FECHA: MOVIBLE CARNAVALES

This section lists the petrographic features of the main litho-types present in the cover and the basement. Sub-sections list, in table form, the assemblages and textures found in thin section. Visual investigation of thin sections showed that the coarse grain size and heterogeneity of the samples precluded meaningful quantitave estimation of phases present. Instead, qualitative estimation of the relative abundance of phases are listed in decreasing order (e.g. Qtz+Kfs+ etc.).

Four cover rock categories are recognised, these are:

1) pelites and semi-pelites; 2) arkosites and psammites; 3) calc-silicates and marbles; 4) amphibolite and amphibolites.

Generally there are fewer lithologies in the basement i.e. the Narubis Granitoid Complex (Brandt 1987) than in the cover. Three categories were recognised:

1) Semi-pelites (uncommon)

2) Quartz-feldspathic gneisses

3) Amphibolites

4.2.1 Petrology of the Basement rocks in the Namibfontein area

A summary of the assemblages and textures of four representative samples of basement rocks in the Namibfontein area are shown in Table 4.1. Samples have been selected from the twin domes and the Valencia dome. The main textural feature is an inequigranular-interlobate arrangment of matrix mineral grains, typically amoeboid and sutured. Quartz and feldspar are recrystallised forming a ~3-4mm in diameter mosaic while aligned biotites define a weak fabric. In samples from the high strain zone however, (see Section 3.9) the general texture departs from perfect 120° contacts and instead a discrete foliation has developed. It should be noted however, that thinsection cut is important when considering textures. Most sections were cut in the x-z plane to show the extent of strain fabric development, sections cut in the y-z plane on the other hand appear more granoblastic.

124 N18.4 Qtz + Kfs + Bt + Sil .basement raft

Texture Interlobate quartz and microcline form an inequigranular-amoeboid migmatitic texture. Corroded biotite with lobate edges and quartz blebs. is overgrown by sillimanite.

GR[O7O,O88]

NP43.15 Qtz+Bt+Kfs+Pl+Grt+Mz basement semi-pelite Texture A granoblastic coarse gneiss. Garnet is corroded and

appears to reacting with potassium feldspar evident from a quartz rim and embayed texture between the two. Inclusions of zircon and monazite are found in garnet and biotite. Euhedral zircons have a corroded growth rim seen with EMP.

GR[034,098]

NP44.11 Kfs+Qtz+Bt+FeTiO+Pl basement augen gneiss Texture Coarse granoblastic schist with aligned biotite.

Quartz and microcline and perthitic orthoclase have interlobate grain contacts. Bt is corroded. FeTiO oxides are pinkish-grey in reflected light with grey exolution lamaellae(Mag-Ilm).

GR[066,105]

NT38.17 Kfs+Pl+Bt+Qtz basement augen gneiss Fabric is granoblastic. Large microcline augens

define a linear fabric. Microcline has large

oligoclase/andesine patches, orthoclase is perthitic. Biotite is weakly aligned. Plagioclase grains have many small quartz blebs..

GR[039,014]

NT43.4 Mc+Qtz+Sil+Bt+Crd+Mag basement gneiss Texture Migmatitic interlobate texture. Fibrolite overgrows

all phases forming bow-tie structures. Twinning in microcline bends.

GR[036,089]

Table 4.1 Summary of assemblages and textures of selected samples from the basement in the Namibfontein area.

4.2.2 Petrology of basement rocks in the Khan Mine area

The textural and phase relations of four samples of basement gneisses from the Khan Mine area are shown in Table 4.2. Acidic gneisses are characterised by the general assemblage Kfs+Pl+Qtz+Bt±Mag while more basic gneisses are characterised by the assemblage Qtz+Pl+Mc+Hbl+Bt+FeTiO±Chl. In the basic gneisses amphibole is hornblendic, its content varies between 10-15%. Feldspar is mainly Kfs constitues -30% of most acidic gneisses and form elongate augens. Perthitic and antiperthitic textures are common.

KP21.6 Kfs > Pl + Qtz +Bt augen gneiss

Texture A inequigranular polygonal texture. Bt envelopes opaques. Opaques are zoned. Inner zone as Magnetite with exolution lamellae with a grey corona. Biotites contain zircons and apatites.

GR[024,042]

KP27.19 Qtz > Pl + Hbl + Bt (Chl)+ Fe Ti O (Chi)

Texture Granoblastic-lobate textures. Hornblende grains large (1­ 3mm) within the matrix.

