wAbstract
@The 1.9 Ga Gwalior Group represents one of the major Paleoproterozoic
depositional basin of Bundelkhand craton and is comprised of `1
km thick unmetamorphosed and undeformed sedimentary succession,
with basal clastic sequence of sandstone-shale association (Par
Formation) and an upper chemogenic BIF rich sequence (Morar Formation).
The geochemical examination of the basal clastic rocks are the
subject of present study. The compositional maturity and extreme
quartz enrichment in sandstones of Gwalior Group suggest slow
upliftment and high in situ weathering of basement terrains in
a stable platformal setting. These clastic sediments are characterized
by strong depletion of mobile constituents like Ca, Sr and Na,
high plagioclase Index of Alteration (PIA) values and strong positive
correlation between Al2O3
and TiO2, indicating severe weathering of
upper crust. Positive correlation between FeO(T) and
TiO2 suggests Fe retention in paleoweathering
profiles and is attributed to oxygenated atmosphere. Post depositional
potash metasomatism is indicated in A-CN-K systematics of these
sediments and K-enrichment from near zero to as high as 38 is
inferred, with most of the shale samples show `30 extraneous
K2O additions. Depleted transition element
(Ni, Cr, Co, Sc) contents, elevated large ion lithophile element
(LILE) [K,Rb, Ba, Th, light rare earth elements (LREE)] contents,
fractionated rare earth element (REE) patterns, strong negative
Eu anomalies in these sediments point toward K-rich intracrustal
granitic source component. Steeper REE patterns (Avg. La/YbN `12.57) compared to basement granites and conspicuous
depletion of Y over other LILEs in AUC normalized spidergram also
suggest a heavy rare earth element (HREE) depleted TTG source
component. Minor contribution from tholeiitic mafic rocks is evidenced
from Th/Sc-Sc, La/Sc-Sc/Th systematics and positive correlation
of transition elements with FeO(T) and TiO2.
Mixing calculations on the REE data suggest that upper continental
crust of Bundelkhand craton was constituted of 40 granite, 40
TTG and 20 mafic rocks. This is in contrast with the present
day composition of Bundelkhand Granite Gneiss Complex (BGGC) that
contains `90 high K granite. The data suggest unroofing of granite
batholiths by means of erosion and upliftment of supracrustal
TTG cover.
Keywords: Sediment geochemistry; Paleoproterozoic; Crustal evolution;
Provenance; weathering; Bundelkhand cratonx
1. Introduction
2. Geological setting
@2.1. Geology of basement complex
@2.2. Geology of Gwalior Group
3. Sampling and analytical techniques
4. Results
@4.1. Major element geochemistry
@4.2. Trace element geochemistry
@4.3. Rare earth element geochemistry
5. Discussion
@5.1. Provenance characteristics
@5.2. Paleoweathering conditions
@5.3. Tectonic setting
6. Conclusions
Acknowledgements
References