Mesoproterozoic crustal evolution in Southern Africa
Sammanfattning: The objects of this thesis were to use geochronology to investigate crustal evolution of part of Southern Africa that formed during a global event, resulting in the ~1 Ga supercontinent Rodinia. The timing of assembly by continent-continent collisions and the position of the individual continental blocks of this ancient landmass is still largely unknown. This research is a contribution to the investigation and understanding of Precambrian tectonism and the evolution of the continental crust. It also provides a basis for future ore and mineral exploration in the Namaqua Sector of the Namaqua-Natal Province. The Namaqua-Natal Province is draped along the western and southern flanks of the > 3 Ga Kaapvaal Craton, South Africa. It formed at 1200-1100 Ma, in relation to the birth of Rodinia. The Natal Sector of the province in the south-east is contiguous with the Namaqua Sector in the west, separated by Phanerozoic cover. The Natal Sector is mainly composed of ~1.2 Ga juvenile crustal material. The Namaqua Sector is composed of six different terranes in which the metamorphic grade varies from greenschist facies to amphibolite and granulite facies. The major ore districts of the Namaqua Sector have been investigated, but the regional geochronology is poorly constrained and few regional studies have been done. We provide new geochronology, including Sm-Nd model ages of crustal extraction from the mantle, microbeam U-Pb zircon dating of sedimentary, magmatic and metamorphic events and Ar-Ar dating of minerals indicating the cooling histories of rocks, to investigate and compare the chronological and geochemical evolution of the entire Namaqua Sector. Sm-Nd model ages show that parts of the Namaqua crust were extracted from the mantle as early as the Archean Era (>2.5 Ga) in the Bushmanland Terrane, with major Paleoproterozoic (2.5-1.6 Ga) crustal growth in the Kaaien, Areachap, Kakamas (or eastern terranes), Bushmanland, Garies Terranes and Richtersveld Subprovince (or western terranes). Mesoproterozoic (1.6-1.1 Ga) juvenile additions to the crust took place in the Areachap, Kakamas and Garies Terranes. Several volcanic,magmatic and sedimentary events contributed to the building of crust, before, during and after the collisional orogeny at 1.21-1.16 Ga. Thickening of the crust led to voluminous grainitoid magmatism at this time, the extent of which strongly suggests a near complete reworking of the crust, accompanied by regional metamorphism in the eastern and parts of the western terranes. After a quiet period, a new pulse of magmatism took place at ~1.1 Ga in the eastern terranes, and a second pulse of high temperature metamorphism at 1.05-1.01 Ga in the western terranes. From this time and onwards to ~965 Ma the crust in the Areachap, Kakamas, Bushmanland and Richtersveld cooled rapidly, likely related to crustal uplift. Hence, the Namaqua Sector has a long history of crustal extraction, in contrast with the essentially juvenile Natal sector. This work represents a regional documentation of the geochronology of the Namaqua Sector and is a significant contribution to the understanding of its complex geological evolution. Keywords: Namaqua-Natal Province, Mesoproterozoic, Geochronology, U-Pb zircon dating, Sm-Nd model ages, Ar-Ar dating, Southern Africa.
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