The Kibaran of southern Africa: Tectonic evolution and metallogeny

Abstract

The tectono-magmatic and metallogenic evolution of the ∼ 1100 Ma (Late Kibaran) high-grade Namaqua-Natal metamorphic belt is reviewed and compared with that of the Grenville Province and the Kibaran Belts of central and eastern Africa. In Namaqualand, 1300–1600 Ma supracrustal rocks are recognised, including arc-related sequences in the east and extensional foreland basin sequences in the west. These were laid down either on a heterogeneous basement of Eburnian (∼ 2000 Ma) age or on juvenile oceanic crust, prior to Late Kibaran deformation, metamorphism and multiple plutonic intrusion. The pre-Namaqua basement infrastructure has been recognised largely in the eastern and NW parts of the orogen, adjacent to areas of exposed Eburnian crust and probably only the Areachap Terrane in the east comprises juvenile Kibaran crust. In Natal, ∼1400 Ma, predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the ∼ 1050 Ma tectonothermal event. The northern terrane is interpreted as an ophiolitic assemblage, obducted to the NE onto the Kaapvaal Craton, whereas the southern domains are predominantly arc-related. No older basement is recognised and the entire belt comprises juvenile crustal material, none of which is apparently older than ∼ 1400 Ma. The kinematic evolution of the entire Namaqua-Natal Belt is comparable. In both areas, early SW- and NE- d directed thrusting and nappe emplacement during collision, were followed by pervasive steep transcurrent ductile shearing. In Natal. the later shearing event gave rise to major WNW-trending oblique sinistral transcurrent “steep belts”, whereas the same event in Namaqualand is manifested as major, subvertical, NW-trending dextral transcurrent shear zones. The difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent Archaean craton margin, within a progressive, consistently oriented, NE-SW directed zone of plate convergence. In eastern Namaqualand and Namibia, the transcurrent shearing gave rise to a north-propagating stricke-slip rift system in which the volcano-sedimentary Koras-Sinclair-Ghansi sequences were deposited. Mineral cooling ages of ∼ 950 Ma, reported from throughout the belt, are considered to signal the end of the Kibaran cycle in southern Africa. The metallogenic evolution is discussed in the light of the tectono-magmatic model presented. Economically important stratiform volcanic and sedimentary exhalative base-metal occurrences are extensively associated with early supracrustal sequences throughout the belt. Other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites, which characterise the younger rocks in both Natal and Namaqualand. The metallogenesis of the dominantly convergent-collisional Namaqua-Natal Belt is similar to that seen in the Grenville Province, but distinct from, and younger than, the lower grade, strike-slip basin-related Kibaran Belt of central and eastern Africa.