page_title: Charnockite, St. Thomas Mount
section_title: Geology
hierarchical_section_title: Charnockite, St. Thomas Mount / Geology
caption_attribution_description: English: Charnockite rock exposure at Pallavaram quarries
context_page_description: Charnockite, St. Thomas Mount, located to the south of the Chennai city, in Pallavaram suburb, Tamil Nadu, is a characteristically profuse exposure of quartz–feldspar–hypersthene rock, illustrated by occurrence of two pyroxene facies metamorphism.
  The name "Charnockite, St. Thomas Mount" originated from the use of the rocks quarried from a central band in the St.Thomas Mount for the tombstone of Job Charnock, the founder of Kolkata in 1679. Several incidents in Charnock's life are linked to Charnockite rock, Chennai city and the St. Thomas Mount.
context_section_description: The constituents of the rock suggests of its origin in particularly dry and high temperature conditions which is deduced to have an important bearing in explicating prehistoric crustal development of the earth.
  From the time Holland, named the rock as Charnockite, its characteristics have been extensively studied by several researches in Earth Sciences over the century. An analysis of three charnockites in the region, extending from St. Thomas Mount – Pallavaram –Tambaram, south of the city of Chennai, carried out on the compositional characteristics of coexisting orthopyroxene, garnet and biotite has established several petrographic varieties within the charnockites–enderbites such as the granulites and gneisses corresponding to adamellite, monzonite, granite and others.
  Another study on the general characteristics of the rocks of this series has recorded that the rocks are in general bluish gray or darkish in colour and extremely fresh in appearance with an even grained granular structure. The mineral composition shows an unvarying presence of pleochroic rhombic pyroxene. The other femic minerals noted are augite and hornblende with minute grains of opaque iron ores and these minerals have been noted to cluster together. Plagioclase feldspars, alkali feldspars and quartz are the sialic minerals present in this series of rocks.
  In order to determine the geological age of the charnockitic region of the Pallavaram area in the Madras (Chennai) Block, palaeomagnetic investigations were carried out at 12 sites, which suggested an age of magnetization in the Pallavaram charnockites as 2600 Ma while in the St. Thomas Mount the period of emplacement was assessed as 1650 Ma (Mesoproterozoic). There was a further inference that the Madras block granulites indicated crustal evolution similar to those in the Eastern Ghats belt with identical palaeopoles from both the areas. The study also noted that "pressure, temperature and the chemical potential of water were constant for assemblages separated by large distances."
caption: Charnockite rock quarry at Pallavaram
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