3 SCOGINGS 1990 NGOYE COMPLEX
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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. 12-371
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Transcript of 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.
12-371
