Geology Civil Services Paper 2 Section- A, Questions 3,4_ Solutions

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3. (a)What are the different types of metamorphism and what are their controlling factors? State characteristic mineral assemblages which appear under different facies during regional metamorphism of pelitic rocks.

Answer: Anisotropic minerals have optical properties that vary depending on the direction of light passing through them. When viewed under crossed polars, anisotropic minerals show interference colors that change as the stage is rotated. At four points during the rotation, the mineral appears to go completely dark, with no interference colors visible. This is known as "complete extinction."

The reason for this is that the mineral has four optic axes, which are directions in the crystal where light is transmitted without splitting into two separate beams. When the mineral is oriented with one of these axes parallel to the polarizers, the light passing through the mineral is completely absorbed, causing the mineral to appear dark.

Pleochroism is the property of certain minerals to show different colors when viewed from different angles. This property arises due to the mineral's crystal structure and the way it absorbs light. As light passes through a pleochroic mineral, it is absorbed differently along different crystallographic directions. The resulting color depends on the direction of light passing through the mineral.

Pleochroism is determined by comparing the color of a mineral when viewed with light polarized in different directions. This can be done using a polarizing microscope equipped with a colonoscopic lens or a Bertrand lens. By rotating the stage and observing the mineral under crossed polars with the colonoscopic or Bertrand lens, the mineral's pleochroism can be determined. The mineral will appear to change color as the stage is rotated, revealing the different colors that are visible along different crystallographic directions.

3. (b) Define different types of zoning observed in minerals. Discuss processes of formation of different types of zoning in plagioclase with the help of the Albite Anorthite system.

Answer: Zoning in minerals refers to the spatial variation in chemical composition within a single crystal. There are several types of zoning that can be observed in minerals:

Oscillatory zoning: This type of zoning is characterized by alternating bands or layers of different compositions within a crystal. These bands can be caused by variations in the conditions (such as temperature or pressure) during crystal growth.

Sector zoning: Sector zoning is characterized by discrete sectors within a crystal that have different compositions. This type of zoning can be caused by fluctuations in the composition of the mineralizing fluid during crystal growth.

Core-rim zoning: Core-rim zoning is characterized by a change in composition from the core to the rim of a crystal. This type of zoning can be caused by changes in the composition of the mineralizing fluid during crystal growth, or by the incorporation of impurities into the crystal over time.

Resorption zoning: Resorption zoning is characterized by the partial dissolution of a crystal, resulting in a change in composition. This type of zoning can be caused by changes in the conditions (such as temperature or pressure) after crystal growth.

In the case of plagioclase, which is a series of minerals ranging from Albite (NaAlSi3O8) to Anorthite (CaAl2Si2O8), several types of zoning can be observed.

Oscillatory zoning: Oscillatory zoning is commonly observed in plagioclase crystals, particularly in magmatic rocks. This zoning is caused by changes in the temperature, pressure, and composition of the magma during crystal growth. For example, during the crystallization of a magma, the temperature and composition of the magma can fluctuate, causing the plagioclase crystal to grow in layers with different compositions.

Sector zoning: Sector zoning is also commonly observed in plagioclase crystals. This zoning can be caused by variations in the composition of the mineralizing fluid during crystal growth. For example, if the composition of the fluid changes during crystal growth, different sectors of the crystal will incorporate different amounts of different elements, resulting in sector zoning.

Core-rim zoning: Core-rim zoning is observed in plagioclase crystals that have been subjected to metamorphism or weathering. This type of zoning is caused by changes in the composition of the fluid or rock matrix over time, resulting in a change in the composition of the crystal from the core to the rim.

Resorption zoning: Resorption zoning is commonly observed in plagioclase crystals that have been partially dissolved and then reprecipitated. This type of zoning is caused by changes in the conditions (such as temperature or pressure) after crystal growth, resulting in the dissolution of some parts of the crystal and the reprecipitation of other parts with a different composition.

