Class 4 Igneous Rocks & Magmatic Deposits - Notes

Igneous Rocks

Igneous rocks are those which form by crystallizing from molten magma (melted rock).   Igneous rocks are one of the most important groups of rocks because they make up about 95 % of the earth’s crust.  Igneous rocks in the form of melts also comprise everything below the crust, for example, the mantle, outer and inner core zones. 

Most of the igneous rocks present at or near the earth’s surface are crystallized from some type of silicate magma, which means the melt was made up mostly of the two elements silicon and oxygen, combined to form “silica”, or SiO2.  The continental areas are made up of close to 60 % silica, and the oceanic areas are made up of about 47 % silica. 

Direct observations of volcanism and the crystallizing of lava at the surface is evidence for crystallization from a melt only in the case of volcanic rocks.  It is not possible to observe the crystallization of plutonic and hypabysal rocks, for obvious reasons.   We just accept that these also form from melt based on indirect evidence, such as similarity of composition with volcanic rocks, and by the fluid nature of the original material indicated by contact relations in the field (ie, the way the material looks injected into other rocks).  In reality magmas often contain more than just liquid.  They also contain a certain amount of gas or vapor, and a certain amount of crystals of some minerals.  This is because each mineral has its own unique temperature at which it passes from a liquid state into a solid state.  This temperature is actually dependent on the pressure being exerted on the molten rock, which is mostly a function of the depth below the surface.  At great depths, the pressure is very high, and at shallow depths the pressure is very low. 

We broadly categorize igneous rocks by:  1) genesis, 2) composition, and 3) texture.  Their genesis refers to the environment where the igneous rock formed, which  is strongly influenced by depth below the surface.  These three categories are called "plutonic, volcanic and hypabysal".  

Genetic Classification of Igneous Rocks

PLUTONIC:

• Crystalline rocks formed by crystallization of magma at great depth below the surface, resulting in slower cooling and/or greater fluid content

• Coarse-grained sizes (greater than a millimeter); equigranular to porphyritic

VOLCANIC:

• Crystalline rocks formed by crystallization of magma at the surface, which rapidly crystallizes (quenches) as it comes into contact with the atmosphere or within a body of water. 

• Fine-grained textures (less than a millimeter); equigranular to porphyritic to porphyry textures.

HYPABYSAL:

• Crystalline rocks formed by crystallization of magma at a shallow depth below the surface
• Mixed fine- to medium-grained textures; equigranular to porphyritic to porphyry textures

Plutonic means deep underground (plutonic).  Another name for plutonic rocks is “intrusive”, because they intrude or invade other rocks.  Volcanic means extruded at the surface.  Another name for these rocks is “extrusive”, because they are extruded onto the surface.  They may be extruded into the air or underwater, which are called “subaerial” and “submarine” environments, respectively.   Hypabysal means intruded into rocks at near surface conditions.  Hypabysal rocks often occur in the form of smaller types of intrusions, like dikes, sills or plugs.  Hypabysal rocks are often found in close association with volcanic rocks because they often form the feeder systems for volcanic lava being extruded at the surface. 

Compositions of Igneous Rocks

The compositions of igneous rocks can be broadly subdivided into four major types.  These are felsic, mafic, intermediate and ultramafic.

Felsic:  “Fel” from feldspar, and “sic” from silicon; generally light colored.  A rock made up of abundant silica, aluminum and alkalis (potassium, sodium, calcium) due to the presence of abundant feldspar and quartz. 

Mafic:  “Ma” from magnesium and “fic” for iron; generally dark colored.  A rock with abundant magnesium and iron, due to the presence of abundant iron-magnesium-bearing minerals (such amphibole or pyroxene).

Intermediate:   Contains a balance between the minerals quartz plus feldspar, and mafic minerals; generally moderate in tone and color.

Ultramafic:  Composed chiefly of the iron-magnesium minerals pyroxene and olivine.

