The physical volcanology and geochemistry of the Nsuze group, Pongola supergroup, of northern KwaZulu-Natal and southeastern Mpumalanga.
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Date
2003
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Abstract
The Nsuze Group forms the lower,
predominantly volcanic succession of the
Pongola Supergroup. The 2.9Ga Nsuze Group
outcrops in southeastern Mpumalanga, northern
KwaZulu-Natal and Swaziland. The volcanic
rocks of the Nsuze Group are basalts, basaltic
andesites, andesites, dacites and rhyolites
preserved as both lava and pyroclastic deposits.
The oldest volcanic sequence of the Nsuze Group
is the basaltic Wagondrift Formation. The
younger Bivane Subgroup represents the main
volcanic component of the Nsuze Group. The
White River Section represents a complex
volcanic history of magma storage, fractionation,
and eruption, supplied by a multi-level system of
magma chambers. The basaltic and basaltic
andesite rocks of the White Mfolozi Inlier
represent the steady and non-violent eruption of
lavas from related volcanic centres. The Nsuze
Group rocks have been metamorphosed by high
heat flow burial metamorphism to lower
greenschist facies. Geochemically, elements
display well-defined fractionation trends, with
evident sub-trends within each phase group of
samples. These sub-trends are related to the
fractionation of key minerals, in particular
plagioclase. The REE patterns show that
evolution of magma was largely controlled by
the fractionation of plagioclase. All REE
patterns show LREE enrichment relative to the
HREE. The Wagondrift Formation was derived
from a more depleted source than the younger
Bivane Subgroup volcanic rocks and exhibits a
within-plate tectonic signature. The volcanic
rocks of the Bivane Subgroup in the White River
Section and the White Mfolozi Inlier are
geochemically similar. The volcanic rocks of the
Bivane Subgroup of both the White River
Section and the White Mfolozi Inlier have a
subduction zone tectonic signature, in particular
a Ta-Nb negative anomaly. Tectonic
discrimination diagrams suggest an enriched
source related to a continental-arc setting. The
geochemistry suggests an eclogitic source for the
Nsuze Group volcanic rocks. The formation of
eclogite in the mantle requires subduction of
basaltic material. Archaean models for
subduction-like processes include decoupling of
oceanic crust and subsequent underplating of the
continental lithosphere, and low-angle
subduction which minimises the effect of the
mantle wedge. It is possible that a combination
of these processes resulted in an enriched
eclogitic source for the magmas of the Nsuze
Group.
Description
Thesis (M.Sc.)-University of Natal, Durban, 2003.
Keywords
Volcanic ash, tuff, etc.--KwaZulu-Natal., Volcanic ash, tuff, etc.--Mpumalanga., Geochemistry--KwaZulu-Natal., Geochemistry--Mpumalanga., Theses--Geology.