Model Answer
0 min readIntroduction
Ophiolites are slices of oceanic crust and upper mantle that have been emplaced onto continental crust, typically during orogenic events. They represent a unique opportunity to study the composition and structure of the oceanic lithosphere, which is otherwise largely inaccessible. India hosts several significant ophiolite suites, remnants of the Neo-Tethys Ocean that existed between the Indian and Eurasian plates. These ophiolites provide crucial evidence for understanding the complex tectonic history of the region, particularly the closure of the Neo-Tethys and the subsequent collision leading to the formation of the Himalayas. Their study helps reconstruct paleogeographic settings and unravel the processes involved in continental convergence.
What are Ophiolites?
Ophiolites are considered to be the obducted (thrust over continental crust) remnants of oceanic crust. A complete ophiolite sequence, as proposed by Black (1962), typically consists of the following layers, from bottom to top:
- Harzburgite/Peridotite: Ultramafic rocks representing the upper mantle.
- Gabbro: Mafic intrusive rocks formed from slow cooling of magma.
- Sheeted Dikes: Vertical to sub-vertical dikes of basaltic composition.
- Pillow Basalts: Basaltic lava flows that cooled rapidly underwater, forming pillow-shaped structures.
- Radiolarian Chert/Sedimentary Rocks: Sedimentary rocks deposited on top of the oceanic crust.
Indian Ophiolite Suites: Major Occurrences
Several ophiolite suites are found in India, primarily associated with the Himalayan orogenic belt and the Gondwana breakup. The major occurrences include:
- Indus Suture Zone (ISZ): This is the most prominent ophiolite belt in India, extending along the Indus-Tsangpo river system. Key ophiolites include the Shyok, Dras, and Padum ophiolites.
- Kohistan-Karakoram Ophiolite: Located in the northern part of Pakistan, but closely related to the Indian plate boundary.
- Zanskar Ophiolite: Found in the Zanskar region of Ladakh, representing a fragment of the Neo-Tethys oceanic crust.
- Semail Ophiolite (Oman): Although located in Oman, it is often studied in conjunction with Indian ophiolites due to its similar origin and tectonic setting. It provides a well-preserved example for comparison.
- Eastern Ghats Mobile Belt: Smaller, dismembered ophiolite remnants are found in the Eastern Ghats, representing older oceanic crust related to the breakup of Gondwana.
Petrological and Structural Characteristics
Indian ophiolites exhibit a range of petrological and structural features that provide insights into their origin and emplacement:
- Harzburgite Composition: The mantle sections are typically composed of harzburgite, depleted in incompatible elements due to partial melting during seafloor spreading.
- High-Mg Basalts: The basaltic rocks are often characterized by high magnesium content, indicative of a relatively primitive magma source.
- Shear Zones and Faults: Ophiolites are commonly associated with major shear zones and faults, reflecting the intense deformation during obduction.
- Serpentinization: The ultramafic rocks are often extensively serpentinized, a process involving hydration of olivine and pyroxene to form serpentine minerals.
- Radiolarian Cherts: The presence of radiolarian cherts indicates a pelagic depositional environment in the oceanic realm.
Significance of Indian Ophiolites
The study of Indian ophiolites has profound implications for understanding the tectonic evolution of the region:
- Neo-Tethys Ocean Closure: Ophiolites provide direct evidence for the existence and eventual closure of the Neo-Tethys Ocean. Their age and composition help constrain the timing and rate of subduction.
- Himalayan Orogeny: The obduction of ophiolites played a crucial role in the Himalayan orogeny, contributing to the thickening and uplift of the continental crust.
- Paleogeographic Reconstruction: Ophiolites help reconstruct the paleogeography of the region, providing insights into the position of continents and the configuration of oceanic basins.
- Geochemical Tracers: The geochemical composition of ophiolites can be used to trace the origin of magmas and identify mantle plumes or other sources of mantle heterogeneity.
- Mineral Resources: Ophiolites are often associated with significant mineral deposits, including chromite, copper, and nickel.
The ISZ ophiolites, in particular, are crucial for understanding the timing of the India-Asia collision. Geochronological studies suggest that subduction initiated around 90 Ma, with obduction occurring between 65-55 Ma, preceding the main phase of Himalayan uplift.
Conclusion
Indian ophiolite suites represent invaluable geological archives, providing critical insights into the complex tectonic history of the Indian subcontinent and the formation of the Himalayas. Their study has revolutionized our understanding of the Neo-Tethys Ocean closure and the processes involved in continental collision. Continued research, including detailed geochemical and geochronological analyses, will further refine our understanding of these fascinating geological features and their role in shaping the landscape of South Asia.
Answer Length
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