The most  common igneous rock types are categorized below on the basis of color and genesis:

  

	Light-colored		Dark-colored	Very dark-colored
	FELSIC	INTERMEDIATE	MAFIC	ULTRAMAFIC
Volcanic (Fine-grained)	Dacite	Andesite	Basalt
Plutonic: (Coarse-grained)	Granite	Diorite	Gabbro	Peridotite Pyroxenite Dunite

This classification scheme, which works well for crude field approximations, is summarized graphically in the diagram below (from Putnam, 1971).

                   

Igneous rocks which are light in color are on the left and those which are dark in color are on the right.  The top row of the chart contains the volcanic rocks and the second row contains the plutonic rock equivalents (in terms of composition).   For example, a volcanic rock which is light in color is called a rhyolite.  According to this classification method, a rhyolite contains approximately 5 % to 25 % quartz,  62 % to 74 % feldspar, and 10 % to 25 % ferromagnesian (iron and magnesian)  minerals.  The coarse-grained equivalent to a rhyolite is called a granite.

Refer to figures 3-3, 3-4, and 3-5 on pages 21-22 of the text to see photographs of three common plutonic igneous rock types, including granite, diorite and gabbro. 

When a more detailed classification is needed, the standard method employed to classify the most common varieties of plutonic rocks is the method proposed by Streckheisen (1977).  This method classifies common plutonic igneous rocks on the basis of the mineral content of four minerals:  quartz, plagioclase feldspar, alkali feldspar and feldspathoids.  The total percentage of each of these minerals is estimated.  These percentage values are then plotted on the diagram to classify the rock.  Quartz occupies the uppermost vertex of the upper triangular portion of the diamond-shaped diagrams, and the two species of feldspars, alkali feldspar and plagioclase, occupy the lower two vertices.  Feldspathoid-bearing rocks are very rare, so the lower triangular areas of the diagrams are seldom used. 

Q =  Quartz

P =  Plagioclase

A =  Alkali Feldspar (Orthoclase)

F =  Feldspathoids

To use the diagrams, consider the following example.  A coarse-grained (plutonic) rock containing 25 % quartz, and having a ratio of alkali feldspar to plagioclase of 75 %, plots as a “granite”.  The volcanic compositional equivalent would plot as a “rhyolite”.   This method can be used for petrographic analysis (using a thin-section of the rock) or it can be used to make more general field identifications.  Its usefulness as a field classification scheme can be limited by the ability to distinguish between the two different species of feldspars, which can be very difficult in some cases.  Typically this distinction requires the observation of twinning habits, or use of mineral staining techniques.

Textures of Igneous Rocks

Textures of igneous rocks are extremely variable, but there are some generalizations which can be made and there are several textural descriptors which are in common usage.  As shown above, the igneous rocks can initially be classified on the basis of the grain size, as to whether the rock as coarse-grained, or fine-grained.  Coarse-grained igneous rocks generally are greater than 1 mm in the average crystal size.  These are referred to as “phaneritic”.  Fine-grained rocks generally have crystal sizes smaller than 1 mm, or too small to observe with a hand lens.   These are also referred to as “aphanitic”.  

Some rocks contain a mixture of both fine-grained and coarse-grained crystals.  In this case, we call the largest grains the “phenocrysts”.  The phenocrysts appear suspended in the mass of finer grained crystals, which we call the “groundmass”.  The common term for a mixture of phenocrysts in a finer-grained groundmass is called “porphyritic” (for example, a porphyritic granite).  In some cases, the phenocrysts sizes are distinctly larger compared to a very fine grained groundmass, causing this type of texture to become highly accentuated.  This is a special group of rocks called “porphyries”.  The convention is to precede the term porphyry with the composition type comprising the fine-grained groundmass. 

It is also useful during the overall description of a rock to indicate how well developed the shape of the crystals, in terms of the shape and number of sides which are present.  The terms normally used to convey this information are:

Some igneous rocks which consist of glass instead of crystals, or glass with minor amounts of phenocrysts, due to extremely rapid cooling or quenching.   This is most common in volcanic environments, where lava can be subjected to rapid cooling by an violent eruption which hurls blobs of magma, which cools very quickly by contact with air.  It can also form where lava comes into contact with water.  Glassy volcanic rocks can only be classified by their chemical composition, since discrete minerals are not present.

