Question

A metamorphosed basaltic assemblage can include the minerals

• A. garnet-omphacite
• B. hornblende-plagioclase
• C. garnet-staurolite
• D. glaucophane-lawsonite

Answer 1

The Correct Option is A, B, D

Mafic rocks like basalt can undergo metamorphism under various conditions, resulting in different mineral assemblages. The question asks which assemblages can typically form in a metamorphosed basaltic composition.

1. Garnet-Omphacite: This mineral pair is typically found in eclogites, which form under high-pressure conditions. Omphacite is a clinopyroxene present in such settings, and garnet commonly accompanies it. Therefore, this is a valid assemblage for metamorphosed basalt.
2. Hornblende-Plagioclase: Amphibolites, a type of metamorphic rock, commonly contain hornblende and plagioclase. These minerals indicate medium-grade metamorphism, typical of regional metamorphic environments affecting basaltic rocks. Thus, this is a plausible assemblage in metamorphosed basalts.
3. Garnet-Staurolite: Garnet can be present in metamorphosed basalts, but staurolite is generally associated with pelitic (aluminum-rich) rocks rather than mafic compositions. Therefore, this is not a common assemblage for metamorphosed basalt.
4. Glaucophane-Lawsonite: Glaucophane and lawsonite are indicative of blueschist facies conditions, forming in low-temperature and high-pressure environments. These minerals can indeed form in basaltic compositions subjected to subduction-zone metaregimes. So, this is a possible assemblage.

Considering the typical metamorphic environments and the resultant mineral assemblages from a basaltic parent rock, the correct assemblages are:

• garnet-omphacite
• hornblende-plagioclase
• glaucophane-lawsonite