Under magmatism is understood a set of phenomena associated with the formation, evolution of composition and the movement of magmas to the surface of the Earth. Magmatism is one of the most important deep-seated 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 foci, mainly in the upper mantle (asthenosphere) and partially in the lower layers of the earth's crust. The formation of a magma chamber occurs with a combination of certain values of pressure and temperature. Such primary magma has a homogeneous composition, including the following components: a liquid (melt) in which a gas or volatile phase (fluid) is dissolved. A certain amount of crystalline solid is also present. As it moves toward the surface, primary magma evolves depending on specific conditions.
The evolution of magma involves several types of processes. Firstly, she experiences differentiations of various kinds:
- segregation, in which it exfoliates into immiscible liquid components;
- crystallization differentiation. This most important process is associated with the precipitation (crystallization) of certain compounds from an amorphous melt under various combinations of temperature and pressure.
Secondly, magma changes its chemical composition as a result of interaction with the host rocks. This phenomenon is called contamination.
Crystallization processes in magma
Since magma is a moving 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 magma. They settle and accumulate in the bottom areas of the magma chamber.
- The main temperature main magmatic phase, in which the main components of rocks are formed, such as feldspars, quartz, mica, pyroxenes, amphiboles. Calcium precipitates, the vast majority of silicon and aluminum. Crystallization in this phase is already accompanied by a deficit of space in the magma chamber, therefore, the formed minerals are finer-grained.
- Low temperature late magmatic (pegmatite) phase. At this stage, the mobile so-called pegmatite magma residue enriched with volatile components propagates through the cavities and cracks remaining in the magma chamber, contributing to the recrystallization of the 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 latter factor plays a particularly important role.
Effusive magmatism is a process during which magma reaches the surface of the Earth through the inlet channel, goes up, forming volcanoes, and freezes. The spilled magma is called lava. Upon reaching the surface, it intensively loses the volatile component. Hardening also occurs quickly; some types of lavas do not have time to crystallize and harden in an amorphous state (volcanic glass).
Intrusive magmatism (plutonism) is characterized in that magma does not reach the surface. Introducing in one way or another into the overlying horizons of the host rocks, magma freezes at a depth, forming intrusive (plutonic) bodies.
Intrusion Classification
The relationships of the host rocks with the products of intrusive magmatism and the types of intrusive bodies are distinguished by many criteria, in particular by:
- Depth of formation. There are near-surface (subvolcanic), mid-deep (hypabyssal) and deep (abyssal) intrusions.
- Location relative to the host rock. According to this criterion, embedded arrays are divided into consonants (concordant) and disagree (discordant).
Also, the nature of intrusive magmatism and types of intrusions are classified according to such signs as the ratio of the structure of the plutonic body to the contact surface (conformal and disconformal), attitude to tectonic movements, shape, size of the array, and so on.
The criteria for distinguishing various types of magmatic implants are closely related. For example, depending on the structure of the host sequence, depth and mechanism of formation of the magmatic massif and other manifestations of intrusive magmatism, the forms of intrusions can vary greatly.
Mechanisms for introducing magma into the rock mass
Magma can penetrate into the enclosing stratum in two main ways: along the planes of the strata of the sedimentary strata or along cracks existing in the rock.
In the first case, roofing layers — overlying areas of the stratum — are raised under magma pressure, or, conversely, as a result of the influence of the mass of intruding magma, the underlying layers bend. Thus consonant intrusions are formed.
If magma penetrates upward, filling and spreading cracks, breaking through layers and breaking down roof rocks, it itself forms a cavity that the intrusive body will occupy. In this way, plutonic bodies that are inconsistently formed are formed.
Forms of Embedded Magmatic Arrays
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 of non-conforming igneous masses are:
- A dike is a plate-like steeply dipping body that cuts enclosing strata. The length of the dikes significantly exceeds the thickness, and the location of the contact surfaces is 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 annular or radial - depending on the location of the cracks filled with magma.
- The vein is a small secant body of an irregular, branched shape.
- A stem is a pillar-shaped body characterized by vertical or steeply falling contact surfaces.
- Batolit is the largest variety of intrusions. Batholiths can be hundreds or even thousands of kilometers long.
Overlying bodies also take various forms. Among them are often found:
- Sill is a reservoir intrusion whose contact surfaces are parallel to the host reservoirs.
- Lopolit is a lenticular massif, convex downward.
