Emplacement and Crystallization Time for the Bushveld Complex

Abstract

The Bushveld Complex formed by the crystallization of successive injections of magma, which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated significantly before the addition of the next one. To constrain the emplacement and crystallization times, a thermal model is presented which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition (and subtraction). Such modelling indicates that magmas injected into the Bushveld Complex were emplaced within 75 000 years. At that time injection into the Complex ceased. The volume of rock in the Eastern and Western limbs is 370 000–600 000 km3. However, a quantitative evaluation of the Cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be accounted for in the preserved rock sequence. Similarly, an evaluation of the incompatible trace-element abundances, such as those for Zr and K, suggests that the chamber was open and that large volumes of differentiated magma escaped. The volume of magma therefore greatly exceeded the preserved volume of cumulate rocks, giving an estimated magma volume of over 1 × 106 km3. An average emplacement rate of 13 km3/year is indicated by these calculations.