Charnockite, St. Thomas Mount
| Charnockite, St. Thomas Mount | |
|---|---|
 | |
  Location within Tamil Nadu | |
| Type | Geological formation |
| Location | St. Thomas Mount, Chennai, India |
| Coordinates | 12°59′42″N 80°11′58″E / 12.99506°N 80.19955°E |
Official name | St. Thomas Mount Charnockite |
| Designated | 1975 |
Designated by | Geological Survey of India |
Charnockite from St. Thomas Mount is a characteristically profuse exposure of quartz–feldspar–hypersthene rock located in southern suburb of Chennai in the Indian state of Tamil Nadu. The name originated from the use of the rocks quarried from a central band in the St.Thomas Mount for the tombstone of Job Charnock in 1679. It was declared as a National Geological Monument by the Geological Survey of India in 1975.
History
The geological formation is located atop St. Thomas Mount, a small hillock in the southern part of Chennai, Tamil Nadu.[1] The hill itself is named after Thomas the Apostle, who died in 72 CE in the region.[2]
The earliest recorded study of charnockite dates to 1892, when Thomas Oldham of the Geological Survey of India reported the occurrence of hypersthene granite in South India. In 1893, Thomas Henry Holland noted that these rocks in Madras were associated with large masses of norite and that the same type of stone had been used for the tombstone of Job Charnock, who died in 1679. The rock used for his tombstone is believed to have been quarried near St. Thomas Mount. Holland later published a paper titled "The Petrology of Job Charnock's Tombstone" and proposed the name "charnockite" for this new type of rock, in honour of Charnock. Later studies in 1950 suggested that the stone used for Charnock's tombstone was quarried from a central band of rock at St. Thomas Mount.[1][2][3] It was declared as a National Geological Monument by the Geological Survey of India in 1975.[1][4]
Geology
Charnockite is an ancient formation similar to granite and is widely distributed across South India and extending into Sri Lanka.[5] Some of these Charnockite formations occur as extensive plateaus, with elevations reaching about 6,000–7,000 ft (1,800–2,100 m).[6]
Studies of charnockites from the region have identified several varieties based on their mineral composition. These include rocks containing different combinations of orthopyroxene, garnet, and biotite, as well as varieties such as enderbites, granulites, and gneisses related to adamellite, monzonite, and granite.[1][7] Charnockites in general are bluish-grey to dark in colour and have a fresh appearance with an even-grained, granular texture. A characteristic mineral is rhombic pyroxene. Other minerals include augite, hornblende, and small amounts of opaque iron ores. Feldspar and quartz are also important components of the rock.[1][8] Charnockite at St. Thomas Mount is a quartz–feldspar–hypersthene bearing metamorphosed igneous rock, characterized by two-pyroxene facies metamorphism.[1]
The mineral composition of charnockite indicates that the rocks formed under particularly dry and high-temperature conditions. These characteristics make them important for understanding the early development of the Earth's crust.[1] Palaeomagnetic studies were carried out at 12 sites in the area to determine the geological age of the Charnockites. The studies indicate that the charnockites at St. Thomas Mount were emplaced about 1.65 billion years ago, during the Mesoproterozoic. The study also suggested that the granulites of the Madras Block underwent crustal evolution similar to that of the Eastern Ghats, based on the similarities in their palaeomagnetic directions. It further indicated that the rock formations, even those separated by large distances, developed under similar pressure, temperature and water conditions.[9]
References
- 1 2 3 4 5 6 7 National Geo-heritage of India (PDF) (Report). Indian National Trust for Art and Cultural Heritage. pp. 110–112. Archived from the original (PDF) on 11 January 2017.
- 1 2 "A memorial at The Mount". The Hindu. 27 May 2007. Archived from the original on 19 October 2003. Retrieved 1 February 2009.
- ↑ Datta, Rangan (24 August 2023). "The discovery of Charnockite – the rock of Charnock". The Telegraph. Archived from the original on 27 August 2023. Retrieved 27 August 2023.
- ↑ "Geo-Heritage Sites". Press Information Bureau (Press release). 9 March 2016. Archived from the original on 29 September 2022. Retrieved 1 September 2018.
- ↑ Proceedings of the 30th International Geological Congress, 4–14 August 1996 (1998). Precambrian Geology and Metamorphic Petrology: Beijing. VSP. p. 354.
{{cite book}}: CS1 maint: numeric names: authors list (link) - ↑ Thurston, Edgar (2005). The Madras Presidency with Mysore, Coorg, and the Associated States. Asian Educational Services. pp. 322–330. ISBN 978-8-12061-716-2. Retrieved 3 February 2009.
- ↑ Sen, S. K.; Sahu, J. R. (10 December 2004). "Phase relations in three charnockites from Pallavaram-Tambaram". Contributions to Mineralogy and Petrology. 27 (3): 239–243. Bibcode:1970CoMP...27..239S. doi:10.1007/BF00385780. S2CID 129633348.
- ↑ Rajagopalan (14 February 1947). "studies in Charnockites from St. Thomas Mount, Madras, Part II" (PDF). Geology Department, Andhra University. Archived (PDF) from the original on 26 March 2009. Retrieved 3 February 2009.
- ↑ V.S. Poornachandra Rao; J. Mallikharjuna Rao (11 April 2005). "A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain". Gondwana Research. 10 (1–2): 57–65. Bibcode:2006GondR..10...57P. doi:10.1016/j.gr.2005.11.015.
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