Geochemistry of Neoarchean Phyllites of the Chitradurga greenstone belt, Dharwar Craton, India: Insights to Provenance, Source Weathering, and Depositional settings
Abstract
Fine-grained siliciclastic sedimentary rocks help in determining the provenance, source weathering, paleo-surface processes, and the evolution of continental crust through time. We carried out geochemical studies on phyllites intercalated with carbonates and banded Fe-Mn rocks of the Vanivilas Formation to understand near-surface processes. In this study, the carbonate and Fe-Mn fractions were leached separately and along with them; residual silicates were analyzed to infer the weathering, extent of oxygenation and provenance of oceans during the Neoarchean.
The major-element Fe and Mn are enriched, whereas, Mg, Ca, K, and Na are depleted compared to Post-Archean Australian Shale (PAAS). The cause of low Ca and Mg could be the dissolution of carbonates and depletion of Na and K due to weathering of feldspars and forming of clay minerals. The chemical index of alteration, CIA (~67%), index of compositional variability, ICV (~83%) and Chemical index of weathering, CIW (~75%) values are moderately high, indicating variable chemical weathering within the source region dominated by tonalite, trondjhemite, granodiorite and granite. The values of Al2O3/TiO2 (23 to 68), high values of La/Sc and Th/Co ratios and TiO2 vs Zr, and TiO2 vs Ni plots implies that these were derived mainly from intermediate to felsic rock provenance. The K2O/Na2O versus SiO2 discrimination diagram suggests that the phyllites got deposited mostly in Island arcs and active continental margins. In most samples of REEs, high field strength elements and their ratios are well preserved. Depleted LREESN and slightly enriched HREESN patterns, superchondritic Y/Ho values, La, Eu and Y anomalies and lack of Ce anomaly in most of the samples indicated preservation of marine signatures. The presence of positive Eu anomaly in the samples indicates the influence of hydrothermal fluids in Neoarchean oceans. The LREE depletion [(Sm/Yb)SN] and Y/Ho, La, and Eu anomalies showed -ve correlations and trends, suggesting that the phyllites were deposited at various depths in the Neoarchean Ocean. Lower (Sm/Yb)SN values and lower Y/Ho, Eu, and La anomalies signal dominantly continental sourcing of sediments and were deposited at shallower depths.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMDI11A..03B