Igneous Petrology

Petrology

The study of the origin, occurrence, composition, and classification of rocks, including the history and geologic processes related to rocks.

Rock

An aggregate of minerals and/or mineraloids.

Petrogenesis

Deals with the generation and origin of rocks

Rock cycle

A model showing the origins, relationships and processes between rock types and other geologic materials.

Monomineralic rock

A rock that consists of multiple crystals of a single mineral.

Polymineralic rock

A rock that consists of multiple types of minerals and/or mineraloids

Igneous rocks

Known as the "primary rocks" that originated from solidified magma or lava.

Sedimentary rocks

Known as "secondary rocks" which formed as a result of lithified fragments of pre-existing rocks

Metamorphic rocks

Rocks that formed as a result of pre-existing rocks undergoing metamorphism.

• heat (temperature)

• pressure

• fluids (mineral-rich)

Factors that facilitate metamorphism (3)

Magma

Molten material that originated from the Earth's interior

Lava

The magma that reached the Earth's surface

By partial melting of the Earth's crust and upper mantle

How is magma generated?

• melt (liquid portion)

• volatiles (gaseous portion)

• crystals (solid portion)

The components of a magma (3)

mobile ions, eight (8)

A melt is composed of _______ of the _____ most abundant elements in Earth’s crust.

1. Oxygen - 46.6%

2. Silicon - 27.7%

3. Aluminum - 8.1%

4. Iron - 5%

5. Calcium - 3.6%

6. Sodium - 2.8%

7. Potassium - 2.6%

8. Magnesium - 2.1%

9. Other elements - 1.5%

What are the eight most abundant elements in Earth's crust & their percentages?

at surface pressures

At what pressure does magma volatiles vaporize & turn into gas?

Anatexis

The partial melting of a source rock in the Earth's crust.

1. liquid melt - low-T minerals

2. residual rock - high-T minerals

Anatexis produces:

1. a liquid ____ - enriched in ____-temperature minerals

2. a residual ____ - enriched in ____-temperature minerals

• quartz,

• muscovite,

• k-feldspar,

• chlorite,

• kaolinite,

• serpentine,

• calcite

Examples of low-temperature minerals

• olivine,

• amphibole,

• pyroxene,

• plagioclase feldspar,

• magnetite,

• chromite

Examples of high-temperature minerals

Low-temperature minerals

Minerals w/ the ff. characteristics:

• form at 100-400 °C

• stable at low T & P

• alteration of pre-existing rocks, hydrothermal systems, surface environments

High-temperature minerals

Minerals w/ the ff. characteristics:

• form at >600 °C

• stable at high T & P

• volcanic settings, deep in Earth's interior

Temperature Increase

A magma generation process involving heat transfer from magma intrusions into surrounding rocks. Commonly associated with plutons and occurs deep within the crust.

Decompression melting / Adiabatic melting (Pressure Decrease)

A magma generation process which involves the rise of hot mantle rock to regions of low pressure, bringing heat & melts surrounding rocks. It is commonly associated with oceanic-ridges and hotspots

ave. of 25 °C/km

Geothermal gradient in the upper 10 km of the crust.

higher

Geothermal gradient in the oceans is _______ than the continents.

5-10 °C/km

Geothermal gradient in continental lithosphere

30-50 °C/km

Geothermal gradient in hotspots, ocean ridges, volcanic arcs

Flux melting (Volatile-Induced)

A magma generation process which involves the introduction of volatiles to the asthenosphere above a subducting slab. It is commonly associated with subduction zones.

Marginal accretion

A mode of magma diversification which results from the crystallization along the walls of the magma chamber in which crystal preferentially form and adhere to the edges.

Roof accretion

A marginal accretion that results from early crystallization of minerals alo**ng the ceiling or the roof.

due to preferential heat loss of minerals that crystallized early

What is the cause of minerals sticking to the ceiling or roof?

Sidewall accretion

A marginal accretion that generates crystals that adhere to the side margins of the magma chamber.

due to magma chamber walls releasing heat to the relatively cold country rock

How does sidewall accretion develop?

Floor accretion

A marginal accretion that occurs as crystals form along the base of the magma chamber.

Gravitational separation

A series of fractionation processes that occur when crystals develop with significantly different densities than the surrounding magma.

Crystal settling

A mode of gravitational separation that occurs when higher density, ferromagnesian minerals settle to the base of a magma chamber relative to the lower density liquid magma.

Crystal flotation

A mode of gravitational separation that occurs when early formed crystals less dense than the magma floats toward the roof of the magma chamber.

Convective flow segregation

A fractional crystallization that occurs where liquid crystals in a magma are separated due to factors like velocity, density, or temperature.

due to convection currents in a magma chamber that move crystals and melt, causing denser crystals to accumulate in certain areas and leaving lighter, more evolved melt in others

How does convective flow segregation occur?

Filter pressing

A process where a magma chamber squeezes out the more mobile liquid into a new chamber and leaving behind a residue of crystals in the original chamber.

Liquid fractionation / Magmatic Differentiation

A process where one magma fractionates to produce two or more distinctly different daughter magmas with different compositions

Differential Diffusion

Type of liquid fractionation that involves the selective diffusion of ions in the magma due to compositional, thermal, or density gradients, and water content.

metallic ore

Differential diffusion may play a role in the generation of ________ deposits in magmatic systems

Liquid immiscibility

Type of liquid fractionation that involves the separation of magma into two or more distinct immiscible liquid phases.

