Earth Materials Lecture 12: Igneous Petrology Flashcards
What are the majority of igneous rocks composed of?
99% silicate-based – SiO4
.
Rare carbonatites – CO3
.
Composed of crystals +/- glass.
List Intrusive types of igneous rocks/behavior
Shallow – hypabyssal
Deep – plutons (adj. plutonic)
List Extrusive types of igneous rocks/behaviors
Effusive - lava flows
Pyroclastic products
List the IUGS classification of Igneous rocks (based on silica content(SiO2)
Subdivisions at:
<45% Ultrabasic / ultramafic
45 – 52% Basic / mafic
52 – 63% Intermediate
>63% – Acid / felsic / sialic.
Define and Describe Ultrabasic/Ultramafic rocks
<45 % silica.
Predominantly
Fe, Mg, Ca silicates.
+/- spinel / garnet.
Plagioclase rare.
Minor Cr, Fe, Ti oxides
accessory phases:
magnetite, chromite.
ltrabasic rocks are made up of mafic minerals, such as olivine, pyroxene, and amphibole
They also contain metal oxides and native metals
They are high in magnesium and iron
List the main Ultramafic rocks
Dunite: An ultrabasic igneous rock with a hexagonal crystal structure
Peridotite: A dark, heavy, and dense igneous rock that’s high in magnesium and iron, and low in silicon
Pyroxenite: A common ultramafic rock
Troctolite: A rare variety of ultramafic rock with a higher percentage of calcic plagioclase
Anorthosite: A rare variety of ultramafic rock
Describe the occurance and distribution of ultramafic rocks
Found in Earth’s upper mantle.
➢ Nodules in volcanic rocks.
➢ Orogenic belts.
⚫ Cumulates; often layered rocks.
⚫ Ophiolites: mantle and cumulates.
⚫ Ancient lavas: komatiites.
⚫ Kimberlites and related volcanics.
Describe and define Mafic/basic rocks
45 – 52 % silica
Produced by partial melting of mantle.
Rocks / magmas / lavas of basaltic composition.
Fe, Mg, Ca, Al, main elements with traces of Na, K, Ti, P.
Below are plutonic rocks; volcanic equivalents are ~basalts
List the main Mafic rocks
Basalt: The most common igneous rock on Earth, basalt is formed when lava cools quickly on the surface.
Gabbro: A common mafic rock that may contain olivine.
Diabase: A common mafic rock.
Andesite: A chemically intermediate rock with a salt and pepper color and texture.
Diorite: An intermediate igneous rock that often contains hornblende
Common mafic minerals
Olivine, Pyroxene, Amphibole, Biotite, Magnetite, Ilmenite, and Apatite.
Describe the occurrence and distribution of Mafic/Basic rocks
Oceanic crust - intrusive and extrusive
Large Igneous Provinces -
Short-lived major outpourings of magma
Describe and define Intermediate to acid rocks
Intermediate rocks are igneous rocks with a chemical composition between mafic and felsic rocks.
52 – >63 % silica
Generally fractionates of basic magma; crustal melts.
Decreasing Fe, Mg, Ca; increasing Na, K, P, trace elements.
Andesine, oligoclase, albite; orthoclase / microcline / sanidine.
Quartz. Fe-olivine (fayalite).
Augite +/- enstatite; hornblende; biotite; muscovite.
Minor zircon, titanite, apatite etc
List the main intermediate rocks
Diorite: An intrusive intermediate rock that looks like a blend of light and dark minerals.
Andesite: An extrusive intermediate rock.
Granodiorite: An intrusive intermediate rock that’s between diorite and granite.
Trachyandesite: A volcanic intermediate rock.
Quartz syenite: An intermediate rock that contains quartz.
Quartz monzonite: An intermediate rock that contains quartz.
Describe the occurrence and distribution of intermediate to acid rocks
Lots in UK
-Ordovician acid tuffs Snowdonia
- Devonian granites Shetland, Cumbria
-Carboniferous granites Devon and Cornwall
What are the various properties for identifying igneous rocks in hand specimens
Colour of fresh and weathered surfaces.
⚫ Determine composition – mineralogy – depends on grain size.
⚫ Texture:
− Granularity – grain size, range, mode.
− Is there a groundmass? Is it porphyritic?
− Crystallinity – crystals/glass, mixture.
− Fabric – massive, lineated, layered?
− If lineated/layered, determine which mineral(s) define these
fabrics.