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 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?
Show Hint
- M₁ and M₂ events refer to regional and contact metamorphism, respectively.
- The S₁ schistosity developed as a crenulation cleavage.
- Garnet grew syntectonic with D₁.
- Andalusite grew during the waning phase of D₂.
The Correct Option is A, C
Solution and Explanation
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
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