Question

Which one of the following sequence of index mineral correctly represents the order of increasing metamorphic grade during regional metamorphism of siliceous dolomitic limestones ?

• A. Tremolite→Diopside→Talc
• B. Diopside→Tremolite→Forsterite
• C. Talc→Tremolite→Diopside
• D. Talc→Foresterite→Tremolite

Hint:

For siliceous dolomitic limestones:
- Low grade $\Rightarrow$ Talc
- Medium grade $\Rightarrow$ Tremolite
- High grade $\Rightarrow$ Diopside

Answer 1

The Correct Option is C

Step 1: Metamorphism of siliceous dolomitic limestones.
Siliceous dolomitic limestones consist of calcite/dolomite + silica impurities. During regional metamorphism, the mineral assemblages change progressively with increasing temperature and pressure. Step 2: Low-grade index mineral.
At low metamorphic grade, the stable mineral is Talc, formed from the reaction of dolomite with silica under greenschist facies conditions. Step 3: Intermediate-grade index mineral.
With increasing grade, Tremolite (an amphibole) forms, replacing talc as temperature increases. Step 4: High-grade index mineral.
At even higher metamorphic grade, Diopside (a clinopyroxene) develops, indicating higher temperature conditions typical of amphibolite facies. Step 5: Order of progression.
Hence, the sequence of index minerals is: \[ \text{Talc (low grade)} \;\;\Rightarrow\;\; \text{Tremolite (intermediate grade)} \;\;\Rightarrow\;\; \text{Diopside (high grade)} \] \[ \boxed{\text{Talc $\;\Rightarrow\;$ Tremolite $\;\Rightarrow\;$ Diopside}} \]