Introduction:
Layered mafic intrusions are geological formations that consist of layers of mafic rocks, which are rich in magnesium and iron, and are typically composed of minerals such as olivine, pyroxene, and plagioclase feldspar. Mafic rocks are denser and darker in color than felsic rocks, which are rich in feldspar and silica.
Layered mafic intrusions are typically found in large igneous provinces, which are regions of the Earth's crust that have experienced extensive volcanic activity. They are formed by the intrusion of magma into the Earth's crust, which then cools and solidifies to form a layered sequence of rocks.
These intrusions are known for their distinctive layering, which is believed to be the result of the settling of minerals in the magma chamber as the magma cooled and crystallized. The layering can be quite complex, with variations in the composition and thickness of the layers. These variations can provide important clues about the history of the intrusion, including information about the timing and duration of the magma intrusion, as well as the conditions under which it cooled and solidified.
Layered mafic intrusions are also of great interest to geologists and mining companies because they often contain valuable mineral deposits, such as nickel, copper, platinum, and chromite. Understanding the geology of these intrusions is therefore important for both scientific and economic reasons.
Here are some common geological terms used to describe layered mafic intrusions:
Mafic rocks: Rocks that are rich in magnesium and iron and contain minerals such as olivine, pyroxene, and plagioclase feldspar.
Layering: The distinctive horizontal bands or layers of different rock types that are found in layered mafic intrusions.
Cumulate rocks: Rocks that form as crystals settle out of a magma chamber and accumulate on the floor of the chamber. Cumulate rocks are typically layered and can have distinctive textures and mineralogy.
Ultramafic rocks: Rocks that are even richer in magnesium and iron than mafic rocks and contain minerals such as olivine and pyroxene.
Plutonic rocks: Rocks that form when magma cools and solidifies underground. Plutonic rocks are typically coarse-grained and can be exposed at the surface by erosion.
Igneous complexes: Large regions of the Earth's crust that are made up of multiple intrusive and extrusive igneous rocks.
Xenoliths: Fragments of pre-existing rock that are incorporated into an intrusive igneous rock during its formation.
Contact metamorphism: The process by which rocks are altered by the heat and pressure generated by the intrusion of magma into the surrounding rock. This can result in the formation of new minerals and textures in the surrounding rock.
The names of the different layering in a layered mafic intrusion can vary depending on the classification scheme used by geologists, but here are some common types of layering that are often observed:
Cumulate layering: This type of layering is the most common in layered mafic intrusions and is caused by the settling of mineral crystals within a magma chamber. Different minerals have different settling rates and can form layers of different thicknesses and compositions.
Graded bedding: This type of layering is caused by the deposition of sediment in a flowing fluid. In a mafic intrusion, it can be caused by the settling of mineral crystals in a magma chamber that is periodically disrupted by magma injection.
Flow layering: This type of layering is caused by the deformation of magma due to its flow through a conduit or channel. The magma can form layers with different orientations and compositions depending on the flow rate and direction.
Sill layering: This type of layering is caused by the emplacement of magma into pre-existing fractures or bedding planes. Sill layering can be continuous over long distances and is often seen in the upper parts of layered mafic intrusions.
Unconformities: This type of layering represents a gap in the geological record due to erosion or non-deposition of sediment. Unconformities can be caused by changes in the environment or tectonic activity.
Compositional layering in layered mafic intrusions refers to the variation in the mineralogical and chemical composition of the different layers within the intrusion. Here are some of the different types of compositional layering that can be observed in mafic intrusions:
Orthocumulate layering: This type of layering is characterized by a vertical sequence of cumulate layers that increase in mineral abundance and decrease in grain size upwards. The lower layers are rich in olivine and the upper layers are rich in plagioclase feldspar.
Symplectic layering: This type of layering is characterized by the presence of symplectites, which are intergrowth of two or more minerals that form during the reaction between different minerals. Symplectic layering can be observed where two contrasting minerals are in contact.
Poikilitic layering: This type of layering is characterized by the presence of large crystals (phenocrysts) that enclose smaller crystals (groundmass) within them. The larger crystals can have a composition different from the smaller crystals.
Chromitite layering: This type of layering is characterized by the presence of chromite-rich layers that are enriched in platinum group elements. These layers are often found in the lower parts of the intrusion.
Gabbronorite layering: This type of layering is characterized by the presence of alternating layers of gabbro and norite. Gabbro is rich in calcium and magnesium, while norite is rich in iron and magnesium.
Cryptic layering in a layered mafic intrusion refers to a type of layering that is not immediately obvious or visible to the naked eye, but can be detected through more detailed examination using specialized techniques. Cryptic layering can be caused by subtle changes in mineralogy, texture, or chemical composition within a layer, which can be difficult to distinguish from the surrounding rock.
One of the most common causes of cryptic layering in mafic intrusions is the presence of interstitial melt or residual liquid trapped within the solidified rock. These melts can be difficult to distinguish from the surrounding rock and can form thin layers or veins that are only visible under close examination. Cryptic layering can also be caused by changes in the size or shape of mineral grains, changes in the distribution of minerals, or the presence of secondary minerals that form after the rock has solidified.
Cryptic layering can be important for understanding the formation and evolution of layered mafic intrusions, as it can provide clues about the processes that occurred during the solidification of the magma. Techniques such as thin section analysis, petrography, and geochemical analysis can be used to detect and study cryptic layering in mafic intrusions.
There are several examples of layered mafic intrusions in India. Here are some of the most well-known ones:
Sittampundi Anorthosite Complex: Located in the Salem district of Tamil Nadu, this intrusion is one of the largest anorthosite bodies in the world. It is composed of alternating layers of anorthosite, gabbro, and granulite, and is believed to have formed around 2.5 billion years ago.
Nuggihalli Layered Complex: Located in the Chitradurga district of Karnataka, this intrusion is composed of layered gabbro and anorthosite. It is believed to have formed around 2.5 billion years ago.
Eastern Ghats Layered Complex: Located in the Eastern Ghats region of Andhra Pradesh, this complex is composed of gabbro, anorthosite, and charnockite. It is believed to have formed around 1.4 billion years ago.
Bundelkhand Massif: Located in the central Indian Shield, this massif is composed of layered gabbro and anorthosite, and is believed to have formed around 1.8 billion years ago.
Chilka Lake Intrusion: Located in the Odisha state, this intrusion is composed of layered gabbro, norite, and anorthosite. It is believed to have formed around 1.1 billion years ago.
Here are some of the major layered mafic intrusions across the globe in a chronologic order:
Stillwater Complex, Montana, USA: Formed about 2.7 billion years ago, this intrusion is composed of layered gabbro and anorthosite, and is one of the largest deposits of platinum group elements in the world.
Bushveld Complex, South Africa: Formed about 2.05 billion years ago, this is the largest layered mafic intrusion in the world, covering an area of over 66,000 square kilometers. It is composed of layered gabbro and anorthosite, and is a major source of chromium, platinum, and vanadium.
Great Dyke, Zimbabwe: Formed about 2.5 billion years ago, this intrusion is composed of layered mafic and ultramafic rocks and is rich in platinum group elements.
Muskox Intrusion, Nunavut, Canada: Formed about 1.2 billion years ago, this intrusion is composed of layered anorthosite, gabbro, and norite. It is a major source of nickel, copper, and platinum group elements.
Skaergaard Intrusion, Greenland: Formed about 55 million years ago, this intrusion is composed of layered gabbro and is one of the best-studied examples of a layered mafic intrusion. It is a major source of palladium and platinum.
Hope it helps.
Regards, GeoKhan.
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