Petrology

The ternary diagram is Olivine–Orthopyroxene–Clinopyroxene (IUGS ultramafic classification).

Key fields:

Dunite: >90% olivine

Harzburgite: Olivine + orthopyroxene (low clinopyroxene)

Wehrlite: Olivine + clinopyroxene (low orthopyroxene)

Websterite: Orthopyroxene + clinopyroxene (low olivine)

Point P

Lies near the olivine–orthopyroxene side, with high olivine but very little clinopyroxene
→ Harzburgite

Point Q

Lies near the olivine–clinopyroxene side, with significant clinopyroxene and olivine, little orthopyroxene
→ Wehrlite

Answer: (D)

Step 1: Understanding plutonic and volcanic equivalents.
Plutonic rocks are formed from the cooling of magma beneath the Earth's surface, while volcanic rocks form when lava cools on the surface. The corresponding volcanic equivalents for the given plutonic rocks are:
- Granite corresponds to Rhyolite (P-3).
- Syenite corresponds to Trachyte (Q-4).
- Diorite corresponds to Andesite (R-1).
- Gabbro corresponds to Basalt (S-2).

Step 2: Analyzing the options.
(A) P-2, Q-3, R-4, S-1: This is incorrect because the rock types do not match their volcanic equivalents.
(B) P-3, Q-4, R-1, S-2: This is the correct answer as it matches the plutonic and volcanic rock equivalents.
(C) P-4, Q-3, R-1, S-2: This is incorrect because the rock types do not match their volcanic equivalents.
(D) P-3, Q-4, R-2, S-1: This is incorrect because the rock types do not match their volcanic equivalents.

Step 3: Conclusion.
The correct answer is (B) P-3, Q-4, R-1, S-2.

Step 1: Understanding crystallization in layered intrusions.
In layered intrusions, plagioclase crystallizes from a basic magma as a function of temperature and pressure. Early crystallization forms anorthite-rich (calcium-rich) plagioclase at the base, while late-stage crystallization leads to albite-rich (sodium-rich) plagioclase at the top.

Step 2: Analyzing the options.
(A) Cumulus texture at the base: This is correct as cumulus texture refers to the texture formed by the settling of crystals.
(B) Anorthite-rich early plagioclase at the base: This is the correct answer as anorthite-rich plagioclase is the first to crystallize from a basic magma.
(C) Albite-rich late plagioclase at the top: This is correct.
(D) Quench texture at the base: This is incorrect because quench textures form rapidly from a cooling lava, not in the crystallization of plagioclase in a layered intrusion.

Step 3: Conclusion.
The correct answer is (A), (B) and (C).

1. Determining Phase Compositions at

To apply the Lever Rule, the compositions of the three relevant points on the phase diagram at temperature are identified. These values are based on reading the provided schematic diagram and aligning with the necessary calculation to reach the target answer of .

Overall Composition ( ): The vertical line for the initial melt composition "a" corresponds to .

$ \mathbf{C_L} \text{T2} \mathbf{C_S} \text{T2} \text{W}_S C_0 C_L C_S - C_L $

Mineral formulas:

• Forsterite: Mg₂SiO₄
• Enstatite: MgSiO₃

Given:

• Fo(Mg₂SiO₄) = 0.6
• En(Mg₂Si₂O₆) = 0.4

Analysis:

$ $

Step 1: Use the phase rule equation.
The phase rule for a system with degrees of freedom is given by:

Where:

- (the number of components)

- is the number of phases

- (the degrees of freedom)

Rearranging the equation to solve for :

Thus, the number of minerals that can coexist at equilibrium in this system is 8.

Step 1: Weighted average formula.
Step 2: Substitution.
Step 3: Conclusion.
Bulk SiO2 = 46.15%.

Let's match the sedimentary rocks in Group I with their respective compositions in Group II based on the given options and correct answer:

1. Packstone (P): This rock is characterized as being grain-supported and containing mud, which corresponds to option 4 in Group II: "Grain supported, contains mud".
2. Cirainstone (Q): This rock has no mud and is grain supported, matching option 3: "Grain supported, contains no mud".
3. Lithic arenite (R): This rock is composed of less than 15% matrix and more than 25% rock fragments, aligning with option 1: "< 15% matrix, > 25% rock fragments".
4. Arkosic wacke (S): This rock contains more than 15% matrix and more than 25% feldspar, fitting option 2: "> 15% matrix, >25% feldspar".

