U-Pb and Ar-Ar geochronology and geochemistry of the south-eastern Basement Complex of South Sudan

No Thumbnail Available

Date

2020

Authors

Tshwane, Itumeleng

Journal Title

Journal ISSN

Volume Title

Publisher

Abstract

A few studies have suggested that the poorly studied and undifferentiated Basement Complex of South Sudan was part of the Congo Craton. In this work, granitic gneisses and biotite schists are examined to enhance our knowledge of the nature and mode of tectonic and geological evolution of this geological unit. Zircon U-Pb, Lu-Hf and Ar-Ar isotope systematics, petrographical observations and whole rock geochemistry reveal a single major crust-forming magmatic or crustal reworking event at 981-924 Ma. The zircons show a mean crustal residence age of 2.3 Ga and εHf(t) values of -24 to -30 suggesting the existence of older Precambrian crust. This magmatic event gave rise to calc-alkaline, peraluminous I-type granites with high SiO2 (74-75wt. %) and K2O (3.8-5.3 wt. %) and low CaO (1.0-1.3wt. %) contents. The gneissic granites also show enrichment of LILE and LREE, depletion in HREE and negative P and Ti anomalies in normalized multi-element diagrams indicating a possible subduction-related origin. Biotite schists were originally lithic arenites derived from mixed felsic and basic source rocks from NE Congo Craton in northern Uganda and deposited in an oceanic island tectonic setting sometime after 920 Maas inferred from geochemical fingerprinting. The granitic gneisses and schists all show evidence of high-temperature peak metamorphism (M1) at 633±17 Ma, attributed to continent-continent collision between the Congo Craton and the Saharan Metacraton. This tectono-metamorphic event was followed by retrograde metamorphism at 555 ± 3 Ma due to cooling (M2) and subsequent crystal-plastic deformation (D1) linked to the ductile-brittle deformation stage of the Aswa Shear Zone. The above data reveal a strong similarity in the geodynamic evolution between the Basement Complex in South Sudan and the Western Mozambique Belt in Tanzania, corroborating earlier speculations regarding the extension of the Mozambique Belt into South Sudan

Description

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science, School of Geosciences, 2020

Keywords

Citation

Collections

Endorsement

Review

Supplemented By

Referenced By