3 scogings 1990 ngoye complex

CATALOGUE OF SOUTH AFRICANLITHOSTRATIGRAPHIC UNITSEditor: M.R. JohnsonSA Committee for

[Mokolian] NGOYE COMPLEX

A.J. Scogingsl

1st edition: 1990

1. PROPOSER OF NAMEMcGarthy (1961) proposed the name "Ngoye Granite Gneiss."

2. DERIVATION OF NAMENgoye Hills southwest of Empangeni, Natal (Fig. 1), which largely

consist of rocks of the Ngoye Complex.

3. TYPE AREATerrain between Ngoye Forest Reserve and Mhlatuze River

(Fis. 1).

4. STRATIGRAPHIC POSITION AND AGESituated 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, GEOLOGICDESCRIPTIONBasic concept and unifying features: Geochemically related

alkaline to peralkaline A-type granitoid gneisses, with minormcnzodioritic and syenitic gneiss phases.

Form and size of intrusion: Prominent easterlytrendingwhaleback massif 30 km in length and up to 4 km wide, rising to300 m above the surrounding terrane. A smaller 3-km-long satellitebody is poorly exposed around Ninians, northeast of the mainmassif (Fig. 1), Apophyses of microgranitc gneiss extend into thecountry rocks along the southern margin of the main massif.

Lithology: Three broad varieties ol granitoid gneiss arerecognized, on the basis of alumina saturation relative to alkalis (fordistribution see Fig. 1).

Metaluminous gneisses (65%): Can be mineralogically subdividedinto three types: (i) Biotite granite gneiss. Grey to pinkish, with pinkmicrocline megacrysts (25-36%) set in a matrix of quartz (23-33%),plagioclase (26-34"A) and biotite (4-77"). Accessory phases includehastingsite, epidote, sphene and allanite. (ii) Biotite-hornblendegranite 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 commonaccessory minerals. (iii) Hornblende monzodiorite gneiss. Coarse-grained; white with black hornblende aggregates; comprisesplagioclase (6G-70%), m icrocl ine (1 O-20"t"), horn blende (4-87"), andcalcite (<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%), andaccessory 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 otbiotite, 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; consistsof microcline (45-55%), albite (10%), quartz (35-40%), withmagnetite (<sz) as the diagnostic mafic phase. Zircon and lluoriteare present in trace amounts. Minor variants within the microgranitegneiss include coarse-grained lenses rich in quartz, magnetite,zircon, sphene and fergusonite, in addition 10 syenitic gneissesconsisting mainly of microcline (6G-70"/"), albite (20-25"/.), quartz(<5ol.) and chlorite (5-10"/").

Geochemistry: The Ngoye granitoid gneisses display a widerange of SiO, contents (65-77y") and are generally very depleted inAl2O3, MgO, DaO, Ba and Sr, but enriched in Nar0, KrO,Zr,Zn,Nband REE relative to average granites (Scogings, 1985; 1986).Although subeconomic, the quariz-magnetite lenses within themagnetite 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 andchemical characteristics typical of A{ype late stage "granite centralcomplexes" according to ihe parameters suggested by Whalen ef a/.(1987). Comparison of the Ngoye Complex with the "youngergranite" complexes of Nigeria, Saudi Arabia and the Sudan revealsmarked similarities with regard to rock types, size ol complex,geochemistry and economically important elements (Scogings,1985). By analogy, therefore, it probably represents ametamorphosed and structurally deformed granite central complex.

Other aspects: The Ngoye Complex has a pervasive east-strikingplanar structural fabric, with steep southerly dips along its southernmargins and moderate to steep northerly dips along the northernflanks of the massif.

Fi1. 1. Distribution of the Ngoye Complex and its constituent rcck types.

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