1. CATALOGUE OF SOUTH AFRICAN
LITHOSTRATIGRAPHIC UNITS
Editor: M.R. Johnson
SA Committee for
[Mokolian] NGOYE COMPLEX
A.J. Scogingsl
1st edition: 1990
1. PROPOSER OF NAME
McGarthy (1961) proposed the name "Ngoye Granite Gneiss."
2. DERIVATION OF NAME
Ngoye Hills southwest of Empangeni, Natal (Fig. 1), which largely
consist of rocks of the Ngoye Complex.
3. TYPE AREA
Terrain between Ngoye Forest Reserve and Mhlatuze River
(Fis. 1).
4. STRATIGRAPHIC POSITION AND AGE
Situated within the Nkomo Nappe flugela Group) near the
northern margin of the Natal Structural and Metamorphic Province
(Charlesworth, 1981), Dated by Barton (1983) at 1067 ! 20 Ma (Rb-
Sr whole+ock isochron).
5, GEOLOGICDESCRIPTION
Basic concept and unifying features: Geochemically related
alkaline to peralkaline A-type granitoid gneisses, with minor
mcnzodioritic and syenitic gneiss phases.
Form and size of intrusion: Prominent easterlytrending
whaleback massif 30 km in length and up to 4 km wide, rising to
300 m above the surrounding terrane. A smaller 3-km-long satellite
body is poorly exposed around Ninians, northeast of the main
massif (Fig. 1), Apophyses of microgranitc gneiss extend into the
country rocks along the southern margin of the main massif.
Lithology: Three broad varieties ol granitoid gneiss are
recognized, on the basis of alumina saturation relative to alkalis (for
distribution see Fig. 1).
Metaluminous gneisses (65%): Can be mineralogically subdivided
into three types: (i) Biotite granite gneiss. Grey to pinkish, with pink
microcline megacrysts (25-36%) set in a matrix of quartz (23-33%),
plagioclase (26-34"A) and biotite (4-77"). Accessory phases include
hastingsite, epidote, sphene and allanite. (ii) Biotite-hornblende
granite gneiss. Light pinkish; medium-grained; comprises quartz
(23-35'6), microcl i ne (30-40%), plag ioclase (20-30%), horn blende
(3-8%) and biotite (<4%). Sphene and opaques are common
accessory minerals. (iii) Hornblende monzodiorite gneiss. Coarse-
grained; white with black hornblende aggregates; comprises
plagioclase (6G-70%), m icrocl ine (1 O-20"t"), horn blende (4-87"), and
calcite (<5%). Accessory constituents include quartz, diopside,
garnet, sphene, allanite and apatite.
Peraluminous gneiss (20%): Consists essentially of muscovite-
biotite granite gnelss: Light pinkish to white; fine- to medium-
grained; well foliated; comprises quartz (30-39%), microcline
(25-35%), albite (25-35%), biotite (<5%), muscovite (<4%), and
accessory garnet, opaques and fluorite.
Peralkaline gneisses (15"6): Two types have been recognized:
(il Riebeckite granite gneiss. Medium-grained; white; prominent
,oliation defined by blue-black riebeckite crystals or aggregates ot
biotite, riebeckite and aegirine. Composed of quartz (30-40"/),
microcline (30%), albite (25-35%), riebeckite (36%), biotite (34olo)
and aegirine (s5%). Common accessories include Iluorite and zircon.
(ii) Magnetite miuogranite gneiss. Fine-grained; light pink; consists
of microcline (45-55%), albite (10%), quartz (35-40%), with
magnetite (<sz) as the diagnostic mafic phase. Zircon and lluorite
are present in trace amounts. Minor variants within the microgranite
gneiss include coarse-grained lenses rich in quartz, magnetite,
zircon, sphene and fergusonite, in addition 10 syenitic gneisses
consisting mainly of microcline (6G-70"/"), albite (20-25"/.), quartz
(<5ol.) and chlorite (5-10"/").
Geochemistry: The Ngoye granitoid gneisses display a wide
range of SiO, contents (65-77y") and are generally very depleted in
Al2O3, MgO, DaO, Ba and Sr, but enriched in Nar0, KrO,Zr,Zn,Nb
and REE relative to average granites (Scogings, 1985; 1986).
Although subeconomic, the quariz-magnetite lenses within the
magnetite granite gneiss unit contain significant amounts of Zr
(0,3-5%), Nb (0,18-1 ,3%), Sn FO,2"/"), Y (0,024,48"/"), Zn
(0,024,23"/"1, U (0,01-{,1%), Th (0,01-{,3%) and REE (0,1-1 ,12"/").
Genesis: The Ngoye gneisses have many of the mineralogical and
chemical characteristics typical of A{ype late stage "granite central
complexes" according to ihe parameters suggested by Whalen ef a/.
(1987). Comparison of the Ngoye Complex with the "younger
granite" complexes of Nigeria, Saudi Arabia and the Sudan reveals
marked similarities with regard to rock types, size ol complex,
geochemistry and economically important elements (Scogings,
1985). By analogy, therefore, it probably represents a
metamorphosed and structurally deformed granite central complex.
Other aspects: The Ngoye Complex has a pervasive east-striking
planar structural fabric, with steep southerly dips along its southern
margins and moderate to steep northerly dips along the northern
flanks of the massif.
Fi1. 1. Distribution of the Ngoye Complex and its constituent rcck types.
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