NEW RB-SR AGES FOR IGNEOUS ROCKS FROM THE AREA OF THE PRETORIA SALTPAN IMPACT CRATER

An age for the Pretoria Saltpan (Tswaing) impact crater of about 200 k a has recently been obtained by C-14 dating of algal debris in a drill core and in a fission-track study of impact glass fragments from suevi tic breccia of the lower crater fill. These relatively young ages for the cratering event are in excellent agreement with the preserved stat e of the crater. However, sceptics of the impact origin of the Tswaing crater have continued to quote alkaline and mafic intrusions in the c rater rim as prime evidence for an internal origin for this crater. In order to establish whether these intrusions are related to the crater ing event, or perhaps belong to the late-Proterozoic Pienaars River Al kaline Complex (PRAC), which is of regional significance, selected sam ples of intrusive rocks from the crater area and from the crater envir ons were dated with the Rb-Sr isotope method. These results prove uneq uivocally that the intrusions in the crater rim are part of a regional magmatic event which appears to have occurred around 1.3 Ga ago, and includes all intrusions of the PRAC. Thus, there remains no reason to relate intrusive magmatism to the formation of the much more recent cr ater structure.