intrusion types and intrusive igneous rocks

intrusion types and intrusive igneous rocks Intrusive igneous rocks are formed where molten rock (magma) cools and solidifies without reaching the surface of the Earth to form a volcano. Large intrusive bodies cool slowly and are coarse-grained, whereas small intrusive bodies can cool quickly to form fine-grained igneous rocks. The coarse-grained rocks of large intrusive bodies are known as plutonic rocks. The edges of large intrusive bodies that have come into contact with the cooler surrounding country rock are, however, fine-grained. This fine-grained margin is known as a chilled margin. Intrusive igneous rocks, by definition, solidify beneath the Earth's surface and are seen at the surface only when uplift and erosion have unroofed them during exhumation.

Intrusion types

There are two main methods by which igneous rocks intrude other rocks: active and passive intrusion. Actively intruding rocks deform the rocks around them to create a space for themselves. Figure 1 shows a rising diapir of buoyant molten rock that has forced its way through the surrounding sedimentary rocks, deforming those above into a dome, and rupturing the lower strata. The diagram also shows the way in which this type of deformation produces fractures in the rocks through which the intrusion is passing. Both radial and concentric dyke swarms can be formed. A section through some concentric cone sheets is illustrated in Fig. 1. A variation of diapirism is the formation of a lopolith, a tree-like igneous intrusion formed where the magma has forced its way not only upwards but also along planes of weakness, such as bedding planes (Fig.2). The Duluth gabbro in Minnesota is a classic example.

Passively intruding rocks are accommodated as the country rocks are broken, without being otherwise deformed, to make the necessary space. Figure 3 shows one mode of passive intrusion. Faults, which are usually circular in plan and steeply inclined outwards, allow a cylindrical block of material to separate and subside. Magma rises into the space created above the subsiding block and begins to solidify as a cauldron-shaped intrusion. This type of passive intrusion is called cauldron subsidence. It can occur repeatedly so that numerous concentric circular intrusions are formed. Classic examples are found in Yellowstone Park (Mount Holmes) and in Glen Coe in Scotland.

Not all passively intruded igneous bodies replace a large block of subsided rock; the igneous body may also make space for itself by splitting off small blocks from the top of the magma chamber or by the intrusion of multiple dykes, side by side (Fig. 4). Notable examples of parallel dyke systems are found in the Tertiary intrusions of the west of Scotland, in Colorado (Spanish Peak), and in Montana (the Crazy Mountains). In the former case, liberated blocks sink into, and can be melted by, the magma, which, being more buoyant than the overlying rock, rises to fill the space above. The removal of individual blocks is called stoping. The blocks are called stoped blocks; pieces of rock that have not fallen away and remain hanging from the roof of the magma chamber are roof pendants. When erosion cuts into an igneous body of this type, stoped blocks can in places be seen preserved within the solid magma. Roof pendants may be preserved as islands of country rock in the middle of the roof of an exposed igneous intrusion.

A dyke is a parallel-sided, sheet-like, igneous intrusive body that cuts vertically through the country rock regardless of its type) and is commonly either perpendicular to sedimentary layers (strata) or vertical. If the sheet intrusion is emplaced along the interface between two beds and is horizontal, it is known as a sill. Well-known examples of sills are those of the Karroo region of South Africa, which extend over areas of thousands of square kilometres, and the Whin sill in northern England, along which much of Hadrian's Wall was built. The rock of which the Whin sill is formed is more resistant to erosion than the surrounding sedimentary rocks and consequently forms a prominent natural feature. Dykes and sills may range from tens of metres thick down to centimetres thick, but intrusive igneous bodies less than a centimetre or so thick are more often refered to as igneous veins.