- Laccolite is a body of similar shape, the convex side of which is located at the top, like a mushroom cap. Ayu-Dag Mountain in Crimea is an example of gabbroid laccolith.
- Fakolit - a body located in the fold of the deflection of the enclosing rocks.
Intrusion Contact Area
The formation of plutonic bodies is accompanied by complex processes of interaction at the boundary with the host stratum. Zones of endocontact and exocontact are formed along the contact surface.
Endocontact changes occur in the intrusion due to the penetration into the magma of rocks of the enclosing sequence. As a result of this, magma near the contact undergoes chemical changes (contamination) affecting mineral formation.
An exocontact zone arises 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, the volatile components of magma can replace the minerals introduced into the exocontact zone by the 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, for example, of mica, and with the participation of water, quartz.
Intrusive magmatism and intrusive rocks
The rocks formed as a result of deep crystallization of magma are called intrusive, or plutonic. Effusive (volcanic) rocks are formed by the outpouring of magma on the Earth's surface (or on the bottom of the ocean).
Intrusive and effusive magmatism generates rows of rocks that are similar in mineral composition. The composition of igneous rocks is based on the silica content of SiO 2 . By this criterion, the rocks are divided into ultrabasic, basic, middle and acidic. The silica content in the series increases from ultrabasic (less than 45%) rocks to acidic (more than 63%). Within each class, the rocks vary in alkalinity. The main intrusive rocks, in accordance with this classification, form the following series (in brackets is a volcanic analogue):
- Ultrabasic: peridotites, dunites (picrites);
- Basic: gabbroids, pyroxenites (basalts);
- Medium: diorites (andesites);
- Acidic: granodiorites, granites (dacites, rhyolites).
Plutonic rocks differ from the effusive rocks and the crystalline structure of the minerals composing them: they are full crystalline (do not contain amorphous structures), are clear-grained and have no pores. The deeper the source of rock formation (abyssal intrusions), the slower the processes of cooling of magma and crystallization, while maintaining a large amount of volatile phase. Such deep rocks are distinguished by larger crystalline grains.
The internal structure of intrusive bodies
The structure of plutonic massifs is formed in the course of a complex of phenomena united under the common name of prototectonics. Two stages are distinguished in it: prototectonics of the liquid and solid phases.
At the liquid phase, the primary striped and linear textures of the resulting body are laid. They reflect the direction of the flow of intruding magma and the dynamic orientation conditions of crystallizing minerals (for example, the parallel arrangement of mica crystals, hornblende , etc.). The textures are also associated with the location of fragments of an alien rock that have fallen into the magma chamber - xenoliths - and isolated mineral clusters - schlieres.
The solid-phase stage of evolution of the intrusion is associated with the cooling of the newly formed rock. Primary cracks appear in the massif, the location and amount of which are due to the cooling environment and structures formed in the liquid phase. In addition, in such a magmatic massif secondary structures develop due to fragmentation of its sections and displacements along the gaps.
The study of prototectonics is important to clarify the conditions for the placement 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. These or other manifestations of intrusive magmatism make a significant contribution to both regional and global tectonic processes.
In case of continental collisions, in the course of increasing the crustal thickness, large batholiths are formed due to active granite magmatism, for example, the Gangdis batholith in Transhimalaya. The formation of large batholiths is also associated with active continental margins (Andean batholith). In general, acidic magma intrusions play an important role in mountain building processes.
When stretching the cortex, a series of parallel dikes are often formed. Such series are observed in the mid-ocean ridges.
One of the characteristic forms of intracontinental igneous intrusions are sills. They can also have a large length - up to hundreds of kilometers. Often, magma, penetrating between layers of sedimentary rocks, forms several tiers of sills.
Deep magmatic activity and minerals
Due to the crystallization features, intrusive magmatism processes form ore minerals 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 are concentrated in the upper regions of the magma chamber. This occurs in the early crystallization phase. At a late stage, a mobile pegmatite residue containing rare earth and rare elements forms vein deposits in the cracks of the intrusion.
So, the Khibiny on the Kola Peninsula are laccolith, exposed as a result of erosion of the host sequence. This body is composed of nepheline syenites, which are ore to aluminum. Another example is the Norilsk sill intrusions, rich in copper and nickel.
Contact areas 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 lopolit in South Africa, known for its gold-bearing halo.
Thus, areas of intrusive magmatism are the most important source of many valuable minerals.