The Anna's Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event on the Kaapvaal Craton and beyond?

The Anna's Rust Sheet (ARS) and a suite of mineralogically and chemically r elated intrusions in the core and collar of the Vredefort Dome (in particul ar, the Vredefort Mafic Complex: VMC) represent a newly recognised type of high Ti gabbro in this central part of the Kaapvaal Craton. This lithology, referred to as the Vredefort Type IV mafic intrusion, is distinguished fro m chemically similar Type V intrusions (the Karoo dolerites) by the presenc e of glomeroporphyritic plagioclase and higher Th content and from Type III intrusions (similar to 1600 Ma gabbro) by the lack of cross-cutting pseudo tachylitic breccia veinlets. Petrographic features and both major and trace element compositions of all Type IV intrusions are very similar. Based on its Rb-Sr isotope age and character, a gabbroic intrusion from Majuba Colli ery (Mpumalanga Province) is also thought to belong to the ARS (Type IV) su ite of tholeiitic intrusions. Rb-Sr isotopic analysis resulted in a preferr ed age of 1052 +/- 11 Ma (2 sigma) for biotite and plagioclase data for ARS , VMC and Majuba samples. The Rb-Sr age for the ARS is further supported by Ar-40-Ar-39 stepheating ages for plagioclase and pyroxene separates from t wo ARS and VMC samples, which favour formation of this gabbroic intrusion a t ca 1000 Ma ago. These results suggest that an similar to 120 m thick shee t intrusion may be present throughout a major part of the Vredefort Dome. W hile Kibaran-age (ca 1-1.2 Ga) alkaline, both mafic and felsic, magmatism, as well as tectonic and hydrothermal activity at that time, have been known in the central Kaapvaal Craton, a widespread tholeiitic magmatic component has now been added to this record. There is a strong likelihood that this magmatic event occurred throughout the southern African subcontinent and pe rhaps into Antarctica. (C) 2001 Elsevier Science Limited. All rights reserv ed.