The Westward Reach of the Eastern
Ghats Belt:
Evidence from the Dharamgarh
Granulites, Kalahandi district, Orissa,
India
S. Gupta, I. Das and A. Bhattacharya
Department of Geology and Geophysics,
Indian Institute of Technology, Kharagpur-721302, India
The Eastern Ghats Belt (EGB) is characterised by high grade,
intensely deformed metamorphic rocks whose principle structural trends in
the western part are parallel to the NNE-SSW elongation of the belt (Murthy
et al., 1971; Naranaswamy, 1975). The dominant rock types in the EGB
include migmatitic quartzofeldspathic and metapelitic gneisses that host
various other types of metasedimentary lithologies, mafic granulites, layered
and massif anorthosites, enderbitic charnockitic massifs, K-rich porphyritic
granitoids and alkaline igneous intrusives. Preserved metamorphic mineral
assemblages testify to attainment of extreme metamorphic conditions
associated with several episodes of partial melting in the granulite facies;
such studies quantify near-peak P-T conditions of 8-10 kb and 800-10000C
(e.g., Dasgupta et al., 1991; Mukhopadhyay and Bhattacharya, 1997).
Several of these workers have reconstructed an anticlockwise P-T path for
the region, involving initial cooling from peak temperature conditions,
followed by decompression.
The EGB is topographically elevated, and its western part is
occupied by a zone of enderbitic charnockite (called the western charnockite
zone) which is alligned parallel to the extension of the belt
(Narayanaswamy, 1975). The western boundary of EGB has been
traditionally assumed to coincide with the contact between western
charnockite zone and quartzofeldspathic biotite gneisses in the low-lying
areas to the west (Crookshank, 1938). The contact has been variably
interpreted as a fault, with the eastern block downthrown (Walker, 1902) or
upthrown (Fermor, 1935), or as an irregular intrusive contact defined
defined by the margins of the charnockite bodies (Crookshank, 1938).
Further to the west, the lithologies are deemed to be distinct from those in
the Eastern Ghats, and a lower grade of metamorphism (amphibolitegreenschist facies) has been tentatively assigned to the rock ensemble
(Crookshank, 1963). The granulite facies rocks of the EGB are believed to
be separated from these amphibolite facies rocks to the west by a narrow
"transitional zone" (Narayanaswamy, 1975; Nanda and Pati, 1989). Discrete
enderbitic charnockitic bosses are intrusive into the amphibolite facies
gneisses of the transitional zone.
The Dharamgarh area of Kalahandi district, Orissa, lies to the
west of the supposed boundary and within the "transitional zone". The area
is dominated by quartzofeldspathic gneisses interbanded with intermittent,
concordant mafic layer composed of biotite and hornblende. Orthopyroxene
occurs sporadically within these gneisses in greasy green patches with
diffuse boundaries (called "patchy charnockites"). Bands of mafic granulites,
rare calc-silicate gneisses and enderbitic charnockite intrusives are present in
places.
The area has suffered at least four deformational events. The
earliest event (D1) resulted in the formation of the gneissic segregation
banding (S1) defined by alternate quartzofeldspathic and mafic layers . S 1
has been strongly refolded during a second, intense deformation event (D2).
F2 fold styles vary from tight to isoclinal, and in places result in the
development of a weak axial planar foliation, particularly in biotite-rich
layers within the gneisses. The third deformation event (D3) is characterised
by open to tight folding with N-S trending, steeply east-dipping axial planes
(S3) which represent the dominant structural grain in the dharamgarh area. A
 pole plot of the S3 foliation over the entire study area shows a remarkably
well-constrained point concentration, with a vector mean of 470-900.
Frequently, the S1 foliation curves into S3, mimicing S-C structures; in
places where the original orientation of S3 is antithetic to the sense of shear,
fold forms are generated. this provides evidence of ductile shearing along S3.
A striping, intersection lineation (S1/S3) is seen on the shear plane; these
show variable attitudes, reflecting varying orientation of S1 after the D2
deformation. The S1/S2 lineation plots are accordingly distributed on a girdle
(470-830N) which broadly coincides with the average S3 foliation. A downdip stretching lineation defined by stretched quartz grains is seen in places.
The shearing may either be a consequence of continuing D3 deformation, or
may be a discrete later event (D4). The latter possibility is more likely, since
symmetric, open F3 folds are observed to become increasingly asymmetric
and tight in the proximity of zones of high shearing strain, which are
however invariably parallel to S3. The final phase of movement during D4 is
represented by brittle deformation along the S3 planes, resulting in
displacement of F3 fold hinges and truncation of earlier structural features.
Characterisation of metamorphic conditions during D1 and D3 is
not possible because of the absence of diagnostic mineral assemblages in the
quartzofeldspathic gneisses, which preserve fabrics related to these events.
Quartzofeldspathic veins rimmed by biotite-rich selvages frequently truncate
S1/S3 fabrics; the veins are cofolded with S1/S2 during the D3 event,
testifying to partial melting before D3. Melts synchronous with D3 occurs in
segregations parallel to S3. Further melting synchronous with the ductile
shearing (D3) can be documented on both mafic granulites and
quartzofeldspathic gneisses. Occasionally, melt pockets overprint the S3
fabric in both lithologies, suggesting the persistence of high-grade
conditions after D3. This is further confirmed from the presence of twopyroxene assemblages in mafic granulites, and pathy charnockites in the
gneisses which overgrow S3. Coronas of garnet around clinopyroxenes
suggest decreasing temperature after D3; the breakdown of these coronas
into symplectitic intergrowths of orthopyroxene and plagioclase, attests to
subsequent decompression.
The ensemble of rock types at Dharamgarh closely resembles
those reported from the EGB, and contrasts sharply with rock types of the
Bastar area to the west (Crookshank, 1963). The sequence of deformation
events, and their association with successive generations of leucosome
formation, are also compatible with those inferred elsewhere in the Eastern
Ghats (Halden at. al., 1982; Mukhopadhyay and Bhattachatya, 1997; Biswal
et. al., 1997). Metamorphic conditions are also characteristic of the granulite
facies, enlike the metamorphic facies ascribed to the Bastar region. The
sequence of post-peak cooling followed by decompression recorded in
Dharamgarh is also characteristic feature of the EGB. From the viewpoint of
lithology, structure and metamorphism, therefore, the Dharamgarh region is
indistinguishable from the Eastern Ghats proper.
Thus contrary to the previous assumptions, the western margin
of the eastern ghats in the kalahandi region cannot be considered coincident
with the topographic break defined by the western charnockite zone. This
study reveals that geologically, the EGB stretches further to the west, well
into the "transitional zone", at least to the west of the Dharamgarh town. The
presence of faults or intrusions along the topographic break are therefore
features present within the Eastern Ghats, and are irrelevant to the nature of
the boundary itself.
REFERENCES:Biswal, T.K., et. Al., (1997)