Under magmatism understand the totality of phenomena associated with the formation, evolution of the composition and movement of magmas to the surface of the Earth. Magmatism is one of the most important deep processes in the earth's interior. According to the form of manifestation, magmatism is divided into intrusive and effusive. The difference between them largely determines the mechanisms of rock formation.
The concept of magma
Magma is a high-temperature fluid-silicate melt that forms in deep chambers, mainly in the upper mantle (asthenosphere) and partly in the lower layers of the earth's crust. The formation of a magma chamber occurs at a combination of certain values of pressure and temperature. Such primary magma has a homogeneous composition, including the following components: liquid (melt), in which the gas or volatile phase (fluid) is dissolved. There are also somesolid crystalline substance. As you move to the surface, the primary magma evolves depending on the specific conditions.
Evolution of magma includes several types of processes. First, she experiences different kinds of differentiation:
- segregation, in which it separates into immiscible liquid components;
- crystallization differentiation. This most important process is associated with the precipitation (crystallization) of certain compounds from an amorphous melt at various combinations of temperature and pressure.
Secondly, magma changes its chemical composition as a result of interaction with host rocks. This phenomenon is called contamination.
Crystallization processes in magma
Since magma is a mobile mixture of many substances and is in changing conditions, the crystallization of its components is a very complex process. It is usually divided into three main phases:
- High temperature early magmatic phase. At this stage, high-density iron- and magnesium-containing minerals fall out of the magma. They settle and accumulate in the bottom areas of the magma chamber.
- Mid-temperature main magmatic phase in which the main components of rocks, such as feldspars, quartz, micas, pyroxenes, amphiboles, are formed. Calcium precipitates, the vast majority of silicon and aluminum. Crystallization in this phase is already accompanied by a shortage of space in the magma chamber, so the resulting minerals are finer-grained.
- Low-temperature late magmatic (pegmatite)phase. At this stage, the mobile so-called pegmatite magma remnant, enriched in volatile components, spreads through the cavities and cracks remaining in the magma chamber, contributing to the recrystallization of host rocks. Pegmatite veins are characterized by the formation of large crystals that can grow into each other. This stage borders and is closely related to the hydrothermal phase of mineral formation.
Volcanism and plutonism
There are such forms of manifestation of magmatism as intrusive and effusive. The difference between them lies in the conditions of evolution of magmas and the place of their solidification. The last factor plays a particularly important role.
Effusive magmatism is a process during which magma reaches the surface of the Earth through a supply channel, rises to the top, forming volcanoes, and freezes. The erupted magma is called lava. When it reaches the surface, it intensively loses its volatile component. Solidification also occurs quickly, some types of lavas do not have time to crystallize and solidify in an amorphous state (volcanic glasses).
Intrusive magmatism (plutonism) is different in that the magma does not reach the surface. Intruding in one way or another into the overlying horizons of host rocks, magma solidifies at depth, forming intrusive (plutonic) bodies.
Classification of intrusions
Relationships of host rocks with products of intrusive magmatism and types of intrusive bodies are distinguished according to many criteria, in particular, such as:
- Formation depth. There are near-surface (subvolcanic), medium-deep (hypabyssal) and deep (abyssal) intrusions.
- Location relative to host rock. According to this criterion, embedded arrays are divided into consonant (concordant) and discordant (discordant).
Also, the nature of intrusive magmatism and types of intrusions are classified according to such features as the ratio of the structure of the plutonic body to the contact surface (conformal and disconformal), relation to tectonic movements, shape, size of the massif, and so on.
The criteria for identifying different types of magmatic intrusions are closely related. For example, depending on the structure of the enclosing stratum, the depth and mechanism of formation of the magmatic massif and other manifestations of intrusive magmatism, the shapes of intrusions can vary greatly.
Mechanisms for the introduction of magma into the rock mass
Magma can penetrate into the host stratum in two main ways: along the planes of stratification of the sedimentary stratum or along existing cracks in the rock.
In the first case, under the pressure of magma, the layers of the roof rise - the overlying areas of the thickness - or, conversely, as a result of the influence of the mass of intruding magma, the underlying layers sag. This is how consonant intrusions are formed.
If magma penetrates upwards, filling and expanding cracks, breaking through layers and collapsing roof rocks, it itself forms a cavity that will be occupied by an intrusive body. In this way, unconformably occurringplutonic bodies.
Shapes of embedded igneous masses
Depending on the specific path along which the process of intrusive magmatism proceeds, the forms of intrusive bodies can be very diverse. The most common unconformably occurring igneous massifs are:
- Dike is a plate-like steeply dipping body that crosses the enclosing strata. The dikes are much longer than thick, and the contact surfaces are almost parallel. Dikes can be of different sizes - from tens of meters to hundreds of kilometers in length. The shape of the dikes can also be ring or radial, depending on the location of the cracks filled with magma.
- A vein is a small secant body of an irregular, branched shape.
- Stem is a column-shaped body characterized by vertical or steeply dipping contact surfaces.
- Batholith is the largest variety of intrusions. Batholiths can be hundreds or even thousands of kilometers long.
Overlapping bodies also take on various forms. Among them are often found:
- Sill is a bedded intrusion whose contact surfaces are parallel to host beds.
