The Eastern Ghats Mobile Belt (EGMB) comprises densely forested hilly tract along the eastern coast of the Indian Peninsula. It extends from Brahmni river in north of Cuttack in Orissa to Ongole in Andhra Pradesh. It comprises gneiss, charnokites and khondalites of largely Neoarchean to Mesoproterozoic ages. These rocks were subjected to multiple events of deformation, magmatism and metamorphism during Proterozoic time.
The western boundary of the belt was named as “charnokite line” since it demarkets “charnookite blet” of the eastern ghats from the non-chrnokitis regions of the west.
The western most zone is named as the transition zone that shows characteristics of ‘orogenic front’.
- The rocks of both blocks, the Eastern ghats orogeny and cratonic foreland, together underwent strong shear deformation during the eastern ghats orogeny. The Nellore and Khammam schist belts that are regarded as part of the eastern dharwar craton represents the transition at the “orogenic front”
Charnokite zone :-
Charnokite zone that borders the western part of the EGMB represents a densely forested region. It is comprised of charnokite and enderbites wit enclaves of mafic rocks, metaplites and cal-silicate gneissses metamorphosed under granulite facies conditions.
- the boundary between the basic charnokite and khondalite is often marked by the presence of cordierite-sillimanite gneiss.
Khondalite zone :-
Khodalite zone occurs both onthe western and eastern sides of the Migmatite zone that forms the central part of the EGMB. Saphirine (green spinal) bearing hypresthene rocks are also reported from the eastern belt of Khondalite zone in the vicinity of Vishakhapatnam.
Khondalite series refers to the rock of gray and red colored foliated garnet-sillimanite schist containig quartz, feldspar and graphite. Khondalites are regarded as the product of ‘dry metamorphism’. The P-T condition of khondalite series are similar to those of the charnokites.
- The highly acidic granulite (leptynite) resembling khondalite is exposed in the hills of Nilgiri, Annamalai and areas of SGT. These rocks sometimes contain 30-50cm thick bands of graphite deposits of metamorphic importance.
- Khondalite also contains manganese rich gneisses known as kodurite. chemical weathering of this leads to litho-marge, chert and ochre.
- Khondalite zone also contains some igneous complexes of alkaline gabbro, cals-silicate syenite and nephaline syenite. Minute crystals of diamond that are known to occur in stream sediments of ‘khondalite terrain’.
Tectonic evolution of EGMB :-
EGMB evolution is linked to the evolution of Purana basins of Mesoproterozoic and Neoproterozoic ages of Dharwar and Bastar craton.
- The evolution started with rifting that leads to the
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