Dykes and sills make space for themselves by pushing the rocks on either side apart because the magmatic pressure exceeds the inward pressure on the walls of the dyke or sill. This process, which is analogous to hydraulic fracturing, causes the rock mass to dilate. Multiple dykes may cause large, cumulative amounts of dilation, in some instances reaching as much as 100 per cent. This means that no original rock remains and the whole of the section being examined is dyke material. Dilation is one means by which an igneous body can be accommodated. The whole of the ocean floor has been formed by means of dilation. In this case, new dykes are added in the middle of the ocean, meaning that each dyke only has one chill margin, on the outside. Younger dykes are emplaced into the centre of the most recent dyke to be formed in the area, splitting the older dyke into two thinner dykes, each with one chill margin, and themselves solidifying to form a two-sided dyke. This type of intrusion formed of numerous dykes is called a sheeted dyke complex.

Passive intrusion requires brittle fracturing of the country rock, in contrast to active intrusion which entails some ductile deformation. Brittle fracturing is said to have occurred when the country rock has been broken, whereas ductile deformation occurs when a rock has behaved as a plastic solid or a viscous liquid. Whether the stresses set up by an intruding magma body result in ductile or in brittle deformation of the country rock depends principally on temperature, the pressure, and the rate at which the stress is applied. The term ‘country rock’ is used in igneous petrology to describe any rock into which an igneous rock intrudes, irrespective of whether it is brittle or ductile. In general, the upper part of the crust is more susceptible to brittle fracture, while magmas deeper within the crust are more likely to cause ductile deformation of the country rock. In some tectonic environments, the deformational environment set up by the tectonic activity may favour one or other type of behaviour. In County Donegal, Ireland, a number of granites have been intruded at the same crustal level and would be expected to have similar intrusion styles. However, the complicated tectonics of the region at the time of intrusion led to a mixture of active and passive styles of intrusion. Those granites intruded into areas of tectonic extension (where brittle fracture occurs more easily) had a tendency to be of the passive type, whereas the granites in areas of compressive tectonics generally exhibit active intrusion by ductile deformation. Passive intrusions at high levels within the crust are thought to be fed by lower-level magma chambers that have been actively intruded.

Contact relations

In an area where there have been multiple igneous intrusions their relative ages can be deduced from the contact relations. The fine-grained selvage called a chilled margin, formed in a younger rock in contact with an older rock, has already been mentioned. Other contact relations that are used to infer relative age are dykes and veins that cut across each other or features of an older rock. (An exception to this is back veining, see below.)

Back veining

Back veining is a feature of some igneous contacts that occurs where molten rock of high melting temperature (gabbro, for example) intrudes a rock of lower melting temperature, for example granite. In this situation the gabbro will intrude the granite at about 1300 °C, forming dykes and veins of gabbro which are then chilled. However, granite, which melts at about 800 °C, will still be melted by the heat of the gabbro after the gabbro has been chilled. The molten granite then intrudes the recently formed gabbro. At a normal igneous contact, the relative dates of the rocks can be deduced from their contact relations. The second rock to be intruded has veins into earlier rocks. Where back veining has occurred, each rock looks as though it has intruded the other. It can thus be concluded that the rock of lower melting temperature was the first to be emplaced.

Intrusive igneous rocks

The grain size of an igneous rock depends upon the rate of cooling. Slow cooling gives time for the nucleation and growth of crystals; fast cooling forces the molten rock to freeze into a fine-grained mass or a glass. Small intrusions, such as narrow dykes and sills, are usually fine-grained, while larger dykes and other intrusions are characterized by their larger grain size. Crystals form when molten rock begins to cool in a magma chamber. Material intruded as dykes from the magma chamber after crystallization has begun will form porphyritic dykes (i.e. dykes with easily visible crystals, some up to a few centimetres across). Many plutonic rocks have names that are distinct from those of their fine-grained compositional equivalents. Gabbros, dolerites, and basalts differ in grain size but are of the same chemical composition.

