Question

A schematic deformation (D)-metamorphism (M)-time map of a metamorphic belt is shown below. The belt recorded two phases of deformation, D₁-D₂ and attendant schistosity, S₁-S₂ and metamorphism, M₁-M₂. The metamorphic minerals stable during these events are shown by solid lines. On the basis of this information, which of the following statement(s) is/are correct? 

• A. M₁ and M₂ events refer to regional and contact metamorphism, respectively.
• B. The S₁ schistosity developed as a crenulation cleavage.
• C. Garnet grew syntectonic with D₁.
• D. Andalusite grew during the waning phase of D₂.

Hint:

In metamorphic studies, it's important to distinguish between regional and contact metamorphism, with regional metamorphism occurring under high pressure and temperature, and contact metamorphism occurring due to the heat from an igneous intrusion.

Answer 1

The Correct Option is A, C

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