Lecture Fifteen - Igneous and metamorphic geology IV Flashcards
What three factors are used to identify and classify igneous rocks?
1) Their chemistry - SiO2 contrent and TAS diagram.
2) Mineralogy - IUGS classifications (ternary diagrams). The minerals that are present and their proportions.
3) Texture - Crystal sizes. Course grained =intrusive
Tectonic setting of different igneous rocks.
How can igneous rocks be classified by their chemistry?
Igneous ricks (and magmas) are often classified according to their silica content.
Very low silica <45% = Ultramafic.
Low silica (45-52% = mafic.
Intermediate silica content (52-63%) intermediate.
High silica content >63% = felsic (or silicic).
SiO2 may be plotted with other major element oxides to classify igneous rocks and magmas.
Total Alkalis (Na2) and K2O) Vs Silica (TAS diagrams) can be used.
The most common sequence is the low (normal) alkali types - sub-alkali or tholeiitic.
Basalt -> rhyolite (extrusive vs intrusive).
Gabbro -> granite (intrusive or plutonic).
What are some pairs of igneous rocks with the same chemical make up but different cooling environements?
How can igneous rocks be classified by their rock mineralogy?
A cooling magma will crystallise different minerals depending on its temperature and composition (e.g. SiO2 %).
The appearances of these minerals tend to overlap so that certain combinations are indicitive of different rock (and magma) types.
It is therefore extremely useful to be able to identify the different minerals in an igneous rock and their relative proportions.
What are the four light and four dark coloured minerals?
Compare a granit and a gabbro with regards to their mineralogy.
Show the igneous classification based on mineral proportions diagram.
Show the different sizes of crystals in some igneous rocks.
What are the intrusive and extrusive textures of igneous rocks?
Intrusive textures:
Phaneritic (course grained texture >1mm) - Slow cooling at depth.
Aphanitic (fine grained texture <1mm) - Fast cooling (near surface). Also common in extrusive settings.
Extrusive textures:
Glassy/aphyric - Magma cools so quickly crystals don’t have time to form.
Vesicular - Full of round holes (vesicles) due to escape (exsolution) of gas during cooling of a magma/lava.
Pyroclastic - Poorly sorted, fragmental, matrix-supported volcanic rock consisting of pumice, lithics, ash and crystal fragments.
Porphyritic texture - Course crystals (phenocrysts) surrounded by fine grained crystals or groundmass crystals (bimodal crystal size range). Two stage cooling -> can see in either intrusive or extrusive igneous rocks.
In general, what kinds of igneous rocks are formed at mid ocean ridges?
Divergent oceanic margins.
Bassaltic lava if erupted, gabbro if intrusive (much of the oceanic crust is basalt-gabbro composition - basalt on top, gabbro undernearth).
Can also be known as mid ocean rides basalt-gabbro (MORB).
Sea floow basalt = pillow basalt.
In general, what kinds of igneous rocks are formed at hot spots?
(Shields).
Also produce basaltic compositions (e.g. Hawaii laval flows).
Can also get continental hot-spots - more complex due to overlying continental crust.
50-100 currently active hotspot volcanoes.
Occur in interiors of tectonic plates due to mantle plumes.
Plumes creasted mainly due to the hotter parts of the mantle rising and melting.
Creates Oceanic Island Basalt (OIB) with shield volcanoes common.
Often see linear chains of volcanoes as crust moves over stationary mantle plume.
In general, what kinds of igneous rocks are formed at subduction zones?
Convergence of two OCEANIC plates:
Magmas have higher SiO2 content because:
1) Magmas created from partial melting of the mantle - more crystal fractionation, evolved to higher SiO2.
2) Magmas may have melted som eof the (already) higher SiO2 oceanic crust.
More explosive due to higher water contedt.
Basaltic andersite to andersie/diorites common.
Fragmental igneous rocks more common here than in MOR or hot spot settings.
Island arcs:
From curved volcanic chains.
Magma able to ecolve to higher SiO2 compositions (more felsic) due to crystal fractionastion - andersite/diorites common.
Subducting ocenaic plate hydrates magmas - higher water content - more explosive.
Subductino of OCEANIC plate under CONTINENTAL plate:
Similar to island arcs except that magmas are more evolved (higher SiO2%) and eruptions more explosive.
Overlying continental crust can also melt.
Intermediate magmas and igneous rocks common (andersite-dacite if extrusive, diorite-granodirite if intrusive).
Why does oceanic lithosphere always subduct under the continental lithosphere?
Thick continental crust slows magma ascent to the surface.
Get more silicic, viscous, cooler magma that either form large intrustions (diorite-granodirite-granite) or large explosive volcanes (andersite-dacite-rhyolite).
In general, what kinds of igneous rocks are formed at continental collisional zones?
Collision of two pieces of continental lithosphere - e.g. India and Tibet.
Produces high mountain rnages with highly thickened crust.
Creates pressures and temperatures hot enough for the continental crust to start melting (migmatites).
Little mass imput from the mantle (but may supply heat).
Rare volcanic (extrusive) felsix igneous rocks (e.g. rhyolites).
Plutonic (intrusive) igneous rocks common (diorite-granodirite-granite).
Extensive metamorphic rocks.
In general, what kinds of igneous rocks are formed at continental rifts?
Breaking apart of continental lithosphere.
Successful rifts (long lived) can form a new MOR with separate tectonic plates separated by an ocean (and oceanic crust/lithosphere).
Rifting starts with thinning of the continental lithosphere causing melting of:
1) The intermediate-felsic continental crust (creating dacite-rhyolite).
2) Uprising of the mafic-ultramafic asthenospheric mantle (basalt).
Magmas produced are complex and varied due to melts produced byt he different sources - basalts and rhyolites common (bimodal magmatism/volcanism).
Mixing of these two can produce intermediate hybrid compositions - andersites.
Large, sustained mantle plumes are though the initiate/help continental rifting.
1) Initial rifting occurs as a series of three-armed triple junctions.
2) Two arms of each rift/fracture link up.
3) The thirf arm fails, and becomes a rift valley (aulocogen).
- > This is thought to be currently occuring on a large scale with the East African Rift.
NOTE: Eachs et of triple junctions need not occur simultaneously, they can form sequentially, with the continent ‘unzipping.’
Continental rifts can often be associated with (and perhaps driven by) mantle plumes.
Plumes are (often) hotter and can create more partial melting and greater magma volumes than in normal asthenospheric mantle.
Creates an abnormally large amount of basaltic magma that when erupted can flow for hunereds of km (due to low viscosity). These are called flood basalts.
Floow basalts are the one type of Large Igneous Provences (LIPs).
Also associated with extensive mafic intrusive rocks.