GR[052,022]

KP42.19 Pl+Qtz+B t+Ch l+(Sphene) mylonite

Finely banded and mylonitic texture, partly annealed. (directly below Opaques are elongate fish structures. Opaque (magnetite) cover)

enclosed by sphene. GR[060,0261

:4 126 i

4.2.3 Petrology of cover pelites in the Namibfontein area

The textural and phase relations of twelve representative pelitic samples from the Namibfontein area are shown in Table 4.3. The majority of pelitic samples were collected from the Kuiseb Schist. The general assemblage observed is Qtz+Bt+Pl+Kfs±Crd±Grt. Two frequently coexisting porphyroblastic minerals occur, these arel) ferro-cordierite (comprising -20% of most samples) and 2) garnet (< 10%). All garnet analysed is almandine-rich with minor proportions of grossular, spessartine and pyrope (see Appendix 4.1).

1) Figure 4.2 B&E shows examples of cordierite in thin section. It is commonly enveloped by a well defined external fabric (Si) and contains straight inclusion fabrics of quartz and biotite. Porphyroblasts in Figure 4.2B exhibit G-type relationships with aysmmetrical quartz- biotite pressure solution shadows (Passchier & Simpson 1986). This relationship is not common in all samples but clearly shows the growth of cordierite is syntectonic and involved some kinematic rotation. According to Nash (1971) who studied the SJ area and Nex (1997) who studied the Goniakontes area potassium feldspar also grew simultaneously with cordierite as both are partly 'rotated' within matrix biotite folia and contain similar inclusions. This is consistent with the observations in this study.

2) Garnet porphyroblastesis on the other hand is somewhat ambiguous. This is partly because the original matrix-porphyroblast relationship is obscured by coarse gnessose fabrics and because garnet is observed in two textural modes i) syn-tectonic and ii) post-tectonic:

i) In some thin sections post-tectonic growth is demonstrated, in Figure 4.2 A for example, it is apparent that garnet has overgrown biotite that is both aligned and in continuity with the external fabric (SJ.

ii) Syntectonic growth, however, is more common. In some sections (e.g. N10.1 Plate 4.1) garnet is enveloped by biotite and quartz, clearly diagnostic of coeval matrix-porphyroblast development. In section NT34.1 garnet, cordierite and biotite form a semplectite texture.(Plate 4.IB), suggesting growth was coeval with syn-tectonic cordierite. Furthermore, in section NP28.14, both garnet and cordierite exhibit G-type relationships suggesting syntectonic growth.

Sillimanite is the only A^SiOs polymorph observed and appears only rarely (e.g. sample (N34.7c, N34.1 & N3.2) as inclusions in cordierite and less commonly, in garnet and quartz. Two varieties of sillimanite have been found. Prismatic and fibrolitic sillimanite is found in the cores of cordierite oriented parallel to the main folia and constitutes -2% of those samples. In section NT34.7c it is notable that matrix prismatic sillimanite appears to be

replacing biotite parallel to the 001 cleavage trace i.e. biotite appears to have been consumed at the expense of sillimanite.

Accessory phases are numerous in pelites. Euhedral zircon clusters were seen in biotites in sample N9.4 displaying radiation halos. Phosphatic phases are common notably (0.5mm) apatite and small size monazite crystals. Semi-quantative EMP analysis showed that xenotime (YPO4) is a widespread accessory mineral. In sample NP 34.7c clean non-inherited euhedral zircons are found with monazite and anhedral ilmenite within a single biotite grain whilst parallel monazite and sillimanite inclusions are found as inclusions in cordierite and garnet. Curiously, monazite in cordierite exhibits radioactive halos whilst in garnet these are not seen. Secondary alteration of cordierite and feldspar to sericite is frequently observed. Hematite and ilmenite usually occur as inclusions in biotite. In sample NP9.2 accessory Fe-tourmaline and flourite was observed. Steven & Moores (1995) reported abundant tourmalinite mineralization in the Kuiseb Formation 30km west of Omaruru, they attribute this to an abundance of diagenetic boron-rich fluids.

5s

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(Opposite) Figure 4.2 (A-E)

A. N3.2 Small (~lmm) post Sj (syn S2?) garnet in an annealed fabric. Note that biotite folia are cut by garnet.

B. N9.4 a type Syn-tectonic cordierite (top left and right) and quartz domains. Note