Shapes of Igneous Intrusions

As mentioned, igneous rocks form when molten rock crystallizes by cooling either on the surface (volcanic), at shallow depth (hyp-abyssal) or deep underground (plutonic).  Plutonic and hypabysal rocks are said to be “intrusive”, because they intrude other rocks, whereas volcanic rocks are said to be “extrusive” because they are extruded onto other rocks or ejected into the atmosphere. 

Igneous intrusions can form many different shapes, but several described here are the most common.  “Dikes” are tabular intrusions at high angles to the layering of the rocks they intrude.  “Sills” are tabular intrusions parallel or very low angle to the layering of the rocks they intrude.  Dikes and sills are usually relatively narrow in width.   Dikes can form in clusters with parallel orientations between each, which is referred to as a “dike swarm”. 

Irregular to rounded intrusions of substantial size are called “stocks”, “plutons” and “batholiths”, and are classified by the aerial extent of their outcrops.

“Laccoliths” are intrusions which are flat like a sill on along the bottom edge, and with an upper edge shaped like a huge bulge upward.  Their shape suggests they originally were intruded as sills, but before cooling was complete, further injection forced the overlying rocks to bend upward, creating a rounded upper edge on the intrusion.

Frequently in plutonic terranes intrusions which were thought to be discrete are found to be actually connected to others at depth.  The shallower, secondary intrusion extending off of the larger one at depth is called an “apophysis”.   

Bowen’s Reaction Series

Bowen’s Reaction Series is a theory concerning the crystallization process which occurs when a magma cools.  As a magma cools, individual mineral species selectively crystallize at some unique temperature.  Thus we have some minerals crystallizing very early in the cooling history when the melt is still extremely hot, and other minerals which crystallize at successively lower and lower temperatures.  The reaction series has two branches, shown below:

The earlier formed crystals can sink to the bottom of the magma by gravitational settling, or tectonic activity (directed pressure) can squeeze the melt out and away from these crystals.  Whatever the case, there is abundant experimental and field evidence which suggests that at each stage of crystal formation, the remaining melt’s composition continues to evolve over time as selective components are removed during the formation of crystals.  This process is called “fractional crystallization”. 

Not only does the melt change composition, but the crystals also change their original composition by reacting with the remaining melt.  When a crystal first forms, it is in equilibrium with the melt.  However, as noted, the melt changes its composition over the course of time as crystals continue forming.  The net result is the earlier crystals become less stable, and so they gradually alter their composition to become more stable minerals.  The alteration process can occur either as a “discontinuous” or a “continuous” process.  Both processes are active at the same time.

A “discontinuous reaction series” is where the early formed minerals react with the melt to form completely new mineral species.  An example of a discontinuous reaction series is the reaction of the very high temperature mineral olivine with the melt to form pyroxene.  With further cooling, pyroxene reacts with the remaining melt to form amphibole.  Amphibole reacts with the melt to form biotite.  Each new mineral forms at a lower and lower temperature.  This sequence is shown graphically on the left side of the diagram.

In a “continuous reaction series” the crystals react with the melt but do not form completely new mineral species.  Instead, ionic diffusion causes an exchange or ions to occur in the crystal lattice of the mineral, resulting in a ‘modified’ composition.   The primary example of continuous reaction is that of Ca-rich plagioclase (or anorthite).  The continuous reaction of anorthite with the melt causes the anorthite to change its composition to become a more Na-rich plagioclase (or albite).  This sequence is shown graphically on the right side of the diagram.  

