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Physical volcanology and compositions of the basaltic lavas in the Archean Nzuse Group, White Mfolozi inlier, South Africa

By
Allan H. Wilson
Allan H. Wilson
1
School of Geological Sciences, University of KwaZulu-Natal, Durban 4041, South Africa
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Claire E. Grant
Claire E. Grant
1
School of Geological Sciences, University of KwaZulu-Natal, Durban 4041, South Africa
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Published:
January 01, 2006

The Mesoarchean Pongola Supergroup is a thick volcano-sedimentary succession that is unique in the early record of Earth's history because of the nature and composition of the lava flows that, in addition to basalts, have a high proportion of intermediate and acid compositions. The succession contrasts with other contemporaneous volcanic deposits in southern Africa and in other parts of the world that gave rise to greenstone belts by having a low state of deformation and a uniformly low grade of metamorphism.

The White Mfolozi inlier is a deeply eroded river section that exposes spectacular structures and textures of two sequences of volcanic rocks, the lower Nhlebela Formation and the upper Agatha Formation. The Nhlebela Formation contains pillowed successions, peperite-like breccias, and extensive fragmental deposits formed as a result of hydroclastic eruption. The shallow water hydromagmatic deposits developed into amygdaloidal pahoehoe units higher in the succession as the lava eruptions raised the ground level. The Agatha Formation, studied in two traverses, commenced with the formation of a hydroclastic tuff formed by the emplacement of a high-Mg basalt magma interacting with unconsolidated sediment. Subaqueous eruptions rapidly progressed into subaerial deposits consisting initially of aa lavas that then developed into pahoehoe lavas higher in the succession.

The pahoehoe lavas in the Agatha Formation preserve spectacular ropy surfaces. The internal structures of the flow lobes and distribution of vesicles indicate that lava inflation was the dominant mechanism of emplacement. Therefore important processes that are observed in modern and recent subaerial and continental basalts also operated in the mid-Archean.

Changes in structures and styles of the pahoehoe lava flow lobes can be linked to magma compositions. Compositions include basalts, Fe-rich basalts, siliceous Mg basalts, andesites, and dacites. Trace elements identify at least four flows made up of 75 flow lobes that make up the 210-m-thick Agatha Formation. Geochemical tectonic discrimination diagrams fail to resolve the tectonic setting because of crustal contamination, but field observations indicate it is likely that the depositional setting in this area was a subsiding continental margin.

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Contents

GSA Special Papers

Processes on the Early Earth

Wolf Uwe Reimold
Wolf Uwe Reimold
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Roger L. Gibson
Roger L. Gibson
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Geological Society of America
Volume
405
ISBN print:
9780813724058
Publication date:
January 01, 2006

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