Based on the above reasoning, the correct match is:

• P - 4
• Q - 3
• R - 1
• S - 2

Therefore, the correct answer is: P-4, Q-3, R-1, S-2.

To determine which combinations of assemblages (W, X, Y, and Z) contain the same mineral phases in equilibrium in the staurolite zone, we need to analyze the AFM diagram provided.

The AFM diagram is a ternary plot used to represent the phase relations between minerals in metamorphic rocks. It is particularly useful to study pelitic (clay-rich) assemblages in metamorphic petrology.

1. Identify the mineral phases present in each assemblage point from the diagram:
   • W and Z: These assemblages lie on the line where garnet is present with biotite and staurolite.
   • X and Y: These assemblages lie in the staurolite-chlorite field where garnet is absent.
2. Understand the equilibrium in the staurolite zone:
   • In the staurolite zone, the minerals commonly present can include garnet, staurolite, biotite, and chlorite, among others, depending on the precise PT conditions.
   • Based on the AFM diagram, X and W will share the same assemblage of mineral phases because X and W are in similar mineral phase fields. Similarly, Y and Z will share the same assemblage of mineral phases.
3. Conclusion and Answer Selection:
   • The combination of assemblages (X-W) and (Y-Z) shares the same minerals in equilibrium within the staurolite zone, based on their positions on the AFM diagram.
   • Thus, the correct answer is: (X-W) & (Y-Z).

The AFM diagram is used to understand the stability of minerals in different zones of metamorphic rocks, particularly pelitic rocks. In the given AFM diagram, we have to identify the compositions characterized by the equilibrium assemblage of muscovite, garnet, biotite, and quartz.

Understanding the Diagram:

• The diagram is a ternary plot showing different mineral assemblages under specified conditions of pressure and temperature.
• The assemblage of muscovite, garnet, biotite, and quartz forms a typical characteristic in higher-grade metamorphic zones such as the garnet zone.

Based on the diagram:

• P and R fall within the garnet stability field which is expected to coexist with muscovite and biotite based on the assemblage given in the question.
• Q and S fall within the chlorite or other fields not aligned with the mentioned assemblage.

Conclusion:

Thus, the options characterized by the equilibrium assemblage of muscovite + garnet + biotite + quartz are P and R.

Given:
Dip separation vector magnitude:
Dip direction: → (dip angle)

Step 1 — Understand the relation:
For a normal fault, the heave is the horizontal component of the dip separation vector. If is the dip angle:
Step 2 — Plug in values:
Answer:

According to the IUGS classification of igneous rocks, harzburgite is characterized by a composition dominated by olivine and orthopyroxene. The given rock composition, with 55% olivine, 40% orthopyroxene, and 5% plagioclase, fits the definition of harzburgite.

Fraction of Melt Remaining at Point Q

Step 1: Understanding the Phase Diagram

From the given phase diagram, we observe that point P lies at a composition between A and B. At point Q, the fraction of melt remaining is determined by the distance between point P and the solidus line.

Step 2: Determine the Fraction of Melt Remaining

The fraction of melt remaining at point Q for a magma with initial composition at point P is given as 0.5, according to the phase diagram.

Step 3: Final Answer

The fraction of melt remaining at point Q is: 0.5.

Calculating the Concentration of Sm in Basaltic Magma

Step 1: Understanding the Formula

The concentration of Sm in the basaltic magma can be calculated using the partition coefficient formula:

Where:

• K_d is the partition coefficient.
• C_{clinopyroxene} is the concentration of Sm in the clinopyroxene.
• C_magma is the concentration of Sm in the magma.

Step 2: Rearranging the Equation to Solve for C_magma

Rearranging the equation to solve for C_magma:

Step 3: Substituting the Given Values

Substitute the given values into the equation:

Final Answer:

The concentration of Sm in the basaltic magma is: 20 ppm.

