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

[Mokotian] BULLS RUN COMPLEX

A.J. Scogings'

1st Edition: 1991

1. PROPOSER OF NAMENicolaysen and Burger (1965 - "Bulls Bun Syenitic Gneiss");

revised by present author (this publication).

2, DERIVATION OF NAMEThe farm Bulls Run Estate 12987, 17 km NNW of Eshowe, Natal.

3. TYPE AREANortheast of Goedertrou Dam (Fig. 1).

4. STRATIGRAPHIC POSITION ANO AGELocated within the Nkomo Nappe of the Tugela Group, near the

northern margin of the Natal Metamorphic Province (Charlesworth,1981). lntrusive into the Tugela Group. Nicolaysen and Burger (1965)

obtained a U-Pb age ol 1140 ! 35 Ma from a single zircon. Morerecent work (author's unpublished whole-rock Rb-Sr data) indicatesan age ol 1 138 I 45 Ma for the nepheline syenite gneiss.

5. GEOLOGICAL DESCRIPTIONBasic concept and unifying features: Geochemically related

undersaturated to saturated alkaline/peralkaline syenitic and minorcarbonatitic gneisses.

Form and size of body: Prominent easterly-trending ridge 15 kmlong and up to 1,5 km wide, rising some 400 m above the MhlatuzeRiver (Fig. 1).

Lithology: A broad subdivision into three ma,ior syenitic gneisstypes (muscovite syenite, nepheline syenite and albite syenitegneiss) is proposed for the bulk of the complex (Fig.1). Three minorrock types (carbonatite, peralkaline microsyenite and alkaline maficgneiss) occur as dyke- or sheet-like bodies in the muscovite syenitegneiss (Scogings and Forster, 19Bg).

Muscovite syenite gneiss (50%): Medium-grained; white toyellowish grey. Consists of muscovite (5-35%), microcline (2C-357.),albite (30-40%) and biotite (<10ol.); calcite, apatite and sphene areaccessory minerals.

Nepheline syenite gneiss (30%): Medium- to coarse-grained;blue-grey to dark grey. Comprises nepheline (5;40"/"), microcline(3G-50%), albite (5-30%) and biotite (2-15y"). Cancrinite, calcite,apatite and ilmenite are characteristic accessories. Nepheline syeniiegneisses within the eastern portion of Goedertrow contain aegirineaugite (5-20%) and are more mafic than those on Black Eyes andBulls Bun; they can be classified as feldspathic ijolite'

Aibite syenib gneiss (20%): Fine-grained; light grey to white; has

siliceous appearance in outcrop. Composed of albite (45-65%),microcline (25-451"), nepheline (<25'/"\ and biotite (2-7%1.

Accessory minerals include zircon, pyrochlore, cancrinite andsericite; nepheline is commonly kaolinized.

Carbonatite gnerss (<1%): Medium-grained; white to brown,Consists of calcite (6H6"6), alkali feldspars (<20%), biotite (3-25"6), apatite (<10%). Accessory minerals are ilmenite, pyrochloreand zircon.

Peralkaline microsyenite gneiss (<1%): Fine-grained; white toblue-green. Consists ot albite (60-70"/"), microcline (10-20'6), quartz(2-6%) and aegirine-augite (2-5"/"). Calcite and riebeckite occur inaccessory amounts.

Alkaline mafic gneiss (<1"6): Fine-grained; dark grey to black.Consists oI biotite (2G-50%), albite (35-50%), microcline (2-15%),calcite (8-12ol") and apatite (<3%).

Geochemistry: Muscovite syenite gneisses are silica-saturated(SiO,: 52-59%), peraluminous and have moderate alkali contents(Nap+tlO : 10-13%), with [O generally in excess of NarO(Na,O/GO c. 0,75). Nepheline syenite gneisses are undersaturated(SiO,: 48--58"/"; normative nepheline: 10-30"/5), metaluminous toperaluminous and have elevated alkali contents (Na.O+QO :

12'15"/0). Potassic and silica+ich in central and western outcrops(Na"O/K"O c. 0,75) but more sodic and undersaturated in the east(Na,O/X,O c. 1,25), Albite syenite gneisses have the highest silicaconients-of the major lithologies (SiOr: 57-65"/") and due to theirsodic character (NarO/KrO : 1-2,5) are moderately undersaturated(normative nepheli-ne: i}-z}y"). FeO (<2"/") and CaO (<17")contents are low. Enrichment in trace elements such as Z;(7OO-2000 ppm) and Nb (20G-16500 ppm) is diagnostic. Carbonatitegneisses have typical sovite chemistry: CaO (30-55%), MgO (c.

0,5olo), P2Ou (<6%), Sr (6000-90C0 ppm), Nb (s|{000 ppm).Peralkaline microsyenite gneisses are silica-saturated (SiOr: 6U7Oo/",normative quad.z: 2-12"/o), sodic (NarO/KrO c. 2) and mtldlyperalkaline (Na+K/Al c. 1,02). Alkaline-mafi-c gneisses have highalkali contents (NarO + QO: 8-1 1%) for relatively low SiO, (45-50"6),trace elements Zn,l.lb, 27, Rb, Sr, Ba, F, CO, and LREE are enrichedand typical of nephelinite or al,faline lamprophyre.

Genesis: The Bulls Run gneisses have many of the mineralogicaland geochemical characteristics typical of nepheline syenite/ijolitecarbonatite central complex magmatism. Nepheline syenitecomplexes are generally intruded into anorogenic or riftedconlinental settings, but minor occurrences have also been noted inwithin-plate oceanic settings. By analogy with undeformed complex-

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F8. 1 Distribution of the Bulls Run Complex and location of its type area.

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