In summary, the different types of zoning observed in plagioclase are caused by a variety of processes, including changes in temperature, pressure, and composition of the mineralizing fluid, metamorphism, weathering, and partial dissolution and reprecipitation. The type of zoning observed in a particular plagioclase crystal can provide important information about the conditions under which the crystal formed and the processes that have affected it since its formation.

3. (c) State the petrographic characters of different types of anorthosites. Write a note on the petrogenesis of anorthosites.

Answer: Anorthosites are a group of intrusive igneous rocks composed predominantly of plagioclase feldspar (>90%) and with minor amounts of other minerals. They are generally divided into two main types based on their texture and mineralogy:

Layered anorthosites, and Massive anorthosites.

The petrographic characters of these two types are as follows:

Layered anorthosites:

Show a layered texture, with alternating bands of plagioclase feldspar and other minerals such as pyroxene, amphibole, or mafic minerals.

Plagioclase feldspar crystals are usually large and range in size from 5-15 mm.

The layers are generally parallel to the base of the intrusion.

They often contain small amounts of other minerals, such as olivine, quartz, or biotite.

The color of the rock is usually light grey, white, or pink.

Massive anorthosites:

Do not have a layered texture and are composed mostly of plagioclase feldspar.

Plagioclase feldspar crystals are usually smaller and range in size from 0.5-5 mm.

May contain minor amounts of other minerals such as pyroxene, amphibole, or mafic minerals.

The color of the rock is usually light grey, white, or pink.

The petrogenesis of anorthosites is still a topic of debate among geologists. However, several models have been proposed to explain their origin. One of the most widely accepted models is the "plutonic to volcanic continuum" model, which suggests that anorthosites can form by the following processes:

Differentiation of mafic magmas:

Anorthosites can form by the fractional crystallization of mafic magmas. As the magma cools, minerals with a higher melting point such as plagioclase feldspar, settle out and crystallize, forming anorthosites.

Assimilation of continental crust:

Anorthosites can also form by the assimilation of continental crust. When mafic magmas intrude the continental crust, they can assimilate some of the crustal rocks, which can trigger the crystallization of plagioclase feldspar, leading to the formation of anorthosites.

Crystallization of melts derived from the mantle:

Anorthosites can form by the partial melting of the mantle. When the mantle undergoes partial melting, a melt is generated that is rich in plagioclase feldspar. As the melt cools and crystallizes, anorthosites can form.

In summary, anorthosites are a group of intrusive igneous rocks composed predominantly of plagioclase feldspar. They can form by different processes, including the fractional crystallization of mafic magmas, assimilation of continental crust, and crystallization of melts derived from the mantle. The exact petrogenesis of anorthosites is still a topic of research, and further studies are needed to better understand their origin.

4. (a) What do you understand by sedimentary depositional environment? Describe the fluvial environment in detail.

Answer: Sedimentary depositional environments are specific geological settings where sediments are deposited and eventually become sedimentary rocks. These environments are influenced by various factors such as climate, water chemistry, sediment source, and the energy of the transporting agent.

A fluvial environment is a sedimentary depositional environment that is characterized by the deposition of sediment transported by rivers and streams. The sediment in fluvial environments can range from boulders to fine sand and mud.

Fluvial environments are divided into two categories: alluvial and fluvial-deltaic. Alluvial environments are characterized by rivers and streams that transport sediment from high-elevation to lower-elevation areas, while fluvial-deltaic environments are characterized by the deposition of sediment at the mouth of a river as it flows into a larger body of water, such as a lake or the ocean.

In alluvial environments, sediment is typically deposited in a series of sedimentary structures known as channels, bars, and floodplains. Channels are typically narrow, elongated features that are cut into the underlying sediment or rock. Bars are sediment accumulations that form within channels, and they can be either linear or crescent-shaped. Floodplains are low-lying areas adjacent to channels that are periodically inundated by floods. They are often characterized by fine-grained sediments such as silt and clay.

In fluvial-deltaic environments, sediment is deposited in a series of sedimentary structures known as distributary channels, mouth bars, and deltaic lobes. Distributary channels are branching channels that carry sediment from the main river channel to the deltaic lobes. Mouth bars are sediment accumulations that form at the mouth of the distributary channels. Deltaic lobes are sediment accumulations that form at the terminus of the distributary channels and can be either linear or fan-shaped.

