Last act of the Rustenburg Layered Suite, Bushveld Complex: from the final major pulse of magma to modern ambient temperatures

The Bushveld Complex in South Africa is the largest, most studied, and most economically important layered intrusion in the world. The magmatic and thermal evolution of such an igneous body has significant consequences for the formation of ore deposits both within the intrusive body itself, as well as in the rocks it intruded into. Despite being vigorously studied, the thermal evolution of the Bushveld Complex, from magmatic temperatures to the present-day ambient geotherm, remains largely unknown. Within this dissertation, I attempt to elucidate the complete thermal history of the Rustenburg Layered Suite (RLS), the predominately mafic to ultramafic portion of the Bushveld Complex. I start by first investigating the lateral variation associated with the final major pulse of magma into the RLS magma chamber. The observed lateral compositional variations produce a general theme which carries on throughout the dissertation. That theme is namely that an awareness of position within the magma chamber is particularly important for all studies of the RLS as considerable variations can occur dependent on location. The majority of the dissertation is then focused on the sub-solidus thermal history of the RLS. To meet this goal, I produce the first comprehensive dataset of 40Ar/39Ar biotite and plagioclase ages throughout the ~7 km RLS stratigraphy. The varying closure temperature between the 40Ar/39Ar system in biotite and plagioclase, as well as the U-Pb system in zircon, allows for the calculation of cooling rates at every level of the stratigraphy. Results from 40Ar/39Ar biotite combined with published U-Pb zircon chronology, show that the RLS cooled rapidly (~1000°C Ma-1) to ~400°C due to enhanced heat loss from hydrothermal circulation associated with the emplacement of the RLS. Numerical heat loss simulations additionally confirm the need for hydrothermal circulation at high-temperatures in order to produce the observed ages. With the aid of an improved 40Ar/39Ar step-heating method, plagioclase 40Ar/39Ar ages and calculated closure temperatures show that hydrothermal circulation stopped within 1 Ma of zircon crystallization, as calculated cooling rates below ~400°C slow substantially (~10°C Ma-1). The initial rapid cooling of the RLS, and cessation of hydrothermal circulation, provides precise age constraints on hydrothermal ore deposits within the host rocks of the RLS, that are thought have formed from fluids related to its emplacement.Ph.D.Includes bibliographical reference