Intrusive rock

Igneous Chemistry Felsic/acid = > 65% SiO2

– lots of Na, K, Al but little Mg, Fe, Ca – e.g. continental crust; granite

Intermediate = 53 - 65% SiO2– e.g. subduction zone volcanic rocks; andesite

Mafic/basic = 45 - 52% SiO2 – lots of Mg, Fe, Ca but silica poor– e.g. oceanic crust; basalt

Ultramafic = < 45% SiO2– e.g. mantle

Igneous Compositions

The type of rock that melts and/or the composition of the parent magma determines the chemistry of the rock that crystallizes.

Other processes affect this too…– Crystallization sequence of minerals– Crystal settling– Assimilation– Magma mixing

Within the field of geology, Within the field of geology, Bowen's reaction seriesBowen's reaction series is is the work of the petrologist, Norman L. Bowen who was the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be able to explain why certain types of minerals tend to be found together while others are almost never associated found together while others are almost never associated with one another. He experimented in the early 1900s with with one another. He experimented in the early 1900s with powdered rock material that was heated until it melted and powdered rock material that was heated until it melted and then allowed to cool to a target temperature whereupon he then allowed to cool to a target temperature whereupon he observed the types of minerals that formed in the rocks observed the types of minerals that formed in the rocks produced. produced.

Bowen’s Reaction Series

He repeated this process with He repeated this process with progressively cooler temperatures and progressively cooler temperatures and the results he obtained led him to the results he obtained led him to formulate his reaction series which is formulate his reaction series which is still accepted today as the idealized still accepted today as the idealized progression of minerals produced by progression of minerals produced by cooling magma.cooling magma.


Bowen’s Reaction Series Helps understand how you can get intermediate and

mafic rocks from mafic magmas.

When a melt cools, different minerals crystallize at different temperatures!

Minerals that cool at nearly the same temperatures tend to occur together in rocks. Only certain minerals can occur together.

If you remove the minerals that have crystallized at a certain temperature, you change the overall chemistry of the system…


Cont

inuo

us

Cont

inuo

us Discontinuous

Discontinuous

Bowen’s Reaction SeriesDiscontinuous Series Only ferromagnesian. One mineral changes to

next as temperature slowly drops.

Each change is a chemical reaction between solids (crystals) and fluids (melt, water, gas) present at the time.

Reactions not always complete.

Bowen’s Reaction SeriesContinuous Series Nonferromagnesian

plagioclase feldspar (Ca to Na)

As magma cools, Ca-rich plagioclase reacts with melt and proportionally more Na-rich plagioclase crystallizes

Continues until all Ca and Na is used up.

Plagioclase is often, therefore, zoned with Ca rich core and Na rich rims.

Bowen’s Reaction SeriesFelsic Minerals Not really part of

series. As Mg, Fe, Ca, and

Na are used up, left over magma has more and more SiO2, K, Al, water, and other exotic stuff (U).

Form K feldspars, muscovite, quartz, accessory minerals.

Increasing Si content

Minerals of Low SilicationMinerals of Low Silication

If there is a considerably deficiency of silica in magma, elements If there is a considerably deficiency of silica in magma, elements may not be able to combine with sufficient silica to form may not be able to combine with sufficient silica to form minerals. This type of minerals are called minerals of low minerals. This type of minerals are called minerals of low silication. Example: K and Na form Leucite and Nepheline silication. Example: K and Na form Leucite and Nepheline instead of orthoclase and albite.instead of orthoclase and albite.

Minerals of High SilicationMinerals of High Silication

In magma, when there is an adequate amount of silica, elements In magma, when there is an adequate amount of silica, elements form the minerals which are rich in silica contents. These form the minerals which are rich in silica contents. These minerals are termed as minerals of high silication.minerals are termed as minerals of high silication.

Mineral of Low silication+silicaMineral of Low silication+silica Mineral of High silicationMineral of High silication

Minerals of Low Silication Minerals of High Silication

Leucite Orthoclase

Nepheline Albite

Analcite Anorthoclase

Olivine Orthorhombic pyroxene

Biotite Augite

Hornblende

Quartz(SiOQuartz(SiO22) doesn’t exist with minerlas of low silication) doesn’t exist with minerlas of low silication

Any excess of silica which may be left over after the bases are Any excess of silica which may be left over after the bases are fully satisfied crystallizes out as quartz. The inter relationship of fully satisfied crystallizes out as quartz. The inter relationship of minerals of low and high silication shows that minerals of low minerals of low and high silication shows that minerals of low silication and SiOsilication and SiO22 combinedly forms minerals of high silication. combinedly forms minerals of high silication. Hence, quartz can’t co-exists with minerals of low silication.Hence, quartz can’t co-exists with minerals of low silication.

