ORCID Identifier(s)

0000-0002-3821-5782

Graduation Semester and Year

2018

Language

English

Document Type

Thesis

Degree Name

Master of Science in Earth and Environmental Science

Department

Earth and Environmental Sciences

First Advisor

Asish R Basu

Abstract

The Iron Ore Group (IOG) greenstone belt is located along the Eastern Indian Shield. It preserves a mid-Archean segment of Earth’s history, which consists of conformable sequences of lower basaltic-andesitic lava at the base (~2 km), followed successively upwards by lower-shales (2.3 km) gradating into a tuffacious horizon (30 m at the top), an extensive Algoma-type banded-iron formation (0.7 km), upper basaltic-andesitic lava and upper-shales (~3 km). This succession provides ideal investigative conditions considering each stratigraphic unit as only been lightly-metamorphosed to the low greenschist facies, lacks penetrative deformation, preserves pristine depositional characteristics and great antiquity (~3.4 Ga). The concentrations of Rare Earth Elements (REEs), High Field Strength Elements (HFSEs) and some Large Ion Lithophile Elements (LILEs) have been determined to represent near-primary features. The lower-lavas exhibit consistent and coherent trace element signatures with respect to the upper-lavas. The rock exhibited moderately light REE-enriched signatures [(La/Sm)N = 1.25-3.69] with variable to non-existing Eu-anomalies [(Eu/Eu*) = 0.33-1.01] and flat HREE patterns [(Gd/Lu)N =0.58 – 2.2] on chondrite-normalized REE diagrams. On primitive-mantle normalized spider-diagrams, the lower- and upper-lavas feature strong enrichments in Th, Pb and Zr-Hf including depletions in Nb-Ta. The high Th/Yb coupled with the low Ta/Yb ratios demonstrate low-K to high-K calc-alkaline and shoshonite characteristics. The low Ce/Pb ratios correspond with an arc signature, whereas the high Zr/Sm and low Nb/Ta ratios suggests the petrogenesis by melting of a low-magnesium amphibolite or metamorphosed hydrous basaltic slab. Trace element and REE systematics of the lower-shales suggests their protolith was the lower-lavas through mechanical and chemical weathering. The stromatolite, which is lithologically associated with the lower-shales, exhibits near-flat REE patterns when normalized to the average lower-lava REE concentrations. This suggests the genesis of the stromatolites were chemically dependent on the lower-shales and its source. The stromatolite also exhibits overall depleted REE concentrations, a strong negative Ce-anomaly and a strong positive-Y-anomaly. This evidence suggests the stromatolite formed in a shallow-sea in the presence of oxygenizing microbilites. The signature for the presence of oxygen has implications for the genesis for the overlying banded-iron formation. The volcanic tuff has been dated to ~3.4 Ga and exhibits highly fractionated REE patterns [(La/Sm)N = 5.22] with a distinctly negative Eu-anomaly [(Eu/Eu*) = 0.22]—indicative of highly evolved magmatic differentiation processes to be occurring contemporaneously with the deposition of the lower-shales. The trace element and REE systematics for the upper-shales favor the protolith for an arc source similar to the composition of the lower-lavas. The positive and negative Ce-anomalies [(Ce/Ce*) = 0.57 – 2.95] represent oxic and anoxic periods following the formation of the banded-iron formation. The average REE concentrations for the lower- and upper-shales were averaged and plotted alongside Post Archean Average Shales (PAAS). The average Archean Indian shales (AAIS, this study) appear to be lower in concentration with respect to PAAS. This suggest the Archean continental crust was fundamentally different compared to modern continental crust.

Keywords

Archean, Craton, Eastern India, Iron ore group, Greenstone Belt

Disciplines

Earth Sciences | Physical Sciences and Mathematics

Comments

Degree granted by The University of Texas at Arlington

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