- © 2003 Geological Society of South Africa
Single-grain U-Pb zircon and monazite geochronology and whole rock Sm-Nd isotopic systematics constrain the timing of metamorphism and protolith ages for granulitic lower crustal xenoliths from the Eastern Namaqualand and Northern Lesotho kimberlites of southern Africa. From these data, inferences regarding the Proterozoic tectonic evolution of the southern African continent, the mechanisms of lower crustal granulite formation, and the present day architecture of the lithosphere are drawn. U-Pb geochronology demonstrates that the granulitization of pre-existing Archean to Mesoproterozoic lower crust is a ~1.1 to ~1.0 Ga phenomenon, resulting from convergent margin tectonic activity represented by the Kaapvaal craton-bounding Proterozoic Namaqua-Natal orogenic belts. Episodic accessory mineral growth related to high-grade metamorphism of the Northern Lesotho granulites occurred at ~1.10, ~1.04 and ~1.00 Ga, first in response to crustal thickening associated with thrust stacking of the Tugela terranes across the southern margin of the Kaapvaal craton and juxtaposition of the adjoined Mzumbe terrane at ~1.10 Ga, and then by subsequent crustal accretion along an enigmatic southern convergent margin between ~1.04 and ~1.00 Ga. In Eastern Namaqualand, where Phanerozoic sediments obscure the basement architecture, crustal xenoliths illuminate the Namaqua front as a narrowing band of Mesoproterozoic Gordonia subprovince lithologies wedged between the Archean Kaapvaal craton margin and a Paleoproterozoic terrane of Bushmanland subprovince affinity. Metamorphism in the lower crust was episodic in Eastern Namaqualand, producing granulites in the margin of the Kaapvaal craton and the adjacent Gordonia subprovince lower crust at ~1.10 Ga, likely related to crustal thickening associated with collision of the Bushmanland microcontinent. In lower crustal xenoliths of the most southerly Bushmanland terrane kimberlites, ~1.00 Ga high-grade metamorphism is significantly younger, and like the contemporaneous metamorphism in Northern Lesotho xenoliths, may be related to continued crustal accretion of terranes along a southern convergent margin. In both Northern Lesotho and Eastern Namaqualand, the timing and spatial patterns of lower crustal granulite metamorphism are more consistent with models for granulite genesis involving crustal thickening related to collisional orogenesis, rather than advective heating through underplating of mafic magmas into the lower crust. With regard to the present-day crustal structure of southern Africa, crustal thickness variations across the Namaqua front at the southwestern margin of the Kaapvaal craton are clearly related to orogenic thickening and the formation of a granulitic lower crust at ~1.1 to ~1.0 Ga. This ≥10km step in crustal thickness is a likely manifestation of late stage transpressional shearing and steepening of the Archean-Proterozoic boundary, and is a long-lived feature of the southern African lithospheric architecture.