5 SCOGINGS 1991 WANGU GRANITE GNEISS

CATALOGUE OF SOUTH AFRICAN LITHOSTRATIGRAPHIC UNITS Editor: M.R. Johnson SA Committee for StratigraPhy [Mokolian] WANGU GRANITE GNEISS A.J. Scogingsl 1st Edition: 1991 1. PROPOSER OF NAME, Scogings (1 989 - "Wangu granitoid gneisses"); modified by author (this publication). 2. DERIVATION OF NAME Wangu Hill, 20 km NW of Eshowe, Natal (Fig. 1). 3, WPE AREAS Wangu Hill to the Mvuzane River 1,5 km to the southwest of Wangu Hill (Fig. 1). 4. STRATIGRAPHIC POSITION AND AGE Situated within the Nkomo Nappe near the northern margin of the c. 11OO Ma Natal Metamorphic Province. No age determinations available. 5. GEOLOGICAL DESCRIPTION Basic concept and unifying features: Predominantly alkaline/ peralkaline granite gneisses, with very minor alkaline mafic intrusive phases. Form and size of intrusion: Elongate easterly-trending body 6 km long and up to 1 km wide. Three-dimensional form unknown. Llthology: Three varieties of gneiss are recognised on the basis ol modal mineralogy, two being granitic and the third mafic. Magnetite granite gneiss (78"k): Fine-grained, light pink to pink. Consists of quartz (25-40'/"1, microcline (20-35%), albite (25-45"/o) and magnetite (3-8%). Biotite and fluorite are accessory phases. OAP classif ication (Streckeisen, 1976): alkali-feldspar granite (Fig. 2). Aegirine granite gnelss (20"6): Fine-grained, light pinkish to white. Consists oI quartz (25-50%), microcline (10-40%), albite (20-50%), magnetite (< 10%) and aegirine (< 15%). Classified as alkali-feldspar granite on the QAP diagram (Fig. 2). Mafic gneiss (2%): Occurs as sheets up to 30 m long and 1 m thick, containing xenoliths ol peralkaline granitoid gneiss implying an intrusiva relationship. Fine-grained; grey to dark grey, with glistening appearance in hand specimen due to high biotite content. Comprises biotite (45-65%), albite (20-40"/"1 microcline (5-10%), calcite (5-10%) and apatite (<5%). Accessory minerals are opaque oxides, pyrite and sphene. Classilied as alkali-feldspar syenite on the QAP diagram (Fig. 2). Geochemistry: Aegirine granite gneiss is peralkaline (Na+ K/Al = 1,Ol-1,33) and silicaoversaturated (SiOr: 67-79%). Scogings (1989) reported low levels of MgO, GaO, Ba aid Sr, elevated Ga/Al ratios and enrichment in incompatible trace elements such as Zn (123-612 ppm), Zr (4s0-1836 ppm), Y (95-159 ppm) and Nb (89-423 ppm). These are characteristic of peralkaline granite chemistry as defined by Whalen ef a/. (1987). Magnetite granite gneiss is metaluminous to almost peralkaline (Na+K/Al = 0,94-0,99) and silica oversaturated (SiOr: 67-78%). Major and trace element chemistry very similar to the aegirine' granite gneisses. Mafic gneiss has low SiO2 contents (c. a0-50%) with relatively Fi1. 1 Distribution of the Wangu Granite Gneiss and location of its type area and type localities. [3-4e]

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Transcript of 5 SCOGINGS 1991 WANGU GRANITE GNEISS

Page 1: 5 SCOGINGS 1991 WANGU GRANITE GNEISS

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

[Mokolian] WANGU GRANITE GNEISS

A.J. Scogingsl

1st Edition: 1991

1. PROPOSER OF NAME,Scogings (1 989 - "Wangu granitoid gneisses"); modified by author

(this publication).

2. DERIVATION OF NAMEWangu Hill, 20 km NW of Eshowe, Natal (Fig. 1).

3, WPE AREASWangu Hill to the Mvuzane River 1,5 km to the southwest of

Wangu Hill (Fig. 1).

4. STRATIGRAPHIC POSITION AND AGESituated within the Nkomo Nappe near the northern margin of the

c. 11OO Ma Natal Metamorphic Province. No age determinationsavailable.

5. GEOLOGICAL DESCRIPTIONBasic concept and unifying features: Predominantly alkaline/

peralkaline granite gneisses, with very minor alkaline mafic intrusivephases.

Form and size of intrusion: Elongate easterly-trending body 6 kmlong and up to 1 km wide. Three-dimensional form unknown.

Llthology: Three varieties of gneiss are recognised on the basisol modal mineralogy, two being granitic and the third mafic.

Magnetite granite gneiss (78"k): Fine-grained, light pink to pink.Consists of quartz (25-40'/"1, microcline (20-35%), albite (25-45"/o)

and magnetite (3-8%). Biotite and fluorite are accessory phases.OAP classif ication (Streckeisen, 1976): alkali-feldspar granite (Fig. 2).

Aegirine granite gnelss (20"6): Fine-grained, light pinkish to white.Consists oI quartz (25-50%), microcline (10-40%), albite (20-50%),magnetite (< 10%) and aegirine (< 15%). Classified as alkali-feldspargranite on the QAP diagram (Fig. 2).

Mafic gneiss (2%): Occurs as sheets up to 30 m long and 1 mthick, containing xenoliths ol peralkaline granitoid gneiss implyingan intrusiva relationship. Fine-grained; grey to dark grey, withglistening appearance in hand specimen due to high biotite content.Comprises biotite (45-65%), albite (20-40"/"1 microcline (5-10%),calcite (5-10%) and apatite (<5%). Accessory minerals are opaqueoxides, pyrite and sphene. Classilied as alkali-feldspar syenite on theQAP diagram (Fig. 2).

Geochemistry: Aegirine granite gneiss is peralkaline (Na+ K/Al =1,Ol-1,33) and silicaoversaturated (SiOr: 67-79%). Scogings (1989)

reported low levels of MgO, GaO, Ba aid Sr, elevated Ga/Al ratiosand enrichment in incompatible trace elements such as Zn (123-612ppm), Zr (4s0-1836 ppm), Y (95-159 ppm) and Nb (89-423 ppm).These are characteristic of peralkaline granite chemistry as definedby Whalen ef a/. (1987).

Magnetite granite gneiss is metaluminous to almost peralkaline(Na+K/Al = 0,94-0,99) and silica oversaturated (SiOr: 67-78%).Major and trace element chemistry very similar to the aegirine'granite gneisses.

Mafic gneiss has low SiO2 contents (c. a0-50%) with relatively

Fi1. 1 Distribution of the Wangu Granite Gneiss and location of its type area and type localities.

[3-4e]