Metamorphic Petrology

To determine which of the given textures indicate the presence of directed stress in a rock, we need to understand the nature of each texture:

• Porphyritic Texture: This texture is characterized by large crystals (phenocrysts) embedded in a finer-grained groundmass. It typically forms during the cooling of molten rock, which is not necessarily related to directed stress, but rather to the cooling rate of the magma or lava. Hence, it is not associated with directed stress.
• Schistosity: This is a type of foliation that results from the parallel arrangement of platy minerals, such as micas, due to directed pressure. Schistosity is a common feature in metamorphic rocks that have undergone significant recrystallization under directed stress, making it a clear indicator of directed stress.
• Gneissosity: Similar to schistosity, gneissosity refers to the alternating bands of differing mineral composition found in gneiss. The formation of these bands is usually due to high-grade metamorphic conditions involving substantial directed stress, which causes mineral segregation and alignment. Therefore, gneissosity indicates directed stress.
• Mylonitic Texture: Mylonites are rocks formed due to intense shearing under directed stress, typically in fault zones. The texture is caused by the plastic deformation of minerals, indicating substantial directed stress. Thus, mylonitic texture is a strong indicator of directed stress.

Based on this explanation, the textures that indicate the presence of directed stress in a rock are:

• Schistosity
• Gneissosity
• Mylonitic texture

Step 1: Understanding blueschist facies metamorphism:

Blueschist facies represents high-pressure, low-temperature metamorphism, typically occurring in subduction zones where oceanic crust is rapidly buried to great depths without significant heating.

Step 2: Evaluating each option:

(A) Hornblende: Common amphibole in amphibolite facies (medium-grade metamorphism). Not diagnostic of blueschist facies

(B) Anthophyllite: An orthorhombic amphibole. Not characteristic of high-pressure conditions

(C) Glaucophane: Sodium-rich blue amphibole. The presence of glaucophane is diagnostic of blueschist facies

(D) Actinolite: Calcium-magnesium amphibole. Not diagnostic of blueschist facies

Answer: (C) glaucophane

Step 1: Analysis of the D-M-time diagram:

Reading the diagram:

• M₁ is a long-duration metamorphic event spanning D₁/S₁ and D₂/S₂
• M₂ is a shorter metamorphic event after D₂/S₂
• Solid lines show mineral stability ranges
• Muscovite, Biotite, Chlorite, and Quartz are stable throughout both M₁ and M₂
• Garnet is stable only during part of M₁ (during D₁/S₁)
• Andalusite is stable only during M₂

Step 2: Evaluating each statement:

(A) - M₁ is a long-duration, broad event associated with deformation (D₁/S₁ and D₂/S₂) - characteristic of regional metamorphism. M₂ is a shorter, localized thermal event without associated deformation, producing andalusite - characteristic of contact metamorphism from igneous intrusion.

(B) - S₁ is the first schistosity formed during D₁. Crenulation cleavage forms when a pre-existing foliation is refolded, creating a new cleavage. Since S₁ is the first fabric, it would be a slaty cleavage or schistosity, not a crenulation cleavage. S₂ might be a crenulation cleavage formed by deforming S₁.

(C) - The diagram shows garnet is stable during the D₁/S₁ peak. "Syntectonic" means growing during deformation. Since garnet's stability coincides with D₁, it grew syntectonically with D₁.

(D) - Andalusite appears only during M₂, which occurs after D₂/S₂ has ended (note M₂ is to the right of the D₂/S₂ peak). Andalusite grew during M₂ contact metamorphism, post-dating D₂, not during its waning phase.

Answer: (A) and (C) are correct