The geology and geochemistry of the volcanic rocks of the Pongola sequence in southern Swaziland.
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Date
1993
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Abstract
The ~3.0 Ga Pongola Sequence, comprising a lower dominantly volcanic Nsuze Group and
an upper largely sedimentary Mozaan Group, crops out in the Mahlangatsha and Kubuta
areas of southern Swaziland. The Nsuze Group consists of basaltic, andesitic, dacitic and
rhyolitic rocks with intercalations of ferruginous shale and pyrophyllitic schists. The Mozaan
Group comprises quartzites, ferruginous shales, basalts and minor amounts of andalusite and
sericitic schists. In the study area in southern Swaziland the Pongola Sequence is represented by a northerly
striking lens of metavolcanic basaltic rocks extending southwards to the Ngwavuma River
valley. These lavas comprise basalts, basaltic andesites and very minor rhyolites that are
amygdaloidal and vesicular in places. Hunter (1952) tentatively correlated these metabasaltic
rocks with the Nsuze Group but the geochemistry indicates that an upper Mozaan correlation
is more likely. In the study area four stages of deformation have been deduced: a cleavage development
(D1); low angle thrusting and bedding-parallel thrust faulting (D2); normal/oblique slip
faulting (D3) and fracturing/jointing (D4). There has been duplication of strata by thrusting
and normal faulting. Absence of marker beds prevents the determination of the degree of
duplication. It is clear from the geochemical analysis that there are two broad groups of data from the
suite, one from the Sigwe Hills in the north and the other from south of the Ngwavuma
River. The samples from south of the Ngwavuma River are enriched in TiO2, AI2O3, CaO,
Cr, Zr and Nb compared to the samples from Sigwe Hills. These volcanic rocks are tholeiitic
in nature and indicate a within plate continental setting.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 1993.
Keywords
Geology--Swaziland., Theses--Geology.