Last year September (2010) I was part of a group hiking the Fish River Canyon. On the way to the canyon, driving north from Cape Town, three of us stopped off in Aussenkehr, just North of the Orange River. It is an area I have only ever driven past and a cursory Google search was unenlightening so I didn't really know what to expect. All I can say is 'what a nice surprise!'. The crags at Aussenkehr are part of the Karoo Large Igneous Province (LIP) with similar ages and geochemistry (Richardson, 1979 and 1984; Reid and Rex, 1994; Marsh, et al. 1997). The Karoo LIP covered and/or intruded most of Southern Africa and part of Antarctica, the Ferrar-Province, current estimates for the size of the LIP are around 3 000 000km2 (Jourdan, et al. 2007). Outcrops of the Karoo LIP can be found in Botswana, Malawi, Mozambique, Namibia, South Africa, Zamibia and Zimbabwe with unconfirmed occurrences in Angola (Jourdan, et al. 2007).
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| Location of Aussenkehr; the 'A' marks the spot. Image from Google Maps |
Much of the present surface expression of the Karoo LIP in Namibia is restricted to sills and dykes. The Aussenkehr rocks are part of the Tandjiesberg dolerite (diabase) sill complex, which has at least 55km of outcrop and is between 80 and 110m thick. The sill consists of two units, the upper and lower sill (Richardson, 1979 and 1984; Reid and Rex, 1994). The columnar jointing in the dolerite cliffs make for some fantastic climbing. I need to go back to do some actual climbing, at the time I was to distracted by the scale of the dyke, clambering over the rocks and hitting things with my geopic.
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| My partner, taking full advantage of the columnar joints |
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| The vertical extent of the sill was rather impressive, car for scale |
The geology/geochemistry
The lower sill, relative to the upper sill, is enriched in incompatible* elements, e.g. Rb, Zr and Y and depleted in compatible** elements such as MgO, indicating that the lower sill has a slightly more evolved*** composition than the upper sill. Trends and ratios between these elements indicate that the two sills could be related by fractional crystallisation (Reid and Rex, 1994).
*Elements which are not easily incorporated into the crystal structure of the major rock forming minerals, in the case of a mafic magma these minerals are olivine, pyroxene and plagioclase feldspar.
**Elements which are either major constituents or easily incorporated into the crystal structure of major rock forming minerals.
***As minerals are crystallised out of a magma they remove elements from the magma. This results in the composition of the magma changing. The concentration of incompatible elements increases while the compatible element concentrations decrease. The remaining liquid has a more evolved composition.
The upper sill has a fine-grained chilled margin caused by the initial contact with the country rock. This is usual for large igneous bodies intruding much colder rock. From the edge to the centre of the sill the grain-size increases as the magma had more time to cool and form larger crystals. The sill groundmass consists of plagioclase, pyroxene and opaque minerals (e.g. iron oxides and/or sulphides depending on whether the magma is oxidised or not) and the microphenocrysts present are plagioclase, augite and olivine (petrography; Richardson, 1979). Richardson (1979) identified changes in magma chemistry and phenocryst concentrations and composition from the edge to the centre of the sill. Often chemical and mineral zoning in sills is caused by minerals settling to the bottom of the magma due to gravity. However, the pattern of chemical change and lack of mineral layering normally associated with gravitational settling, in upper sill, argues against this as a mechanism for the chemical changes (Richardson, 1979). The internal chemical variation observed is also inconsistent with fractional crystallisation, assimilation and multiple intrusions of magma. The chemical zoning is continuous along the 20km length sampled by Richardson (1979) and is attributed it to the migration of phenocrysts to the centre of the sill, flow differentiation. Similar differentiation has been observed in Hawaii in the Makaopuhi lava lake (Moore and Evans, 1967). The lower sill has not been described in much detail or at least I couldn't find the journal article describing it.
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| Just to demonstrate the difference between phenocryst and groundmass. This is from a quartz porphyry dyke. A phenocryst is a large/larger crystal in a fine-grained groundmass. In this image the phenocryst is quartz. |
There is an unsolved conundrum for the sills in Namibia. No feeder dykes have been observed so how did magma originating near the east coast of South Africa travel so far? Reid and Rex (1994) analysed the largest and most prominent dyke in the area, the Mehlberg dyke, to determine if it was the magma conduit for the large Tandjiesberg sill complex. Chemical composition and age ruled out this option. The dyke is 133Ma and has a higher silica content than the sill (Reid and Rex, 1994). The connecting sills and dykes between the Karoo rocks in South Africa and those in Namibia have yet to be identified or perhaps they have already been eroded away.
References
Jourdan, F., Feraud, G., Bertrand, H., Watkeys, M.K. And Renne, P.R. 2007. Distinct brief major events in the Karoo Large Igneous Province clarified by new 40Ar/39Ar ages on the Lesotho basalts. Lithos, pp 195-209
Marsh, J.S., Hooper, P.R., Rehacek, J., Duncan, R.A., Duncan, A.R., 1997. Stratigraphy and age of Karoo basalts of Lesotho and implications for correlation within the Karoo igneous province. In: Mahoney, J.J., Coffin, M.F. (Eds.), Large Igneous Province: Continental, Oceanic and Planetary Flood Volcanism. Geophysical Monograph series, Washinton D.C., pp. 247–272
Moore, J. and Evans, B. 1967, The role of olivine in the crystallization of prehistoric Makaopuhi lava lake, Hawaii: Contributions to Mineralogy and Petrology , pp. 202-223
Richardson,S.H. 1979. Chemical differentiation induced by flow differentiationin an extensive Karoo dolerite sheet, Southern Namibia. Geochem.Cosmochim. Acta, pp 1433-1441
Richardson, S.H. 1984. Sr, Nd and O isotope variation in an extensive Karoo dolerite sheet, southern Namibia. Special Publication, Geological Society of South Africa, pp 289-294
If anyone is interested in reading about geochemistry and how it all works this online text book by William White isn't bad.
Some more photos from the crags
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| Cool village on the bank of the Orange River. The Spar shop was reasonably stocked and having forgotten my hairbrush in Cape Town it was most useful! |
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