Model Answer
0 min readIntroduction
Metamorphism is the alteration of pre-existing rocks by heat, pressure, or chemically active fluids. Metamorphic facies are defined by specific pressure-temperature (P-T) conditions under which particular mineral assemblages are stable. The mineral assemblage formed during metamorphism is directly related to the composition of the protolith (parent rock) and the P-T conditions experienced. A mafic protolith, rich in magnesium and iron, will yield distinct mineral assemblages as it progresses through increasing metamorphic grades – greenschist, amphibolite, and granulite facies. Understanding these changes is crucial for deciphering the geological history of a region.
(i) Greenschist Facies
The greenschist facies represents relatively low-temperature and low-pressure metamorphic conditions (typically 300-500°C and 1-4 kbar). A mafic protolith undergoing greenschist metamorphism will typically produce the following mineral assemblage:
- Chlorite: A key indicator mineral of this facies, formed by hydration of mafic minerals.
- Actinolite: An amphibole stable at these conditions.
- Epidote: A calcium-aluminum silicate, also indicative of moderate temperatures.
- Albite: A plagioclase feldspar.
- Quartz: Present as a secondary mineral.
- Magnetite/Maghemite: Iron oxides formed from the breakdown of primary ferromagnesian minerals.
The reaction involves the breakdown of original mafic minerals like pyroxene and olivine, leading to the formation of these hydrous phases.
(ii) Amphibolite Facies
The amphibolite facies represents intermediate temperature and pressure conditions (typically 500-700°C and 4-8 kbar). Metamorphism at this grade results in:
- Hornblende: A dominant amphibole, replacing chlorite from the greenschist facies.
- Plagioclase: Typically andesine or labradorite composition.
- Garnet: Almandine or pyrope-rich garnet becomes stable.
- Quartz: Continues to be present.
- Biotite: May be present in smaller amounts.
- Magnetite/Ilmenite: Iron-titanium oxides.
The increase in temperature and pressure leads to dehydration reactions and the formation of more stable, anhydrous minerals like hornblende and garnet. The assemblage reflects a shift towards higher-grade metamorphism.
(iii) Granulite Facies
The granulite facies represents high-temperature and moderate to high-pressure conditions (typically 700-1000°C and 6-10 kbar or higher). A mafic protolith metamorphosed to granulite facies will exhibit:
- Pyroxene: Orthopyroxene (enstatite) and clinopyroxene (diopside) become stable.
- Plagioclase: Anorthite-rich plagioclase.
- Garnet: Granate-rich garnet.
- Quartz: Present.
- Feldspar: K-feldspar may appear.
- Magnetite/Ilmenite: Iron-titanium oxides.
At these high temperatures, hydrous minerals are unstable and are replaced by anhydrous phases. Partial melting may also occur, leading to the formation of granoblastic textures. The granulite facies assemblage indicates the highest metamorphic grade among the three.
The following table summarizes the key differences:
| Facies | Temperature (°C) | Pressure (kbar) | Characteristic Minerals |
|---|---|---|---|
| Greenschist | 300-500 | 1-4 | Chlorite, Actinolite, Epidote, Albite |
| Amphibolite | 500-700 | 4-8 | Hornblende, Plagioclase, Garnet |
| Granulite | 700-1000+ | 6-10+ | Pyroxene, Plagioclase, Garnet |
Conclusion
In conclusion, the mineral assemblages formed during the metamorphism of a mafic protolith are directly linked to the prevailing pressure-temperature conditions. Progressing from greenschist to amphibolite and finally to granulite facies results in a shift from hydrous to anhydrous mineral assemblages, reflecting increasing metamorphic grade. Identifying these mineral assemblages allows geologists to reconstruct the thermal and tectonic history of a region and understand the processes that have shaped the Earth’s crust.
Answer Length
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