Abstract

Ion probe U-Pb dating of different age domains within detrital zircons, distinguished using cathodoluminescent images, has been applied to three metasedimentary gneiss samples from the Jannelsepan, Copperton and Kenhardt Formations in the Areachap and Kakamas Terranes of the Namaqua-Natal Province. The Areachap Group samples yielded largely coherent ages which are considered to reflect the age of extrusion of the surrounding volcanic rocks. These are 1275 ± 7 Ma for the Jannelsepan Formation at Boksputs (with a component at ~1301 Ma) and 1282 ± 45 Ma for the Copperton Formation at Prieska Copper Mines. Direct dates of volcanic rocks in these formations in the literature are 1241 ± 12 Ma for the Jannelsepan Formation at Upington and 1285 ± 14 Ma for the Copperton Formation. The Kenhardt Formation sample contained three age domains. Concordant zircon cores dated at 1568 ± 23, 1351 ± 24 and 1296 ± 14 Ma reflect the presence of older crustal material in the Kakamas Terrane, whereas another six cores are coeval with the main, oscillatory zoned phase, dated at 1194 ± 7 Ma. This is the age of the magmatic rocks from which this metasedimentary sample was derived.

The identity of the Areachap Group as defined in the 1980s has been questioned on the grounds that the Boven Rugseer Shear Zone is a terrane boundary that cuts across the Areachap Group. This work shows that the Jannelsepan and Copperton Formations, which crop out to the north and south of this shear zone, both formed in the interval ~1.30 to ~1.24 Ga. Published geochemical data show that these formations have a common origin as arc-related volcanosedimentary sequences containing volcanic massive sulphide ore deposits. Furthermore the Pb and sulphur isotopic data point to very similar sources of mineralization in the Areachap Mine (Jannelsepan Fn) and Prieska Copper Mines (Copperton Fn). We conclude that the integrity of the Areachap Group as originally defined rests on very strong evidence.

The new data are compatible with a tentative plate tectonic model for the region. The ~1.57 Ga Kakamas fragment and the juvenile ~1.3 Ga Areachap Arc are envisaged as minor crustal blocks in a major ocean basin bounded by the major Bushmanland continental block and the Kaaien-Kheis-Kaapvaal block. The Wilgenhoutsdrift Group formed at the same time as the Areachap Group in a back-arc or rift setting on the margin of the Kalahari Craton, from which it derived Palaeoproterozoic and Archean sedimentary material. Assembly of all the terranes by collision following closure of the major ocean basin gave rise to a collision-related cycle of D1-D2 deformation, metamorphic events and migmatisation at different localities between 1194 ± 23 and 1158 ± 7 Ma. A much later thermal event originating in the mantle affected the whole region at ~1.1 Ga, leading to extensive magmatism, D3 deformation and local migmatisation.

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