Model Answer
0 min readIntroduction
The Deccan Traps, a massive flood basalt province covering much of western and central India, represent one of the largest volcanic features on Earth. Formed during the late Cretaceous and early Paleogene periods (around 66 million years ago), these basaltic lava flows are crucial for understanding the geological history of the Indian subcontinent and are linked to the Cretaceous-Paleogene extinction event. The petrological characteristics of the Deccan Traps are vital for deciphering their origin, evolution, and impact on the environment. This answer will enumerate these characteristics, providing a comprehensive overview of their petrology.
Petrological Characteristics of the Deccan Traps
The Deccan Traps are predominantly composed of basaltic lava flows, exhibiting a range of petrological features. These can be broadly categorized into composition, texture, mineralogy, and classification.
1. Composition
- Major Elements: The basalts are typically tholeiitic in composition, characterized by relatively low TiO2 (typically <1.0 wt%), MgO (7-18 wt%), and high Al2O3 (10-16 wt%). SiO2 content generally ranges from 48-53 wt%.
- Trace Elements: The lavas are enriched in incompatible trace elements like Rb, Ba, and K, indicating a mantle source with some degree of prior metasomatism.
- Isotopic Signatures: Radiogenic isotope ratios (Sr, Nd, Pb) suggest a complex mantle source, potentially involving both depleted mantle and recycled crustal components.
2. Texture
- Flow Structures: The lava flows exhibit well-developed flow structures, including pahoehoe (ropy) and ‘a’a (blocky) textures, indicative of varying lava viscosity and cooling rates. Pahoehoe flows are common in the lower parts of the traps, while ‘a’a flows dominate the upper layers.
- Vesicularity: Many flows are highly vesicular, containing abundant gas bubbles (vesicles) formed during degassing of the lava. Vesicle size and density vary depending on the lava's viscosity and gas content.
- Columnar Jointing: Cooling and contraction of thick lava flows resulted in the development of characteristic columnar jointing, with hexagonal columns perpendicular to the cooling surfaces.
- Amygdules: Secondary minerals like zeolites, calcite, and chalcedony often fill vesicles, forming amygdules.
3. Mineralogy
- Plagioclase Feldspar: The most abundant mineral is plagioclase feldspar (typically labradorite to bytownite), occurring as laths and phenocrysts.
- Pyroxenes: Augite is the dominant pyroxene, present as phenocrysts and groundmass crystals. Other pyroxenes like pigeonite are also found.
- Olivine: Olivine is present in varying amounts, often altered to serpentine and iddingsite. Its abundance decreases with stratigraphic height.
- Iron-Titanium Oxides: Magnetite and ilmenite are common accessory minerals, contributing to the magnetic properties of the basalts.
- Apatite: Apatite is present as small, disseminated crystals.
4. Classification
| Classification Scheme | Description | Deccan Trap Basalts |
|---|---|---|
| TAS (Total Alkali-Silica) Diagram | Classifies volcanic rocks based on their total alkali (Na2O + K2O) and silica (SiO2) content. | Generally plot in the basalt field, often exhibiting a tholeiitic trend. |
| Hawaiian Classification | Based on lava viscosity and flow morphology. | Displays both tholeiitic and alkali basalt characteristics, with a dominance of tholeiitic types. |
| MgO Content | Categorizes basalts based on magnesium oxide (MgO) content. | Ranges from low-Mg to high-Mg basalts, with a general decrease in MgO content with stratigraphic height. |
Furthermore, the Deccan Traps exhibit variations in petrology with stratigraphic height. Lower flows tend to be more primitive (higher MgO, lower TiO2), while upper flows are more evolved (lower MgO, higher TiO2). This suggests a fractionation trend during the prolonged volcanic activity.
Conclusion
In conclusion, the Deccan Traps exhibit a diverse range of petrological characteristics, primarily reflecting their tholeiitic basaltic composition, varied textures resulting from different flow regimes, and a mineral assemblage dominated by plagioclase and pyroxenes. The isotopic signatures and compositional variations suggest a complex mantle source and a prolonged period of magmatic activity. Understanding these characteristics is crucial for reconstructing the geological history of the Indian subcontinent and the environmental consequences of the Deccan Traps volcanism.
Answer Length
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