Overall, the fluvial environment is an important depositional environment that contributes significantly to the formation of sedimentary rocks. The sedimentary structures formed in fluvial environments provide valuable information for interpreting ancient environments and can be used to reconstruct the history of the Earth's surface.

4. (b) Explain different processes of diagenesis in clastic sedimentary rocks. Describe common diagenetic structures.

Answer: Diagenesis refers to the physical, chemical, and biological changes that occur in sediment after it has been deposited but before it is buried and lithified into sedimentary rock. In clastic sedimentary rocks, diagenesis can affect the size, shape, sorting, and mineralogy of the sediment grains, as well as the pore spaces between the grains. Here are some common processes of diagenesis in clastic sedimentary rocks, as well as some common diagenetic structures:

Compaction: The weight of overlying sediment can cause the grains in the lower layers to be squeezed closer together, reducing the volume of pore space and increasing the density of the rock. This process is called compaction, and it is one of the most important diagenetic processes in sedimentary rocks. Compaction can result in the formation of horizontal bedding planes and other structures, such as stylolites.

Cementation: Minerals can precipitate from pore fluids and bind the sediment grains together, forming a cement. Common cements include silica, calcite, and iron oxides. Cementation can fill in pore space, making the rock less permeable, and can create structures such as concretions and nodules.

Dissolution and replacement: Minerals in the sediment can dissolve and be replaced by new minerals. For example, calcite can be dissolved and replaced by silica, or feldspars can be replaced by clay minerals. This process can alter the composition and texture of the rock and create structures such as pseudomorphs, where one mineral is replaced by another but retains the original crystal shape.

Recrystallization: The original mineral grains in the sediment can grow and change shape, forming larger, more equidimensional grains. Recrystallization can result in the formation of new minerals and the loss of original textures and structures.

Common diagenetic structures in clastic sedimentary rocks include:

Bedding planes: Horizontal layers that reflect changes in depositional conditions over time.

Graded bedding: Layers in which the grain size and/or sorting decrease upward, indicating that the sediment was deposited by a turbidity current or other sediment gravity flow.

Cross-bedding: Layers in which the sediment is deposited at an angle to the horizontal, indicating the direction of the current that transported the sediment.

Concretions: Rounded or irregularly shaped masses of cemented sediment within the rock.

Nodules: Similar to concretions but usually smaller and more spherical.

Stylolites: Irregular, wavy surfaces that result from pressure solution between sediment grains.

Pseudomorphs: Mineral grains that retain the shape of the original mineral but are composed of a different mineral.

4. (c) Enumerate the sedimentary basins of India based on their petroleum prospects.

Answer: There are several sedimentary basins in India with significant petroleum prospects. Here are some of the major ones:

Mumbai Offshore Basin: Located along the west coast of India, this basin is one of the most productive petroleum regions in the country, with vast reserves of oil and natural gas.

Krishna-Godavari Basin: Located off the east coast of India, this basin is known for its deep-water oil and gas reserves, and has been the site of several major discoveries in recent years.

Cauvery Basin: Located in southern India, this basin is known for its oil and gas reserves, and has several active drilling sites.

Assam-Arakan Basin: Located in northeastern India and adjoining Myanmar, this basin is one of the oldest and most productive in the country, with significant reserves of crude oil and natural gas.

Cambay Basin: Located in western India, this basin is known for its onshore oil and gas reserves, and has been the site of several major discoveries in recent years.

Rajasthan Basin: Located in the northwest of India, this basin is known for its onshore oil and gas reserves, and has seen significant exploration activity in recent years.

Andaman-Nicobar Basin: Located in the Bay of Bengal, this basin has significant potential for oil and gas reserves, but remains largely unexplored due to its remote location and challenging geology.

Overall, India has significant petroleum prospects across its various sedimentary basins, with the Mumbai Offshore Basin and the Krishna-Godavari Basin being two of the most productive and important regions for petroleum exploration and production in the country.

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