Assimilation

The reaction that occurs when the intruding magma chemically reacts with the surrounding wall rock, and incorporates xenoliths within the magma.

Stoping

The fracturing of wall rock due to the forceful injection of the magma.

Stopes

Country rock fragments

Xenoliths

Stopes that fall into the magma

Magma mingling

Occurs when two or more distinct magmas coexist and interact, but do not combine or mix.

Magma mixing

The thorough mixing of two or more magmas, where individual characteristics are no longer recognizable.

Magma series

Consists of genetically-related magmas with a composition that evolved from a common, original, parental magma.

Calc-alkaline magmas

Type of magma that record a progressive decrease in Fe & Mg with increasing SiO₂ and alkali concentrations

Due to the early crystallization of ferromagnesian minerals.

What is the reason calc-alkaline magmas are enriched in alkaline minerals?

• andesites

• dacites

• rhyolites

• high-alumina basalts (BADR)

Types of rock Calc-Alkaline magmas produce

convergent margin environments (volcanic arcs & subduction zones)

Geologic/Tectonic environment of Calc-Alkaline magmas

Tholeiitic magmas

Type of magma that is enriched in Fe, but low to moderate SiO₂ concentrations with increasing fractionation.

MgO & CaO

Tholeiitic magmas are depleted of? (2)

due to early crystallization of forsterite olivine & Ca-plagioclase

What is the reason Tholeiitic magmas are depleted of MgO & CaO?

large volumes of basalt with little variations in composition

Type of rocks Thoeliitic magmas produce

• extensional environments (ocean ridges, continental rifts)

• intraplate settings (hotspots)

• immature arcs in thin volcanic arc crusts

Geologic/Tectonic environment of Tholeiitic magmas

Alkaline magmas

Type of magma that is less common, highly enriched in Na₂O and/or K₂O and contains an extremely diverse compositions with SiO₂ contents ranging from 0-65%.

varied environments which include:

• stable cratons

• continental rifts

• subduction zones

Geologic/Tectonic environment of Alkaline magmas

Bimodal magma suites

Magma suite characterized by high concentrations of silicic & basic rocks, with little intermediate rocks.

continental rifts

Bimodal magma suites are associated with?

from partial melting of mantle rock

In a Bimodal Magma Suite, where is the basic component derived?

from partial melting of continental crust due to heating of rising basic magmas

In a Bimodal Magma Suite, where is the silicic component derived?

• 45-55% SiO₂

• 🔼 Fe, Mg, Ca | 🔽 K, Na

Basaltic / Mafic magma (Composition)

1,000 - 1,200 °C

Basaltic / Mafic magma (Temperature)

fluid

Basaltic / Mafic magma (Viscosity)

low

Basaltic / Mafic magma (Gas Content)

• 55-65% SiO 2

• ~Fe, Mg, Ca, K, Na

Andesitic / Intermediate magma (Composition)

800 - 1,000 °C

Andesitic / Intermediate magma (Temperature)

viscous

Andesitic / Intermediate magma (Viscosity)

intermediate

Andesitic / Intermediate magma (Gas Content)

• 65-75% SiO 2

• 🔽Fe, Mg, Ca | 🔼 K, Na

Granitic / Felsic magma (Composition)

650 - 800 °C

Granitic / Felsic magma (Temperature)

very viscous

Granitic / Felsic magma (Viscosity)

high

Granitic / Felsic magma (Gas Content)

Ultramafic

Magma silica content with <45% SiO₂

Mafic

Magma silica content with 45-52% SiO₂

Intermediate

Magma silica content with 52-63% SiO₂

Dacitic

Magma silica content with 63-68% SiO₂

Felsic

Magma silica content with 68- 77% SiO₂

Intrusive igneous structures / Intrusions

Plutonic structures that form when magma intrudes, cool, and solidifies beneath preexisting country rocks.

Batholith & Stocks

Plutons with an irregular shape

Batholith

Intrusion that has surface exposures ≥100 km²

Stock

Intrusion that has surface exposures ≤100 km²

Concordant structures

Igneous structures oriented parallel to the pre-existing layering in the surrounding country rock.

Sill

A tabular, concordant pluton that is parallel to the country rock that is formed through the injection of magma along a plane.

Laccolith

A blister-like concordant pluton with a flat floor and domed roof.

Lopolith

Dish- to funnel-shaped, concordant plutons resembling a champagne glass.

Veins

Small-scale sills or dikes that can either be concordant or discordant and forms when hot fluids flow thru fractures, cool, and crystallize.

Vein swarms

Veins that occur in great abundance that may display random or preferred orientations.

Discordant structures

Igneous structures that cut across any layering within the country rock.

Neck / Volcanic neck

Cylindrical dikes exposed at the surface due to subsequent erosion, which represent ancient conduits pipes of ancient volcanoes whose summits have long been eroded.

Diatreme

Carrot-shaped, cylindrical pipes that can extend up to 200 km and develops through explosive intrusions originating deep within the mantle.

Kimberlites

Ultramafic, diamond-bearing rocks

Diatremes

Kimberlites are typically associated with?