Page 96 - BE Book PESD 2021 22

Two greenstone belts, namely the Badampahar-Gorumahisani belt (BGB) in Singhbhum
craton and the Sonakhan greenstone belt in Central India have been remapped, and the
stratigraphic and structural details of the volcano-sedimentary successions have been worked
out. The tonalite-trondjhemite-granodiorite (TTG) plutons as well as the composite
Singhbhum granite batholith surrounding the BGB have also been investigated to constrain
the coupled evolution of the Paleoarchean ocean and adjacent felsic cratonic rafts. New
geochemical data from the BGB mafic and ultramafic volcanics, and adjacent granitoids have
been utilized for tectonic discrimination. The major new findings are given below.

The lower greenstone sequence of eastern Iron Ore Group (IOG) is composed of peridotitic
komatiite, and komatiitic basalt with minor chert and BIF, whereas the upper sequence has
komatiitic tholeiites varying to tholeiitic andesite together with clastic rocks. First direct
dating of the Al-depleted Karanjharan spinifex-textured komatiite from basal IOG by LA-
ICPMS U-Pb analysis of zircon grains yielded consistent weighted mean 207 Pb/ 206 Pb age of
3340 Ma, agreeing with Hf depleted mantle (TDM) age between 3357 Ma and 3432 Ma. The
Badampahar komatiites have Gd/YbN in the range 2-3, and show depletion in incompatible
elements including LREE, slight enrichment of MREE and near chondritic HREE, suggesting
derivation from a depleted mantle source with garnet residue. Juvenile Paleoarchean mantle
206
source is also suggested by positive εHf(T) (3.0-4.6) and broadly similar 207 Pb/ Pb age and
TDM age.

Plutons of the Singhbhum granite batholith adjoining the BGB, varying in composition from
tonalite-trondhjemite (relatively older component) to granite-granodiorite (younger), were
derived from low- to high-K mafic source, with some reworking of TTG for generation of
younger granites. Discriminations based on major, REE and trace element data suggest that
older strongly deformed plutons compare well with Archean TTG suite while the younger
intrusive granites are comparable with volcanic arc granites. However, structural geometry of
BGB and adjoining granitoid plutons, komatiite to komatiitic basalt association, absence of
typical signatures of derivation from subduction wedge and heterogeneity of source for the
BGB mafic volcanics, possibly point to arc-plume interaction.

The Neoarchean Sonakhan greenstone belt in central India is constituted of three distinct
NNW-SSE trending domains within a fold-fault belt. The Baghmara domain in the west
consisting of a mafic metavolcanic succession with repeated cycles of massive to pillowed
basalts, pillow breccia and thin chert-BIF-shale, represents MORB proximal oceanic realm.
The Bilari domain in the east has mixed mafic and felsic metavolcanics with minor clastic
rocks. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of
the Arjuni Formation occurs in the central part intruded by syn- to late-tectonic granitoids.
Interpretations based on new geochemical data and structural studies suggest tectonic
juxtaposition of crustal segments of MORB (Baghmara) and intra-oceanic arc setting (Bilari),
related to probable double-sided subduction, while the Arjuni Formatation is comparable to
an accretionary wedge.

Results from this project suggest gradual change over from mantle plume related tectonics in
the Paleoarchean (e.g. BGB, Singhbhum) to a plate tectonic framework of crustal evolution
in the Neoarchean (e.g. Sonakhan, Bastar).

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