A new 1.9 Ga age for the Trompsburg intrusion, South Africa

The Trompsburg intrusion of South Africa is a large layered intrusion, measuring approximately 2500 km(2). Little is known about its age and composition. Boreholes drilled in the 1940s to constrain a strong gravimetric and magnetic anomaly intersected up to 2 km of gabbro-troctolite-anorthosite containing up to 19 massive magnetite layers. Sr isotopic analyses performed in 1970 indicated an age of 1372+/-142 Ma for the intrusion, suggesting no direct genetic link to the 2054 Ma Bushveld Complex. No further work was conducted on the Trompsburg intrusion during the last decades. Our results for a secondary ion mass spectrometry U/Pb isotope study on zircons from two gabbroic samples of the Trompsburg intrusion indicate a crystallisation age of 1915+/-6 Ma, supporting the occurrence of a global 1.9 Ga superplume event. Using the new age, we recalculated available Sr isotope data. The results suggest that the Trompsburg intrusion has a lower crustal component than the Bushveld Complex, with Sr-i approximately 0.704. A genetic relationship between the Trompsburg and Bushveld intrusions remains, therefore, unlikely. (C) 2003 Elsevier Science B.V. All rights reserved.