Most parent magmas, that is magmas whose composition has not been modified since the time of melt generation, are roughly gabbroic in composition. If a gabbroic melt crystallizes completely to a coarse-grained igneous rock, it is called a gabbro. Rocks that are both poorer and richer in silica than a gabbro are formed during the process of fractional crystallization, in which silica-poor material is crystallized and drops out of the magma, whose composition becomes more evolved and richer in silica. If the crystals in the magma chamber are denser than the liquid of the melt, they sink; others may remain in suspension, while some plagioclase crystals, which are of low density, are thought to float, forming rafts at the top of the magma chamber. Crystals that fall to the bottom of the magma chamber are usually those that are rich in magnesium and poor in silica, such as olivine and pyroxene. The crystals settle to form a cumulate, which is an ultrabasic rock (that is, it contains less than 45 per cent SiO₂ by weight).

Cumulates, being deposited by the liquids of the magma chamber, commonly display sedimentary structures that are the result of fluid dynamic processes acting upon the piles of crystals. Sedimentary features typical of this type of rock are graded bedding, developed when larger, heavier crystals were deposited at the bottom of a layer, and channel deposits, where a current of magma flowed across the surface of the crystal pile, forming deposits like those in a river channel. Ultrabasic cumulates are common in rock masses that appear to represent the remains of magma chambers.

The altered remains of igneous cumulates that form a large proportion of an ophiolite have been interpreted as the result of crystal fractionation during ocean-floor formation. The melt that remains after fractional crystallization is richer in silica than the parent magma. The formation of some very silica-rich granites has been attributed to the process of fractional crystallization.

During crystallization, melt may escape and be erupted at the surface, or new melt may arrive in the magma chamber, mixing with the melt already present and altering its composition back towards the composition of the parent magma. Through the cycle of fractional crystallization and replenishment, layered igneous intrusions may develop, in which each layer represents the products of crystallization of the parent magma, followed by the crystallization of the more evolved magma. The Skaergaard intrusion of east Greenland is a famous example of a layered ultrabasic intrusion showing chilled margins and coarse-grained crystalline layers.

The composition of a magma may also be changed by the addition (assimilation) of new material that has been melted from the sides of the intrusion. Assimilation entails the loss of heat from the magma in order to melt the contaminating rocks. It therefore seems unlikely that large amounts of contamination can occur in this manner without the solidification of the parent magma.

Granites

Granites are pale-coloured, silica-rich, intrusive igneous rocks. Current theory ascribes granitic magmas to two possible sources: some granites are interpreted as the result of partial melting of the crust to form a granitic melt; others are seen as the result of extreme fractional crystallization which removes mafic (dark-coloured, silica-poor) components from the melt. Large quantities of granite are known to be formed in the mountain ranges that result from continental plate collision where they intrude the cores of the mountains as a granite batholith. Along the subducting margin of the Pacific Ocean are found the batholith belts of western North and South America. The largest of these is the coastal batholith of Peru, which is over 1600 km long and 60 km wide. One such batholith in England is the batholith of Devon, Cornwall, and the Isles of Scilly. The moorlands of the area are each apophyses rising from the top of a large batholith at depth. The roofs and edges of these granites are associated with the mineral and metal wealth of Cornwall, which has drawn traders and miners to the area for over 4000 years. The Phoenicians are known to have traded in the area and the Beaker People, who were associated with mining operations in continental Europe, also found their way to the west of England.

Contact metamorphism

Country rock intruded by magma is subjected to sudden heating, usually at low pressures. The mineralogical and textural alteration in the country rocks resulting from this heating by igneous bodies is called contact metamorphism. Contact metamorphism often produces hardening of the country rock, which is then known as a hornfels. New mineral growth is particularly obvious in shales that are rich in aluminium. The new minerals that grow in the dark shales include spots of the mineral cordierite (Al₃(Mg,Fe²⁺)₂ [Si₅AlO₁₈]), white needles of sillimanite (Al₂SiO₅), and crystals of andalusite (Al₂SiO₅) and, where high temperatures have been sustained, garnet (e.g. almandine (Fe³⁺₂Al₂Si₃O₁₂)) may grow.

Judith M. Bunbury