Model Answer
0 min readIntroduction
Metamorphic facies represent a range of pressure and temperature conditions under which rocks are formed. Granulite and amphibolite facies are two high-grade metamorphic facies, representing progressively higher metamorphic conditions. Understanding the differences between these facies is crucial for deciphering the tectonic history of a region. While both involve the recrystallization of pre-existing rocks, they differ significantly in their mineralogy and texture, reflecting distinct P-T paths. This answer will delineate these differences, providing a comprehensive understanding of how to differentiate between granulite and amphibolite facies rocks in the field and laboratory.
Defining Metamorphic Facies
A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressure and temperature conditions. The facies are named after a characteristic rock type. The stability of different mineral assemblages is dependent on P-T conditions, allowing geologists to infer the conditions under which a metamorphic rock formed.
Amphibolite Facies
The amphibolite facies is characterized by temperatures ranging from 450-700°C and pressures from 1-1.5 kbar. It is commonly found in regional metamorphic belts associated with orogenic events. Key index minerals include hornblende, plagioclase feldspar, and sometimes garnet. The typical rock type is amphibolite, hence the name.
Texture of Amphibolite Facies Rocks
- Foliation: Generally exhibits a well-developed foliation due to the alignment of platy minerals like hornblende and mica.
- Grain Size: Medium to coarse-grained.
- Mineral Shape: Minerals are often elongated and aligned.
Granulite Facies
The granulite facies represents even higher temperatures (700-900°C) and pressures (1.5-10 kbar). It is often associated with continental collision zones and deep crustal processes. Key index minerals include pyroxene (hypersthene, diopside), plagioclase feldspar, garnet, and sometimes sillimanite or kyanite. Water content is significantly lower than in amphibolite facies rocks.
Texture of Granulite Facies Rocks
- Foliation: Foliation may be present but is often less pronounced than in amphibolite facies rocks. It can be granoblastic (equigranular) or banded.
- Grain Size: Coarse-grained, often with large, well-formed crystals.
- Mineral Shape: Minerals are generally equigranular and less elongated than in amphibolite facies rocks.
Comparative Analysis: Amphibolite vs. Granulite Facies
The following table summarizes the key differences between amphibolite and granulite facies rocks:
| Feature | Amphibolite Facies | Granulite Facies |
|---|---|---|
| Temperature (°C) | 450-700 | 700-900 |
| Pressure (kbar) | 1-1.5 | 1.5-10 |
| Water Content | Relatively High | Low |
| Index Minerals | Hornblende, Plagioclase, Garnet | Pyroxene, Plagioclase, Garnet, Sillimanite/Kyanite |
| Foliation | Well-developed | Less pronounced, granoblastic or banded |
| Grain Size | Medium to Coarse | Coarse |
| Mineral Shape | Elongated, aligned | Equigranular |
| Rock Type Example | Amphibolite | Granulite, Charnockite |
Distinguishing Features in the Field
In the field, granulite facies rocks often appear more massive and less obviously foliated than amphibolite facies rocks. The presence of pyroxene, which is rarely found in amphibolite facies rocks, is a strong indicator of granulite facies. The coarser grain size of granulite facies rocks also makes them easier to identify. The lack of hydrous minerals like hornblende in granulites is another key distinction.
Conclusion
In conclusion, differentiating between granulite and amphibolite facies rocks relies on a careful assessment of their mineral assemblages, textures, and the broader geological context. Granulite facies rocks, formed under higher temperature and pressure conditions, are characterized by coarser grain sizes, reduced foliation, and the presence of minerals like pyroxene, while amphibolite facies rocks exhibit well-developed foliation and a mineral assemblage dominated by hornblende and plagioclase. Accurate identification of these facies provides valuable insights into the metamorphic history and tectonic evolution of a region.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.