NaAl(SiONaAl(SiO44) + 2(SiO) + 2(SiO22) = NaAl.Si) = NaAl.Si33OO88

Nepheline Quartz AlbiteNepheline Quartz Albite

Essential Igneous rock forming mineralsEssential Igneous rock forming minerals

Felsic/Silicic/Light colored Felsic/Silicic/Light colored mineralsminerals

QuartzQuartz

Potash FeldsperPotash Feldsper

Mica(muscovite)Mica(muscovite)

Mafic/Ultramafic/Mafic/Ultramafic/Dark colored Dark colored mineralsminerals

OlivineOlivine

PyroxenePyroxene

AmphibolesAmphiboles

BiotiteBiotite

Plagioclase feldsparPlagioclase feldspar

Intrusive Igneous Rock Bodies Magmas crystallized beneath the Earth's

surface form intrusive bodies of igneous rock known as plutons.

The term pluton (after the Greek god Pluto) refers to any igneous intrusion regardless of size, shape or composition of the magma.

Classification of plutons is based on: 1. Geometry of intrusion: size and shape

2. Relationship to surrounding rocks: – concordant or boundaries parallel to layering in surrounding rocks – discordant or boundaries cut across layering in surrounding rocks

Intrusive Igneous Rock Bodies


Concordant Igneous Bodies

Sill: A sill is a concordant body, few cm to >1 km thick, produced when magma is injected between layers of older sedimentary or volcanic rock, and are generally composed of intermediate to basic composition magma.


Laccolith: A laccolith represents magma that pushes overlying rock layers upward to form a condordant, mushroom-shaped, sill-like body, typically comprising magma of intermediate to granitic composition.

It causes folding of the overlying It causes folding of the overlying rock layersrock layers

Flat bottom and convex upward Flat bottom and convex upward dome shaped igneous bodydome shaped igneous body


Lopolith: A lopolith is a spoon-like shaped concordant body similar to a sill except the floor and roof sag downward. The intrusions are generally magma of intermediate to basic composition.

These are basin or saucer shaped concordant bodies with top These are basin or saucer shaped concordant bodies with top nearly flat and convex bottom.nearly flat and convex bottom.

PhacolithsPhacoliths These are concordant These are concordant

bodies that occurs along the bodies that occurs along the crests and troughs of the crests and troughs of the folded sedimentary strata.folded sedimentary strata.

In folding, the crest and In folding, the crest and trough are regions of trough are regions of weakness and tension, weakness and tension, magma often intruded into magma often intruded into these trough and crest.these trough and crest.

Southern Patagonia Phacoliths, ChileSouthern Patagonia Phacoliths, Chile

Discordant Igneous Bodies

Dike:Dike: A dike is a discordant body, few cm A dike is a discordant body, few cm to >100 m thick, produced when magma is to >100 m thick, produced when magma is injected along fractures in surrounding rock injected along fractures in surrounding rock layers. Dikes ftypically form from magmas layers. Dikes ftypically form from magmas of basic to granitic composition. Ring and of basic to granitic composition. Ring and Radial dikes are discordant bodies having Radial dikes are discordant bodies having either a concentric (circular) or radial either a concentric (circular) or radial distribution; develop above a large distribution; develop above a large subsurface intrusive body (batholith or subsurface intrusive body (batholith or stock) or adjacent to volcanic pipes or stock) or adjacent to volcanic pipes or necks. necks.


Batholith:Batholith: A batholith is a discordant magma body with A batholith is a discordant magma body with exposed surface area of more than 100 square kilometers; exposed surface area of more than 100 square kilometers; typically consists of multiple intrusions. Batholith are typically consists of multiple intrusions. Batholith are usually magma of granitic composition with minor usually magma of granitic composition with minor intermediate varieties. intermediate varieties.

BatholithsBatholiths are the largest bodies of are the largest bodies of

Igneous rock, irregular in Igneous rock, irregular in shape and occupies large shape and occupies large area. Their occurrence is area. Their occurrence is commonly associated with commonly associated with the mountain-building the mountain-building process. process.


Stock:Stock: A stock is a discordant magma body with exposed A stock is a discordant magma body with exposed surface area of less than 100 square kilometers; may represent surface area of less than 100 square kilometers; may represent exposed portion of a much larger intrusion. It is usually magma exposed portion of a much larger intrusion. It is usually magma of granitic composition with minor intermediate varieties. of granitic composition with minor intermediate varieties.

StocksStocks Are irregular Are irregular

masses of masses of batholiths habit of batholiths habit of smaller dimension.smaller dimension.

Discordant Igneous BodiesVolcanic pipes and Volcanic pipes and necks:necks: are discordant are discordant bodies that represent bodies that represent the upper part of the the upper part of the conduit that connects conduit that connects the volcanic vent the volcanic vent (crater) with an (crater) with an underlying magma underlying magma source (magma source (magma chamber or reservoir). chamber or reservoir). Volcanic necks are Volcanic necks are erosional remnants of erosional remnants of magma that solidified in magma that solidified in the pipe or conduit.the pipe or conduit.


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Intrusive rock