Igneous Rocks

IGNEOUS ROCKS (Chapter 3) Magma, Lava and the Formation of Igneous Rocks Magma is: completely or partially molten material, Contains: a) melt = mobile ions mostly Si, O determines the viscosity (resistance of fluid to flow) also Al, K, Ca, Na, Mg b) gasses (volatiles) H20, CO2

SO2

Lava is: molten rock that is extruded to the earth’s surface Igneous Rocks are: rocks that formed by the crystallization of magmas or lavas Intrusive (plutonic) Extrusive (volcanic) Then two most important igneous rocks: GRANITE Intrusive Light SiO2 Al K Continental

BASALT Extrusive Dark Fe Mg Ca Oceanic

Origins of Magma Two main sources of magma: a) divergent plate boundaries (mid-ocean spreading centers, rift valleys) - basaltic composition (like upper mantle) b) convergent boundaries (subduction zones) Ocean-Continent Boundary - complex composition (re-melting of continental crust) Ocean-Ocean Boundary (Island Arc) - basaltic compositions Will discuss magmatic processes later.

Crystallization Reverses the process of melting Chemical bonds form as melt cools down Silica tetrahedral form first, then Ions are added to form high-temperature minerals, then Lower temperature mninerals form Last minerals my envelope the earlier (hi-temp) ones Factors: Chemical composition of melt, especially Si content, also volatiles and pressure Igneous environment (plutonic, volcanic) Note that magma chemistry and environment of formation are often related, And that igneous rocks are mainly classified on their texture (crystal size) and their mineralogy (chemical composition)

Igneous Textures Texture of igneous rocks refers to the sizes of the crystals they are made of. Major difference is between rocks with large crystals and those with small crystals.

Plutonic (Intrusive) Textures Phaneritic (coarse-grained) granite best example; also gabbro, diorite Pegmatitic - extremely large crystals (rare) House-sized feldspars Often associated with precious minerals and rare metals such as gold Volcanic (Extrusive) Textures Glassy (obsidian) - lava cools very fast (quenching) Ions stop moving before many crystals can form Most common for granitic melts, rare for basaltic ones Aphanitic (very fine grained) Rapid cooling of lava - rhyolite good example may be vesicular (holes left from expanding gasses) pumice, scoria Porphyritic ( bimodal crystal sizes) Groundmass - fine-grained matrix - cooled last and fast) Phenocrysts - large crystals enveloped by the groundmass Usually formed prior to eruption/ rapid cooling Usually higher temperature minerals, but not always

Chemistry of Igneous Rocks Chemistry of melt is correlated with the origin of the magma- refer to the convergent versus divergent plat boundaries mentioned earlier. Key chemical parameters: SiO2 content Light versus heavy ions

End members of the compositional spectrum: FELSIC

MAFIC

SiO2 K Na

Fe Mg Ca

Lo temp minerals Light color Continental

Hi temp minerals Dark color Oceanic

Granite/rhyolite Quartz K- spar Mica

Basalt/Gabbro Ca-spar Olivine pyroxenes (enstatite)

Important Plutonic (Intrusive) Rocks Felsic (granitic) Rocks Granite - “pink or gray tombstones” Quartz K feldspars (microcline- pink, orthoclase- gray) Accessories Mica, amphibole Phaneritic to pegmatitic texture Only on continents Associated with regional scale mountain building (batholiths) Extrusive chemical equivalent = rhyolite

Andesitic (intermediate) Rocks Diorite “the black and white rock” Na- and Ca- feldspars Amphiboles Biotite mica Extrusive chemical equivalent = andesite Mafic (Basaltic) Rocks Gabbro - “black rock with big crystals” Ca- feldspars Pyroxenes Extrusive chemical equivalent = basalt

Magmatic Processes Bowen’s Series Magmatic Differentiation Settling and Layering of Magmas Partial Melting Enrichment in ions of low-temp minerals (Si, O, Al, K) Assimilation and Mixing Incorporation of country rock Merger of magma chambers

ECONOMIC IMPORTANCE OF IGNEOUS ROCKS

Metals Hydrothermal deposits Pegmatites Placers

Chromium Copper, Mercury Gold, Platinum, Silver Lead, Zinc, Titanium