The physical volcanology and geochemistry of the Nsuze group, Pongola supergroup, of northern KwaZulu-Natal and southeastern Mpumalanga.

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.