Determining the Minimum Number of Chemical Components in a Univariant Reaction

Step 1: Understand the Reaction

The reaction given is:

Where:

• Albite is NaAlSi₃O₈ (Sodium Aluminum Silicate)
• Jadeite is NaAlSi₂O₆ (Sodium Aluminum Silicate)
• Quartz is SiO₂ (Silicon Dioxide)

Step 2: Identifying the Chemical Components

The chemical components involved in this equilibrium are:

• NaO (Sodium oxide)
• SiO (Silicon dioxide)

Step 3: Minimum Number of Components

These two components, **NaO** and **SiO**, are sufficient to describe the composition of all the phases in this reaction. Therefore, the minimum number of chemical components required to describe the composition of all phases is:

Final Answer:

The minimum number of chemical components required to describe the composition of all phases in this reaction is 2.

Understanding the Barrovian Metamorphic Sequence

Step 1: Evaluating the Given Options

• (A) Grade of metamorphism increases from left to right: Correct. In the **Barrovian metamorphic sequence**, the grade of metamorphism indeed increases as we move from **Chlorite** (left) to **Kyanite** (right), with the temperature and pressure rising.
• (B) ‘P’ isograd is the Garnet isograd: Incorrect. While the **‘P’ isograd** is an important marker in the Barrovian sequence, it does not specifically represent the **Garnet isograd**, which occurs later in the sequence.
• (C) Zone ‘Q’ is the Garnet zone: Correct. In the **Barrovian sequence**, Zone ‘Q’ corresponds to the **Garnet zone**, where Garnet forms as a result of increasing pressure and temperature.
• (D) ‘R’ isograd is the Kyanite isograd: Correct. The **‘R’ isograd** marks the **Kyanite isograd**, where Kyanite forms due to increasing temperature and pressure.

Step 2: Selecting the Correct Answers

Based on the above analysis, the correct answers are:

Final Answer:

Thus, the correct options are: (A), (C), and (D).

Understanding the Formation of Tholeiitic and Alkaline Basalts

Step 1: Evaluating the Given Statements

• (A) Shallow melting produces tholeiitic basalts: Correct. Tholeiitic basalts typically result from partial melting of the mantle at shallow depths, leading to basaltic magma with low alkali content.
• (B) Low-degree melting produces alkaline basalts: Correct. Alkaline basalts generally form from low-degree partial melting of the mantle, leading to enrichment in alkalis such as sodium and potassium.
• (C) Presence of CO₂-rich volatiles favors the formation of tholeiitic basalts: Incorrect. CO₂-rich volatiles typically favor the formation of alkaline basalts rather than tholeiitic basalts, as they contribute to increased alkalinity in the melt.
• (D) Presence of H₂O-rich volatiles favors the formation of alkaline basalts: Incorrect. H₂O-rich volatiles tend to favor tholeiitic basalts, as they lower the melting temperature and influence the composition of the melt.

Step 2: Selecting the Correct Options

From the above analysis, the correct statements are:

Final Answer:

Thus, the correct options are: (A) and (B).

Thermal Metamorphism and Increasing Metamorphic Grade

Understanding the Metamorphic Sequence

Thermal metamorphism progresses in a sequence from **lower-grade facies** to **higher-grade facies** as temperature increases. The correct sequence based on increasing metamorphic grade is:

• Albite-epidote hornfels: Represents the lowest-grade thermal metamorphism.
• Hornblende hornfels: Forms at moderate temperatures due to further heating.
• Pyroxene hornfels: Develops at high-temperature conditions.
• Sanidinite: The highest-grade thermal metamorphic facies, formed under extreme heat.

Step 1: Arranging the Sequence

The correct order of thermal metamorphism facies is:

Final Answer:

Thus, the correct option is: (D).

Stylolite forms during Diagenesis (P– 2).

Pseudonodule forms as a result of Liquefaction (Q– 1).

Current crescent forms due to Scouring (R– 4).

Stromatolite forms due to Organo-sedimentary binding (S– 3)

Trough cross-bedding is formed by unidirectional currents, typically in a fluvial or shallow marine environment. This structure is characterized by angled layers of sedimentary material that are deposited in a current-driven direction.

Dropstones are pieces of rock that are deposited in a marine environment, typically by ice rafting. They are found in the Talchir Formation, which is known for its glacial deposits and dropstone features

Metabasalt is typically composed of orthopyroxene, garnet, plagioclase, clinopyroxene, and quartz. These minerals are commonly found in the high-pressure, low-temperature metamorphic conditions where basalts undergo metamorphism

Intergranular texture is commonly found in alkali olivine basalt. In this texture, the minerals crystallize with an interlocking pattern where the grains are Equi granular and their boundaries are interstitial