The Proterozoic Eastern Ghats Mobile Belt (EGMB) extends for about 1000 km in a NE direction from Ongole in Andhra Pradesh in the south to Brahmani River in Orissa in the north, with a maximum width of about 300 km. It lies along the eastern margin of the Deccan Province, comprising the Archean Dharwar, Bastar, and Singhbhum cratons. The belt is divided by the prominent Mahanadi and Godavari grabens.
Geographic and Geological Significance
Geographic Extent
The term “Eastern Ghats” includes:
Nallamalai Hills of Cuddapah basin
Charnockite massifs of the Southern Granulite Terrain
Geologically, it is restricted to the mountain belt near the East Coast, known as the Eastern Ghats Granulite Belt.
Geological Importance
Rodinia Supercontinent Assembly: EGMB was part of the SWEAT orogen (Southwest United States and East Antarctica) during the assembly of Rodinia (~1.1 billion years ago).
Gondwana Supercontinent Breakup: EGMB separated from Antarctica during the Gondwana breakup (800-600 Ma).
Principle Age of Orogeny
1.1 Billion Years (Greenville Orogeny)
Metamorphism
Ultrahigh Temperature (UHT) Metamorphism: ~1000°C at 9-10 kbar corresponding to lower crustal depths of 30 to 35 km.
Dominated by granulite facies rocks such as charnockite, khondalites, and leptynite.
ZONATION OF THE EASTERN GHATS MOBILE BELT
Crossection of EGMB
Crossection of EGMB
Division
Different divisions of the Eastern Ghats Mobile Belt
Ramakrishnan et al. 1998
Rickers et al. 2001
Dobmeier and Raith, 2003
Western Charnockite Zone
Domain IA, IB (2.9-3.9 Ga) and Domain IV (2.5-2.9 Ga)
Krishna Province (including Nellore-Khammam schist belt and Ongole domain)
Western Khondalite Zone
Domain II (2.2-2.5 Ga) and (south of Nagavalli-Vamsadhara shear zone)
Jeyporc Province
Central Migmatite Zone
Rengali Province
Eastern Khondalite Zone
Domain III (1.8-2.2 Ga) (north of Nagavalli-Vamsadhara shear zone; no Archaean imprint)
Eastern Ghats Province
The event stratigraphy of the Eastern Ghats Mobile Belt (EGMB) over different geological time periods.
Age (Ma)
Event
550-650
Exhumation and stabilisation (=Pan-African)
800-850
Emplacement of anorthosite massifs, some alkaline rocks (?), younger granitoids, and charnockites
950-1100
Main Eastern Ghats orogeny (=Grenville)
Garnet-sillimanite-graphite gneiss (khondalite) with minor cordierite-sapphire-spinel gneiss (Mg-Al gneiss)
khondalite group
Calc-silicate rocks and rare marbles
Quartzites (garnet-sillimanite)
~1500
Emplacement of alkaline rocks along the rift margins
1800-1600
Evolution of platform (‘Purana’) basins like Cuddapah, Chhattisgarh, Indravati etc.
Evolution of Nellore-Khammam schist belt in Dharwar craton
2600-2800
Charnockites and gneisses of the basement (WCZ)
Mineral Resources
Key Minerals and Deposits
Bauxite: Found in red and yellow blankets forming mesa-type plateaus in the western and southern granulitic terrain.
Graphite: Medium- to high-grade ores in bedded form within khondalite.
Manganese: Small deposits within banded quartzite and at contacts of quartzites and calc-granulites near Vishakhapatnam, Srikakulam, Vijayanagaram, and Koraput.
Sillimanite: Low-grade deposits near Palasama in the Sambalpur district.
Precious and Semi-Precious Minerals: Insignificant occurrences of aquamarine, yellow beryl, sapphire, and amethyst in nepheline syenites and aegirine syenite along the western tectonic border of the EGMB.
Beach Placers: Main provenance for Ti-Th rich beach placers along the East Coast of India.
Paleoproterozoic Vestiges: Preserved at 1700-1800 Ma.
Metamorphism
Low Pressure Metamorphism in charnockites.
Tectonic Units
Northern Granulite Terrain (NGT)
Southern Granulite Terrain (SGT)
Blocks and Subdivisions
Northern Block (Salem Block): Located between the Fermor line and the Palghat-Cauvery Shear Zone, comprising pyroxene-bearing granites (charnockites), granite gneisses, and migmatites.
Southern Zone Divisions:
Marginal Zone
Madurai Block (Central Zone)
Trivandrum Block, consisting of Ponmudi Sub-Block (Kerala Khondalite Belt) and Nagercoil Sub-Block
Key Shear Zones
MBASZ Moyar-Bhavani Shear Zone
PCSZ Palghat-Cauvery Shear Zone
Southern Granulite Terrain (SGT)
Economic Importance
In the EGMB important mineral deposits are bauxite, graphite, manganese, and sillimanite.
Bauxite: occurs in southern and western granulitic terrain. Consists of gibbsite, goethite, and hematite.
Medium to high-grade ore of graphite occurs in bedded form in khondalite.
Small deposits of manganese. + low-grade sillimanite
Insignificant occurrences of precious and semiprecious minerals such as aquamarine, yellow beryl, and amethyst.
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