Compare the mineral groups that you would expect to form from intermediate-grade metamorphism of shale, basalt, and limestone.
Describe the major compositional differences between shale and basalt. How are these differences reflected in the minerals formed during metamorphism?
The Correct Answer and Explanation is :
Comparison of Mineral Groups Formed from Intermediate-Grade Metamorphism of Shale, Basalt, and Limestone:
- Shale: Intermediate-grade metamorphism of shale produces minerals typical of the greenschist to amphibolite facies. Common minerals include muscovite, biotite, garnet, staurolite, and sometimes kyanite. These minerals form as the clay minerals in shale recrystallize into more stable silicates under increased pressure and temperature.
- Basalt: Basalt metamorphosed at intermediate grade also yields minerals of the greenschist to amphibolite facies, but with a distinct composition reflecting its mafic nature. Typical minerals include amphibole (hornblende), plagioclase, chlorite, and sometimes epidote or garnet. These minerals are derived from the alteration of pyroxene and plagioclase in the basalt.
- Limestone: Limestone, being primarily composed of calcite (CaCO₃), transforms into marble under intermediate-grade metamorphism. Recrystallization produces coarse-grained calcite or dolomite, but it does not significantly change in mineral composition unless impurities (silicates or clay) are present, which could lead to minor amounts of garnet, diopside, or wollastonite.
Compositional Differences Between Shale and Basalt:
- Shale: Shale is rich in aluminosilicates and has a high content of clay minerals, along with minor amounts of quartz and organic material. Its chemical composition is dominated by SiO₂, Al₂O₃, K₂O, and trace elements.
- Basalt: Basalt is a mafic rock, rich in iron (Fe), magnesium (Mg), and calcium (Ca), with primary minerals like plagioclase and pyroxene. It contains lower amounts of SiO₂ compared to shale.
Reflection in Metamorphic Minerals:
The aluminosilicate content in shale leads to minerals like mica, garnet, and staurolite under metamorphism. In contrast, the mafic composition of basalt forms minerals like amphibole and chlorite due to its higher Fe and Mg content. These differences highlight how protolith composition directly influences the mineralogy of metamorphic rocks.