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M. Knoper¹, Lewis D. Ashwal¹, and M.A.G. Andreoli²
¹Dept. of Geology,
Rand Afrikaans University, P.O. Box 524,
Auckland Park 2006, South Africa
²Atomic Energy Coporation, P.O. Box 582,
Pretoria 0001, South Africa; and
Schonland Research Centre, BPI Building,
University of the Witswatersrand, P.O. Box 3,
Wits 2050, South Africa
Steenkampskraal, located in southwestern Namaqualand, South Africa, is well known for a monazite orebody situated within polydeformed Mesoproterozoic peraluminous granulite gneisses (Table 1.). Recent mapping of Steenkampskraal at a scale of 1:2500 indicates that the gneissic fabrics are composite and resulted from progressive or episodic and spatially variable deformation. U-Pb age determinations on monazite and zircon from granitic-gneisses, leucosomes, and from transgressive, penecontemporaneous intrusions (associated with the monazite orebody) are required to bracket deformation at Steenkampskraal and thereby develop a well-constrained structural-intrusive scheme. The ion microprobe (SHRIMP) is the best technique for dating the complexly-zoned monazite and zircon occuring in the Steenkampskraal granitic-gneisses and intrusions. The SHRIMP technique can also date monazite and zircon "in-situ" within a single thin section fragment, thereby providing the opportunity to further constrain deformation at the microfabric scale. Implementing "in-situ" dating is not always practical because U(Th)-bearing minerals typically occur in most rocks at low abundances. But since Steenkampskraal contains high abundances of accessory U(Th)-bearing mineral phases in virtually all rock types, using an "in-situ" method of dating will bracket deformation at a much finer scale. Thin sections will be made from paleomag drill cores of precisely sampled rock types at Steenkampskraal, including difficult-to-sample thin intrusive bodies. A well-bracketed structural-intrusive scheme will highlight similarities and/or differences of deformation at Steenkampskraal with other parts of the Namqualand Metamorphic Complex. Because the Steenkampskraal monazite orebody appears to postdate an earlier phase of deformation, yet has been deformed along with the (granulite) granitic-gneisses during later deformation, precisely understanding structural-intrusive relationships will help target extensions to the monazite orebody.
| Rock Unit | Structural Features | Ages |
| Namaqualand Metamorphic Complex | ||
| minor paragneisses, leucosomes, megacrystic gneiss, leucogneiss, minor banded gneiss, garnet-bearing quartzofeldspathic leucosomes | gneissic foliation striking NE-SW to E-W, regionally dipping 20-30° N | Mesoproterozoic (?) |
| monazite orebody, granodiorite, intermediate- to mafic-composition intrusive suite, minor charnockitic veins | cuspate E-W folding, steep E-W striking cleavage transposes earlier gneissic fabrics, transgressive, penecontemporaneous intrusive relationships | Mesoproterozoic (?) intrusive suite: 1278-1029 Ma NdCHUR monazite ore: 1150±15 Ma U-Pb whole rock 1140 Ma NdCHUR |
| pegmatitic veins | 2-5mm thick, subvertical with a 130° strike | Mesoproterozoic (?) |
| Nama Group, Kuibis Formation | ||
| quartzite, minor shale, arkosic 1m above erosional unconformity with underlying Namaqualand Metamorphic Complex | steep E-W striking faults, brittle reactivation of earlier gneissic fabrics | Neoproterozoic-Cambrian |
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in:
Namaqualand
·Web publication date: July 20, 1998·
"Rockscapes" 1996-2001 M Knoper
