Thermal stabilization of the Zimbabwe Craton deduced from high-precision Rb-Sr chronology, Great Dyke

Abstract

Dating low temperature events such as magmatic cooling or (hydro-)thermal surges in Archean and Proterozoic terranes is crucial in defining cratonal thermal stabilization after episodic continental growth during the Archean and Early Proterozoic. Rubidium-Sr chronology is potentially a powerful tool in this regard because of its low closure temperature, i.e., ∼<400 °C in most minerals, but has until now been hampered by its relatively low precision compared to high-temperature chronometers. Consequently, Rb-Sr age investigations have so far failed to provide high-precision age constraints on the cooling of rocks older than ∼2 Ga. Here, it is demonstrated that internal Rb-Sr microchrons can yield important, high-precision age constraints on the cooling history of Archean intrusions. After careful mineral selection and chemical treatment, a Rb-Sr age of 2543.0 ± 4.4 Ma was obtained from the Archean Great Dyke, Zimbabwe Craton, in contrast to the intrusion age of 2575.8 ± 1 Ma, yielding an ambient average cooling of ∼5 ± 2 °C/Ma. The non-disturbed magmatic Rb-Sr cooling age of the Great Dyke marks the final stage of Zimbabwe craton stabilization and that the greater craton area did not experience any intensive later reheating event during metamorphic or tectonic events.