Emplacement and Crystallization Time for the Bushveld Complex

Abstract

The Bushveld Complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years. For example, the Columbia River Basalts (Hooper, injections of magma, which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988) were erupted in the period 17–12 Ma, with minor eruptions for a further 5 my, although most outpouring significantly before the addition of the next one. To constrain the emplacement and crystallization times, a thermal model is presented occurred within the first 2 my. It is now recognized that large intrusions were not emplaced in a single pulse, but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition (and subresult from multiple magma injection. The question is how rapidly were magma chambers, such as the BC, traction). Such modelling indicates that magmas injected into the Bushveld Complex were emplaced within 75 000 years. At that filled and how much magma was involved. Answers are relevant to the dynamics of melt production, storage and time injection into the Complex ceased. The volume of rock in the Eastern and Western limbs is 370 000–600 000 km. However, transport in the mantle and crust. This paper describes a thermal modelling technique (not previously applied a quantitative evaluation of the Cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers) which can be used to analyse this process, and to obtain an estimate of the emplacement accounted for in the preserved rock sequence. Similarly, an evaluation of the incompatible trace-element abundances, such as those for Zr time. To present this model it is necessary to discuss the stratigraphy, size, and connectivity of the different limbs and K, suggests that the chamber was open and that large volumes of differentiated magma escaped. The volume of magma therefore of the Bushveld Complex, and to consider the extent of tapping of the magma chamber as well as its filling. greatly exceeded the preserved volume of cumulate rocks, giving an estimated magma volume of over 1× 10 km. An average The term ‘Bushveld Complex’ has been given several meanings in the literature, and according to the South emplacement rate of 13 km/year is indicated by these calculations. African Commission on Stratigraphy (1980) includes not only the ultramafic–mafic layered rocks, but also the sills beneath the intrusion, volcanic rocks which pre-date the