Precambrian Crustal Evolution of the Rehoboth Province, Southern Africa

摘要

This thesis uses U-Pb, Sm-Nd Lu-Hf, Ar-Ar and Oxygen isotope data on well-documented rocksamples, to investigate crustal evolution of the Precambrian Rehoboth Province of Southern Africa. Thisprovince is defined by its smooth magnetic character, reflecting deep magnetic basement in contrast tothe adjoining mobile belts and the Kaapvaal Craton. Most of the area is covered by sedimentarysequences and the Kalahari sands.A first indication of old crust in the Rehoboth Province was provided by granitoid and mafic cobblesfrom the ~300 Ma Dwyka glacial diamictite. The granitoid cobbles have Archaean ages between 3100 to2500 Ma and a Palaeoproterozoic group between 2050 and 2020 Ma. The mafic cobbles yield ages of1123–1111 Ma. The likely provenance of the Dwyka cobbles is the basement of the Rehoboth Provinceor the Kalahari Line east of Rietfontein. A source within the Kaapvaal Craton is excluded by the absenceof typical Kaapvaal cover rocks in the diamictite cobbles. Previous theories of crustal growth orcollision between the Rehoboth Province and the Kaapvaal Craton at either 1800 or 1200 Ma are notsupported by these data.The Rehoboth Basement Inlier (RBI) is a tectonic terrane at the northern margin of the RehobothProvince, thrust up in the ~600 Ma Damara foreland. U–Pb, Lu–Hf and Oxygen isotope data for zirconfrom metasedimentary and magmatic rocks provide new insights on the crustal evolution of theRehoboth Province. A small group of 3.41 to 2.45 Ga U-Pb zircon ages found in the metasediments of theRBI strengthens the concept of an Archaean foundation to the Rehoboth Province. A group ofPalaeoproterozoic ages ranging from 2.2 to 1.92 Ga have not been identified in outcropping magmaticrocks. The Lu-Hf isotope character of these zircons requires mixing between Archaean crustal sourcerocks and juvenile mantle material. This again points to the presence of an Archaean nucleus within theRehoboth Province. The 2.05 Ga event, seen in both Dwyka cobbles and detrital zircons, corresponds inage to the Bushveld event and thermal peaks in the Kaapvaal Craton and Limpopo Belt. It suggests thatthe Rehoboth Province was attached to the Kaapvaal Craton before 2.05 Ga, but after 2.45 Ga.A large age peak at 1.87 Ga corresponds in age to the largely metabasaltic 1870 ±5Ma ElimFormation, now the oldest dated unit in the RBI. However, the Lu-Hf isotope data of the detrital zirconsfrom metasediments shows a source with distinctly older crustal residence and it is likely that themagmatic event at 1.87 Ga was widespread over the Rehoboth Province.The detrital zircon ages that correspond to the younger Palaeoproterozoic (1.83–1.72 Ga) magmaticages of the RBI show similar Lu-Hf character and the source for these zircons was thus mainly providedby rocks related to the same subduction phase. The Palaeoproterozoic magmatic rocks of the RBI reveala complex arc-related tectonic history which probably represents an Andean subduction setting.The Kaaien Terrane in South Africa is part of the complex suture zone between the KaapvaalCraton, Kheis Province, Rehoboth Province and the Namaqua–Natal Province related to a ~1200 Macollision event. New metamorphic pressure-temperature (PT)-calculations combined withgeochronology for unusual garbenschiefer rocks of the Groblershoop Formation reveal an unique burialand uplift history. A segment of the Kaaien Terrane reached depths around 40 km. This caused peakmetamorphism at 1164 Ma (Lu-Hf on garnet), followed by rapid exhumation to hornblende and whitemica Ar-Ar closure temperatures by ~1140 Ma, thought to be controlled by a change in the tectonicregime. This is the highest pressure found thus far in the Namaqua-Natal Province, most others beingless than 5kb. Other parts of the Kaaien Terrane remained at the surface during this period.The Rehoboth Province is thus revealed as an ancient crustal block with Archaean foundations,which may have been attached to the Kaapvaal Craton prior to 2.05 Ga. Major Palaeoproterozoic eventswithin the Rehoboth Province involved mantle additions mixed with reworked Archaean crust. Finally,the Rehoboth Province played a major role during the evolution of the Mesoproterozoic Namaqua–Natal Province, which led to the formation of the Kalahari Craton.