ISOTOPIC MODELING OF FLUID/MELT MIXING IN THE BUSHVELD COMPLEX AND SUBDUCTION ZONE ANALOGUES

Crystallization of the 2.06 Ga Bushveld magma formed a 9 km (maximum) sequence of ultramafic and mafic rocks that generated a large volume of country fluid as it thermally metamorphosed a 3+ km section of previously unaltered underlying sedimentary rocks of the Transvaal sequence. Initial Sr and Nd isotopic ratios in different zones of the complex vary substantially, while mineral assemblages show extensive isotopic disequilibrium. Stable isotopic compositions also show evidence of alteration from mantle signatures. Conventional explanations for this involve variable crustal contamination; one oft-overlooked possibility is contamination introduced by crustal fluids generated via dehydration of the country rock during emplacement. Using a variety of radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotopic systems, two-component mixing diagrams are generated between estimated Bushveld parental melts and Bushveld country rock fluids. The results of these mixing calculations suggest that as a little as 1 wt. % of a country rock fluid can pull the mantle isotopic composition toward a more typical Main Zone composition. Assuming the 3.5 km Bushveld aureole generated 3-6 wt. % H₂O, this would require roughly a third to half of the vapor generated by dehydration. Along with geochemically demonstrating the viability of the country fluid circulation model, we draw analogies to certain subduction zone processes that are not fully understood. The geometry of the Bushveld Complex—hot mantle material overlying cold crustal material—is similar to that seen as subducting lithospheric slabs are heated by overlying mantle rocks. Dehydration of subduction slabs is responsible for melt generation in the mantle and the creation of volcanic arcs at the surface. The process of fluid migration out of the slab and into the mantle wedge is poorly understood; layered intrusions can lead to a better understanding of unobservable subduction zone processes such as fluid migration.