Model Answer
0 min readIntroduction
Anorthosites are intrusive igneous rocks composed almost entirely of plagioclase feldspar, typically labradorite or bytownite. They represent a significant component of the Earth’s crust, particularly in Precambrian shields, and provide valuable insights into the early evolution of the planet. Globally, prominent anorthosite provinces are found in Canada (Laurentian Shield), Greenland, and Australia. In India, these rocks are less extensive but occur in several distinct provinces, offering a unique window into the country’s geological history. Understanding their origin is crucial for deciphering the processes that shaped the continental crust.
Definition and Characteristics of Anorthosites
Anorthosites are characterized by their leucocratic (light-colored) appearance due to the dominance of plagioclase feldspar. They typically contain less than 5% mafic minerals (like pyroxene, olivine, and amphibole). Their formation requires specific conditions that favor the accumulation of plagioclase crystals, often involving slow cooling at depth within the Earth’s crust. The texture can range from coarse-grained to gabbroic, and they are often associated with other igneous rocks like gabbros and norites.
Anorthosite Provinces of India
Several anorthosite provinces are identified in India, each with unique geological characteristics:
- Eastern Ghats Mobile Belt (EGMB): This is the most significant anorthosite province in India, extending across Andhra Pradesh, Odisha, and Tamil Nadu. The Charnockite series within the EGMB contains substantial anorthosite bodies, often associated with enderbites and rapakivis. These anorthosites are typically Proterozoic in age (around 1.8-1.6 Ga).
- Bundelkhand Granite Complex (BGC): Located in Madhya Pradesh and Uttar Pradesh, the BGC contains smaller anorthosite intrusions within the granite suite. These are also Proterozoic in age and are often associated with iron ore deposits.
- Southern Granulite Terrain (SGT): Found in parts of Tamil Nadu and Kerala, the SGT hosts localized anorthosite occurrences, often linked to high-grade metamorphic rocks.
- Western Ghats: Minor occurrences of anorthosites are found within the Western Ghats, associated with the Deccan Trap volcanics and older basement rocks.
Origin of Indian Anorthosites: Proposed Theories
The origin of anorthosites has been a long-standing debate in geological circles. Several hypotheses have been proposed, and the Indian anorthosites likely have a complex origin involving multiple processes:
1. Magmatic Differentiation
This is the most widely accepted theory. It suggests that anorthosites form through the fractional crystallization of mantle-derived magmas. As the magma cools, plagioclase crystals preferentially crystallize and accumulate due to their density, forming layers or lenses of anorthosite. This process requires efficient removal of other minerals (mafic minerals) to maintain the high plagioclase content. The EGMB anorthosites are often explained by this mechanism, with the initial magma being modified by crustal contamination.
2. Impact Origin
The impact theory proposes that large meteorite impacts can generate melt sheets that undergo fractional crystallization, leading to the formation of anorthosites. This theory gained prominence with the discovery of anorthositic breccias at impact structures. While no definitive impact structure is directly associated with the major Indian anorthosite provinces, the possibility of ancient, eroded impact structures cannot be ruled out.
3. In-situ Differentiation
This theory suggests that anorthosites can form through the differentiation of pre-existing crustal rocks. Partial melting of crustal rocks, followed by fractional crystallization, can produce anorthositic magmas. This process is particularly relevant in regions with pre-existing felsic crust.
4. Liquid Immiscibility
This process involves the separation of a magma into two or more immiscible liquids. In some cases, a plagioclase-rich liquid can separate from a more mafic liquid, leading to the formation of anorthosites. This is considered a less dominant process but may contribute to the formation of localized anorthosite bodies.
The anorthosites of the Eastern Ghats are particularly interesting as they show evidence of multiple episodes of magmatism and deformation, suggesting a complex history involving both magmatic differentiation and possible crustal reworking. The association with Charnockites, which are high-temperature, high-pressure metamorphic rocks, further complicates the understanding of their origin.
| Province | Age | Dominant Rock Type | Proposed Origin |
|---|---|---|---|
| Eastern Ghats | Proterozoic (1.8-1.6 Ga) | Anorthosite, Charnockite, Enderbite | Magmatic Differentiation, Crustal Contamination |
| Bundelkhand | Proterozoic | Anorthosite, Granite | Magmatic Differentiation |
| Southern Granulite Terrain | Archean-Proterozoic | Anorthosite, Granulite | In-situ Differentiation, Metamorphism |
Conclusion
In conclusion, anorthosites in India, though less extensive than their global counterparts, represent significant geological features providing insights into the Precambrian crustal evolution. Their origin is likely polygenetic, involving a combination of magmatic differentiation, potential impact events, and crustal processes. Further research, including detailed geochemical and isotopic studies, is crucial to unravel the complete history of these fascinating rocks and their role in the formation of the Indian subcontinent. Understanding these formations is also important for resource exploration, as they are often associated with valuable mineral deposits like iron ore and rare earth elements.
Answer Length
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