- Lopolith is a lenticular array, convex facing down.
- Laccolith is a body of a similar shape, the convex side of which is located at the top, like a mushroom cap. Mount Ayu-Dag in Crimea is an example of gabbroid laccolith.
- Phacolite is a body located in the fold of the host rock trough.
Intrusion Contact Zone
The formation of plutonic bodies is accompanied by complex processes of interaction at the boundary with the enclosing stratum. Zones of endocontact and exocontact are formed along the contact surface.
Endocontact changes occur in the intrusive due to the penetration of host rocks into the magma. As a result, magma near the contact undergoes chemical changes (contamination) that affect mineral formation.
The exocontact zone occurs in the host rock as a result of the thermal and chemical effects of magma and is characterized by active processes of metamorphism and metasomatism. Thus, volatile magma components can replace minerals in the exocontact zone with introduced compounds, forming the so-called metasomatic halos.
Mineral compounds carried out by volatile components can also crystallize directly in the contact zone. This process plays a significant role in the formation of, for example, micas, and with the participation of water, quartz.
Intrusive magmatism and intrusive rocks
Rocks formed as a result of deep magma crystallization are called intrusive, or plutonic. Effusive (volcanic) rocks are formed when magma erupts on the surface of the Earth (or on the ocean floor).
Intrusive and effusive magmatism gives rise to series of rocks similar in mineral composition. The classification of igneous rocks by composition is based on the content of silica SiO2. According to this breed criterionsubdivided into ultrabasic, basic, medium and acidic. The silica content in the series increases from ultrabasic (less than 45%) rocks to acidic (more than 63%). Within each class, rocks differ in alkalinity. The main intrusive rocks in accordance with this classification form the following series (volcanic analogue in brackets):
- Ultrabasic: peridotites, dunites (picrites);
- Main: gabbroids, pyroxenites (bas alts);
- Medium: diorites (andesites);
- Acidic: granodiorites, granites (dacites, rhyolites).
Plutonic rocks differ from effusive ones by the conditions of occurrence and the crystal structure of the minerals that compose them: they are full-crystalline (do not contain amorphous structures), clear-grained and have no pores. The deeper the source of rock formation (abyssal intrusions), the slower the processes of magma cooling and crystallization proceeded, while maintaining a large amount of the volatile phase. Such deep rocks are characterized by larger crystalline grains.
Internal structure of intrusive bodies
The structure of plutonic massifs is formed in the course of a complex of phenomena united under the general name of prototectonics. It distinguishes two stages: prototectonics of the liquid and solid phases.
At the liquid-phase stage, the primary striped and linear textures of the resulting body are laid. They reflect the direction of flow of the intruding magma and the dynamic conditions for the orientation of crystallizing minerals (for example, the parallel arrangementmica crystals, hornblende, etc.). Textures are also associated with the location of fragments of alien rock that fell into the magma chamber - xenoliths - and isolated mineral accumulations - schlieren.
The solid-phase stage of intrusive evolution is associated with the cooling of the newly formed rock. Primary cracks appear in the massif, the location and number of which are determined by the cooling environment and the structures formed in the liquid phase. In addition, secondary structures develop in such a magmatic mass due to fragmentation of its sections and displacements along ruptures.
The study of prototectonics is important to clarify the conditions for the location of mineral deposits within intrusions and in the surrounding rocks.
Magmatic intrusions and tectonics
Rocks of intrusive origin are widespread in various areas of the earth's crust. Some manifestations of intrusive magmatism make a significant contribution to both regional and global tectonic processes.
During continental collisions in the course of increasing the thickness of the crust, due to active granitic magmatism, large batholiths are formed, for example, the Gangdis batholith in the Trans-Himalayas. Also, the formation of large batholiths is associated with active continental margins (Andean batholith). In general, silicic magma intrusions play an important role in mountain building processes.
When the crust is stretched, series of parallel dikes often form. Such series are observed in mid-ocean ridges.
Sills are one of the characteristic forms of intracontinental magmatic intrusions. They can also have a large extent - up to hundreds of kilometers. Often magma, penetrating between layers of sedimentary rocks, forms several layers of sills.
Deep magmatic activity and minerals
Due to the peculiarities of crystallization in the processes of intrusive magmatism, ore minerals are formed for chromium, iron, magnesium, nickel, as well as native platinoids in ultrabasic rocks. In this case, heavy metals (gold, lead, tin, tungsten, zinc, etc.) form soluble compounds with volatile magma components (for example, water) and concentrate in the upper regions of the magma chamber. This occurs in the early phase of crystallization. At a later stage, a mobile pegmatite residue containing rare earth and rare elements forms vein deposits in intrusive fractures.
Thus, the Khibiny on the Kola Peninsula are a laccolith, exposed as a result of erosion of the enclosing stratum. This body is composed of nepheline syenites, which are an ore for aluminum. Another example is the Norilsk sill intrusions rich in copper and nickel.
Contact zones are also of great practical interest. Deposits of gold, silver, tin and other valuable metals are associated with metasomatic and metamorphic halos of intrusive bodies such as the Bushveld lopolith in South Africa, known for its gold-bearing halos.
Thus, areas of intrusivemagmatism are the most important source of many valuable minerals.
