Crustal evolution of the Rehoboth Province from Archaean to Mesoproterozoic times: Insights from the Rehoboth Basement Inlier

摘要

The Rehoboth Province of southern Africa is defined by its smooth magnetic character by contrast to the adjoining mobile belts. It has been slow to yield information about its crustal evolution because most of it is covered by Kalahari sands. A combined microbeam study of U-Pb, Lu-Hf in zircon from metasedimentary and magmatic rocks of the well-exposed Rehoboth Basement Inlier (RBI) provides new constraints on the crustal evolution of the Rehoboth Province. The detrital zircons provide evidence for five U-Pb age groups corresponding to magmatic events within the crust, whereas Lu-Hf zircon data allows estimates of crustal residence age. Zircon oxygen isotope data for the magmatic rocks implies diversity in their genesis. The sparse 3.41-2.45 Ga U-Pb zircon age group I (6 zircons) strengthens the concept of an Archaean foundation to the Rehoboth Province, previously suggested from kimberlite xenoliths and granitic cobbles transported by the Dwyka icesheet from further south in the Rehoboth Province. Detrital age group II from 2.15 to 1.92 Ga has not been identified in outcropping magmatic rocks. However several Dwyka cobbles have 2.05 ages, similar to the Bushveld Complex and thermal event seen in the adjoining Kaapvaal Craton. This suggests that the Rehoboth Province may already have been attached to the Kaapvaal Craton at this time. Age group II has Lu-Hf isotope character requiring mixing between Archaean crustal source rocks and juvenile mantle material. Age group III (1.92-1.83 Ma) corresponds in age to the largely metabasaltic 1870 ± 6 Ma Elim Formation, now the oldest dated unit in the RBI. The detrital Lu-Hf data again requires an Archaean crustal source mixing with mantle-derived material. However the magmatic Elim Formation zircons originated from a distinctly more juvenile source than the detrital zircons of the same age (group III), and they have a low mantle δ~(18)O_(zrc) value of 4.83 (assuming mantle 5.3 ± 0.6). This suggests a mantle or lower oceanic crustal affinity for the Elim Formation, whereas the detrital zircons may have come from a more evolved crustal source located in the continental hinterland. Detrital age group IV (1.83-1.61 Ga) corresponds to eight precisely dated volcanic and plutonic rocks which have similar Lu-Hf character and probably provided the detrital zircons. Zircon oxygen isotope values for the magmatic rocks show a considerable range. The sub-mantle values of the 1826 ±5 Ma Kalkbrak Gneiss and 1753 ±6 Ma Marienhof meta-rhyolites (δ~(18)O_(zrc) 4.30 and 4.31 respectively) are thought to have originated from the same low-δ~(18) O crustal source as the Elim Formation.