~(40)Ar/~(39)Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana

摘要

In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New ~(40)Ar/~(39)Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6 ± 1.2 and 178.4 ± 1.1 Ma. Our data are concordant with previous ~(40)Ar/~(39)Ar ages for Northern Karoo dykes and lava flows exposed in western Zimbabwe. The data are tightly clustered, indicating a short-lived (179-181 Ma) flood basalt magmatism in this region. The new radiometric dates allow the definition of a diachronous Jurassic flood basalt activity in southern Africa. A significant south to north younging at the scale of the Karoo igneous province correlates with a chemical zonation from low-Ti (south) to high-Ti (north) mafic rocks. Structural measurements on the ODS/N110° and SLDS/N70° Karoo dykes of NE Botswana suggest that: (1) most of the host fractures are inherited Precambrian structures; (2) dyke emplacement occurred under unidirectional tensional stresses; (3) significant syn- and post-volcanic extensional tectonics are lacking. Combined with regional geology, these geochronological and structural data do not confirm unambiguously the triple-junction hypothesis usually put forward to support a mantle plume model for the evolution of the Karoo igneous province, prior to Gondwana breakup.