altered to chlorite. Muscovite found in some granites, but rarely in volcanic rocks. Perthitic feldspar, reflecting slow cooling and exsolution, is widespread.

Rhyolite Hand Specimen

Granite Hand Specimen

More on Fabric Classification Porphyritic texture: magmatic rocks with bimodal grain size distributions. –Larger grains are called phenocrysts –Smaller grains constitute the groundmass or matrix –Porphyritic aphanitic rocks are more common than porphyritic phaneritic rocks Glassy or vitric texture: rocks that contain variable proportions of glass. –Holocrystalline rocks: wholly composed of crystals –Vitrophyric rocks: porphyritic rock with phenocrysts in a glassy matrix

Magmatic Intrusion

Deeply Eroded Intrusions GRANITE INTRUSION SHALE COUNTRY ROCK Torres del Paine, Chile

Coarse Grained Intrusive Rock Texture Potassium Feldspar (stained yellow)

Photomicrograph - Phaneritic Texture Photo credit: C.C. Plummer Interlocking grains

Andesite Hand Specimen Plagioclase Feldspar phenocrysts Matrix or groundmass

Diorite Hand Specimen Interlocking grains with uniform size

Classification based on Mineralogy & Chemistry Felsic rocks: mnemonic based on feldspar and silica. Also applies to rocks containing abundant feldspathoids, such as nepheline. GRANITE Mafic rocks: mnemonic based on magnesium and ferrous/ferric. Synonymous with ferromagnesian, which refers to biotite, amphibole, pyroxene, olivine, and Fe-Ti oxides. BASALT Ultramafic rocks: very rich in Mg and Fe. Generally have little feldspar. PERIDOTITE Silicic rocks: dominated by quartz and alkali fsp. Sometimes referred to as sialic (Si + Al).

Color Index Defined as the modal proportion of dark- colored minerals in a rock. Should really be based on the proportion of ferromagnesian minerals as feldspars may range in color. –Leucocratic: 0-30% mafics –Melanocratic: % mafics

Gabbro Hand Specimen

Basalt Hand Specimen Fined grained mafic volcanic rock

Summary Classification for Igneous Rocks

Igneous Rock Identification: Summary

Intrusive Bodies Shallow Intrusive Structures –Sills –Dikes –Volcanic necks Intrusive Rocks That Crystallize at Depth Abundance and Distribution of Plutonic Rocks Sierra Nevada Batholith Pegmatites

San Cristobal Volcano, Nicaragua Photo Credit: G. Mattioli

Volcanic Neck and Dike

Aerial Photo of Shiprock, New Mexico From:

Dikes vs. Sills Sills: Concordant structures Parallel to pre-existing layers Dikes: Discordant structures Cross-cut pre-existing layers

2 m Dike in Dominica, West Indies Photo Credit: G. Mattioli

Dominica Dike along Strike View Photo Credit: G. Mattioli

Dikes in northern Victoria Land, Antartica

Edinburgh Sill From:

Magmatic Diapirs A diapir is a dome that is cored by plastic material, in this case, partially molten rock. Concept first applied to salt domes.

Coalescing Diapirs and Plutons Rise because of buoyancy-magma is lower density than rocks. But siliceous magmas have high viscosity or resistance to flow, which makes eruption difficult without high gas contents.

Pegmatite Outcrop

Sierra Nevada Batholith