Micro-Facies Analysis and Geochemistry of Shaley-Banded Iron Formations
(S-Bifs) from Late-Archaean Kushtagi-Hungund Schist Belt (KHSB),
Karnataka, South Indian Shield (SIS)
Abstract
Present work deals with paleogeographic reconstruction through sequence
modelling, facies and micro-facies analysis and bio-geochemical
investigations of late Archaean Banded Iron Formations (BIFs) from
Kushtagi-Hungund Schist Belt (KHSB) of Eastern Dharwar Craton (EDC),
South Indian Shield (SIS). This comparatively less metamorphosed schist
belt of EDC is correlated to the Bababudan Group of Dharwar Supergroup,
Western Dharwar Craton (WDC). The lower and upper age constraints have
been established from dating of 3.4-3.1 Ga old underlying Sargur Group
and TTG gneisses, and 2.5 Ga old younger granites, intruding the KHSB
metasediments, respectively. Further, metavolcanics intercalating with
the metasediments of KHSB have been dated at 2750-2670 Ma. In this work,
special emphasis has been given to the shaley-BIFs of this schist belt
which is comprised of thin (few millimeters to a maximum of 1.2-1.5 cm
thick) alternating units of iron rich shale, chert and hematite. Facies
analysis and sequence model reveal deep water, offshore paleogeography,
where proximal outer shelf is dominated by shaley siliciclastics and
distal outer shelf and further deep oceanic succession is occupied by
chert-hematite-dominated chemogenic sediment suits. Micro-facies
analysis of the shaley-BIFs elucidated the interaction between
chemogenic and deep water siliciclastic and volcaniclastic shaley
sediments within micro domains. Primary chert layers contain several
permineralized structures associated with carbonaceous matters. Recent
RAMAN spectrometric analysis (compared to previous data in provided
figure) and Carbon-isotopic values (d13C values ranging from -22.08 to
-30.84 ‰ -VPDB, n= 12) of corresponding Total Organic Carbon (TOC)
(ranging from 0.03 to 0.14 %) from cherts and shales indicate
preservation of Archean biogenic remnants. Recent elemental (major and
trace) and oxygen isotopic data, associated with the micro-facies
systematics of shale-chert-iron oxide units, have been compared to
previously published geochemical data sets derived from KHSB and other
BIFs of SIS to provide important clues and new insights regarding late
Archean ocean water chemistry, redox state, paleoclimate and control of
tectonics and provenance on sedimentation pattern, prior to Great
Oxygenation Event (GOE).