 That the composition of melt changes, and that crystals change their composition over time is evidenced by the observation of rims of albitic plagioclase on anorthitic plagioclase in some plutonic rocks.  Furthermore, it is also known that anorthite is the most common plagioclase in basaltic (or mafic) igneous rocks, and albite is the most common plagioclase in rhyolitic (or felsic) igneous rocks.  Thus, mafic igneous rocks have higher melting temperatures, because they are comprised mostly of high temperature minerals.  Felsic igneous rocks have lower melting temperatures, because they are comprised mostly of low temperature, and even hydrous minerals.  At one time it was believed that the evolution of a basaltic magma, by continuous modification of its melt composition by fractional crystallization, could result in the formation of a residual magma of rhyolitic composition.  Currently, this type of extreme differentiation is thought to be very rare, although it is widely accepted that fractional crystallization plays an important role in creating a wide variety of igneous rock compositions from the same parent magma.

Rock-Forming Minerals

Although any of hundreds of different minerals can group together to form an igneous rock, only six are very common.  These common minerals are referred to as the “rock-forming minerals” and they include quartz, feldspars, micas, amphiboles, pyroxenes and olivine.  Note that these are all present in Bowen’s reaction series.  Since they are so common, the abundances of these minerals form the basis for most igneous rock classification schemes. 

Most rocks contain some amount of ‘minor’ or ‘accessory’ minerals, which are those comprising a very small percentage of the rock.  These are often so fine-grained that they require magnification with a microscope to observe.  The most common accessory minerals present in igneous rocks include zircon, sphene, magnetite, illmenite, hematite, apatite, pyrite, rutile, corundum and garnet.  Accessory minerals often contain some of the more rare elements from the periodic table.  It these can be extracted, and there is market demand for such elements, then rock containing these valuable accessory minerals may constitute an ore deposit.

Magmatic Ore Deposits

Magmatic ore deposits are those which are formed during crystallization of a magma, deep underground.  The host rock for the mineralization can range from ultramafic to felsic.  The deposit can consist of massive ores in some cases, and disseminations of rare minerals in others.  In the case of more massive ores, there are three primary means of concentrating minerals of value during the formation of these deposits:

The process of gravitational settling causes early-formed minerals to sink to the bottom of a magma chamber.  This process is best exemplified in magmas with ultramafic and mafic compositions, and the best examples are chromite deposits.  Chromite is a very high temperature mineral which is also quite heavy.  As a result, in some situations it will tend to sink and form layers of massive ore in the bottom of a magma chamber.  The intrusion itself tends to be layered, with rocks like dunite (massive olivine) forming the lowest layers, overlain by gabbro layers, overlain by norite (plagioclase-rich rock).  These intrusions are often funnel-shaped, with the neck forming a feeder system.  Large magmatic deposits of this type are located at Stillwater, Montana, in south Africa, and in Manitoba.

Differentiation causes a concentrating effect resulting in a concentration of selected elements in the residual magma.  These elements are the ones which did not fit well inside of common rock forming minerals.  Instead, they become included in the final liquid present, which forms “pegmatite”.  Pegmatites are very coarse grained rocks and form at the very last stage of crystallization.  The final fluids tend to have a very high water content, which also contributes to forming large crystals.  Pegmatites also contain accessory minerals of special interest because they trap the rare elements in their crystal structure.  Because of the rarity of some of these elements, the accessory minerals in the pegmatite can be be quite valuable and constitute an ore deposit.

Immiscibility is a physical separation of a portion of a magma.  Immiscible melts form irregular shaped segregations or may be injected as a dike into previously crystallized material.  Where the immiscible melt material consists of massive chromite or sulfide ore which will probably migrate downward with gravity, due to the abnormally high specific gravity.  The famous nickel, platinum and sulfide ores of Sudbury, Ontario are prime examples.

Another important deposit type which is classified as magmatic “diamond-bearing kimberlite”.  Kimberlites are rocks of ultramafic composition which are generally fine-grained.  Diamonds occur as accessory minerals in the kimberlite, which is frequently highly altered.  Kimberlites are thought to represent mantle rocks emplace near the surface by forceful, possibly explosive, intrusion.  The shapes are often like a vertical pipe, and some occur as apophyes connected to larger dike structures at depth.  The world’s most famous diamond-bearing kimberlite deposits occur in South Africa; many also occur in central and eastern Canada.