Alkaline intrusives from the Tugela terrane, Natal metamorphic province.
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Date
1991
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Abstract
Three gneissose alkaline granitoid intrusives at Ngoye, Bulls Run and
Wangu are described. They are located within the Nkomo Nappe of the
Tugela Terrane, near the northern margin of the Natal Metamorphic
Province.
The Ngoye Complex comprises alkaline granites, with minor syenite and
monzodiorite phases. According to modal am geochemical criteria the
Ngoye granites range from peraluminous (muscovite-bearing), through
metaluminous (biotite- and/or hornblende-bearing), to peralkaline
(riebeckite-, aegirine- and magnetite-bearing). The granites are
A-types according to their modal and geochemical characteristics. Rb-Sr
isotopic data for the hornblende granites indicate an age of 1063 ± 17
Ma and the initial ratio (R๐ = 0.7025) provides evidence for
derivation from a mantle source. Plotting of the Ngoye geochemistry on
tectonic-discrimination diagrams suggests intrusion into rifted
continental crust. It is concluded that the gneissose Ngoye granites
constitute a deformed central complex, similar to anorogenic complexes
in Nigeria and the Sudan.
The Bulls Run Complex is situated 30 km west of the Ngoye Complex. A
concentric outcrop pattern has been mapped, according to which an
envelope of silica-saturated biotite-muscovite syenite surrounds a core
of nepheline-bearing syenites. Minor intrusive phases include
biotite-rich dykes, sovite carbonatite sheets, silica-oversaturated
microsyenite dykes and feldspathic ijolite. The outer envelope of
muscovite-rich syenite is interpreted as fenitised pelitic country
rock. An alkali-lamprophyre origin is suggested for the biotite-rich
dykes. Geochemically the syenites are predominantly miaskitic, apart
from the microsyenite dykes which are mildly peralkaline. Rb-Sr
isotopic data for the nepheline syenites indicate an age of 1138 ± 45 Ma
(Ro = 0.70322). Carbonate separates from the carbonatites provide a
similar low initial ratio (Ra = 0.70319) which supports a comagmatic
mantle origin. A comparison is drawn between the Bulls Run Complex and
miaskitic nepheline syenite gneisses in the mid-Proterozoic Grenville
Province of canada. From this, it is suggested that the Bulls Run
Complex is pretectonic and was intruded into the rifted passive margin
of a continent.
The Wangu Granite Gneiss is situated 3 km southwest of the Bulls Run
Complex. The granites are fine grained and contain aegirine-augite
and/or magnetite, and classify as alkali-feldspar granite. Peralkaline
chemistry is characteristic of the Wangu granites, with trace-elenent
contents indicating a distinct A-type signature. Biotite-rich mafic
dykes intrude the southern part of the Wangu outcrop and, on the basis
of major- and trace-element signatures, are suggested to be
metamorphosed volatile-rich alkaline lamprophyres similar to those at
Bulls Run. Geochemical similarities between the Wangu granites and
certain comendites from the Kenya Rift are noted. It is suggested that
the Wangu granites were emplaced as high-level dykes, within rifted
continental crust.
It is proposed that the Ngoye, Bulls Run and Wangu intrusives be united
as the Nkwaleni Suite.
Comparison of the Tugela Terrane with the Grenville Province reveals
many similarities, particularly their mid- to late-Proterozoic age and
the occurrence of pre-tectonic anorogenic continental magmatism. It is
concluded that, unlike the current model which would have the Tugela
Terrane as obducted ophiolite, these new data indicate that the Tugela
Terrane is a metamorphosed continental rift system.
Description
Thesis (Ph.D.)-University of Durban-Westville, 1991.
Keywords
Alkalic igneous rocks., Geology., Theses--Geology.