https://commons.ru.ac.za/vital/access/manager/Index en-us 5 https://commons.ru.ac.za/vital/access/manager/Repository/vital:4582 Wed 12 May 2021 22:38:02 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5062 Wed 12 May 2021 20:12:43 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4616 Wed 12 May 2021 19:49:06 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5507 Wed 12 May 2021 19:36:46 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:3819 Wed 12 May 2021 19:04:45 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4908 Wed 12 May 2021 18:52:51 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4808 Wed 12 May 2021 18:35:20 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4238 Wed 12 May 2021 18:32:09 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4909 11Oppm) are the most evolved. They are tholeiitic andesites which display fairly limited compositional variation. It is likely that more evolved compositions are present in other areas where the porphyritic lavas which characterize this unit are better developed. The overlying Orkney lavas are characterized by 110ppm>Zr>90ppm. They are tholeiitic andesites of similar composition to the Alberton lavas, but have lower incompatible element levels, higher siderophile element levels, and are of extremely uniform composition. The uppermost Loraine/Edenville lavas range from magnesian tholeiites to tholeiitic andesites. They are distinguished by Zr< 90ppm, and contain the most primitive magmas af the Witwatersrand triad, with up to 17,5% MgO, 2600ppm Cr, 600ppm Ni and M-values up to 77. The most primitive liquids evolved by 38% fractional crystallization of orthopyroxene ∓ chromite, followed by 35% fractional crystallization of an extract containing clinopyroxene and plagioclase. The absence of olivine precipitation is a result of the inherently high Si0₂ content of the magma. The Loraine/Edenville, Orkney and Alberton lavas do not lie on a common liquid line of descent, but are probably consanguinous. The Platberg Group overlies the Kliprivierberg Group, and has a coarse-clastic sedimentary unit, the Kameeldoorns Formation, at the base. Three petrographically distinct porphyritic lava sequences overlie the Kameeldoorns Formation, namely the informal "Goedgenoeg formation", the Makwassie quartz-feldspar porphyries and the Rietgat Formation. Despite petrographic differences, the Goedgenoeg and Rietgat lavas are chemically indistinguishable and thus form a single magma-type. The Makwassie porphyries are dacitic in composition with a high proportion of feldspar and quartz phenocrysts. Rational variation trends are attributed to a nett loss of Si0₂ during secondary alteration. The porphyries are probably of crustal origin. The Goedgenoeg/Rietgat lavas display unusual chemistry and a broad, irrational compositional spectrum. They contain very high incompatible element levels, high nonnative quartz, as well as high MgO, M-values, Cr and Ni relative to the other tholeiitic andesites of the Witwatersrand triad. It is tentatively suggested that they are hybrid magmas containing both crust and mantle components, the former possibly represented by the Makwassie porphyries. Field evidence suggests that Platberg volcanism commenced directly after Klipriviersberg volcanism ceased, and was accompanied by a period of enhanced tectonic activity. The Platberg lavas thus probably reflect a crustal melting cycle associated with the Klipriviersberg magmatic event. The Allanridge lavas are the youngest rocks of the Witwatersrand triad. They are separated from the Platberg Group by a unit of flat-lying sediments, the Bothaville Formation, which was deposited after an extended period of peneplanation. The Allanridge lavas form a separate magma-type. They are tholeiitic andesites of similar composition to the Alberton lavas, but have higher incompatible element levels and are not consanguinous. The compositional similarities amongst the basic magma-types of the Witwatersrand triad suggests that all were generated in an hydrous mantle. Interelement ratio differences between the various magma-types nevertheless support the concept that the mantle was chemically heterogeneous during the early Proterozoic.]]> Wed 12 May 2021 18:23:53 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5605 Wed 12 May 2021 17:08:20 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:3892 Wed 12 May 2021 16:34:03 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:3813 Wed 12 May 2021 15:46:44 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4104 Thu 13 May 2021 07:27:26 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5633 Thu 13 May 2021 06:44:03 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5040 Thu 13 May 2021 06:09:54 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5632 Thu 13 May 2021 06:04:07 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5811 Thu 13 May 2021 04:06:16 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:3732 Thu 13 May 2021 03:50:21 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5010 Thu 13 May 2021 02:55:17 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4932 Thu 13 May 2021 02:01:08 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5630 Thu 13 May 2021 01:31:45 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:4178 Thu 13 May 2021 00:09:24 SAST ]]>