Contributor

Asish Basu

Abstract

This thesis presents new data on the origin of the Western Iron Ore Group (W-IOG), a Paleoarchean succession deposited on sialic rocks of the Singhbhum Craton. U-Pb ages from zircons from a tuff overlying the Lower Shales just below a banded iron formation (BIF) pin the age of the section to 3.39 Ga. Unconformably overlying the whole IOG section is the Darjing Group, which contains the Birtola Sandstone (on the basis of this work, now estimated to be 2.35 Ga). The lack of penetrative deformation and low-grade metamorphism makes these rocks excellent subjects for combined radiogenic and stable isotope studies. Data on the W-IOG constrain the development of earth systems operating during the Archean into the Proterozoic (4.0-2.4 Gyr). 143Nd/144Nd isotopes for basal Lower Lava greenstones suggest a whole-rock isochron with an apparent age of 3.42 ± 0.14 Ga exhibiting perhaps the highest positive initial εNd (εNd = +5.7 ± 2.5) observed thus far for an Archean greenstone, and suggesting it is a possible remnant of early Earth differentiation. The initial εNd of 2.7 at 2.65 Ga for the Upper Lava (i.e., 3 εNd units lower than the greenstones of the Lower Lava) might reflect the cumulative effect of recycling of less Light Rare Earth Element (LREE) depleted sediments into the mantle by subduction processes. The Lower Lava and the Lower Shale plot on a common whole-rock 143Nd/144Nd versus147Sm/144Nd line. When combined with REE data, this indicates that the provenance for the Lower Shale is from mafic rocks similar to the underlying Lower Lava, perhaps a subaerial constructional pile of mafic rocks (i.e., the Lower Lava). Lower Shale rocks are rich in iron (avg. 18 Fe2O3 wt.%, n = 7) and with diachronous sedimentation could be the proto-ore for the BIF. The spread in iron isotope ratios (-0.693 ≤ δ56Fe ≤ 1.735 ‰) suggests the oxidation of iron may have been biologically mediated 3.4 billion-years ago. The Upper Shales share a common line with Singhbhum’s granitic basement (with an apparent age of 3.36 ± 0.09), implying that these iron-poor shales formed primarily from the unroofing of subvolcanic granitic basement. New zircon U-Pb, Lu/Hf and 18O/16O along with the d18O of host rock sandstones of the Birtola Formation demonstrates that mantle-like zircon d18O values are consistent with source regions that have been affected by the addition of subducted recycled crustal Hf. If so, the additions would have been in proposrtions implying significant refertilization of the MASH source regions that are the presumed igneous source for the Birtola Sandstone. Using the D18O quartz-zircon, forward modelling of the d18O values of the quartz from the zircons and zircons from the quartz distributions are consistent with temperatures of silicic extrusive rock magmatic temperatures. Some low 18O Archean zircons (d18O18O of the IOG greenstone succession requires surface temperatures more consistent with the faint early Sun. The climate implications and the evidence of recycling down into the mantle all suggest plate tectonics in the early Earth.

Degree Date

Spring 5-11-2024

Document Type

Dissertation

Degree Name

Ph.D.

Department

Earth Sciences

Advisor

Roy M. Huffington Department

Second Advisor

ROBERT T. GREGORY

Number of Pages

137

Format

.pdf

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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