THE HYDROTHERMAL SYSTEM OF THE BUSHVELD COMPLEX, SOUTH AFRICA - AN ANALOG FOR SUBDUCTION ZONE HYDROTHERMAL SYSTEMS

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 - a geometry similar to that seen as subducting lithospheric slabs are heated by overlying mantle rocks. The presence diatremes (breccia pipes) and other large, pipe-like features in the Bushveld Complex located above diapiric domes and periclinal folds of the floor rocks suggest that overpressured fluids generated during dehydration of the footwall sediments and focused by the diapiric structures can rapidly penetrate the lower portions of the complex. Similar diapiric structures have been suggested to form in subduction zones. Numeric modeling of the footwall dehydration similarly suggests that most of the country fluids will be confined to pipe-like channels as it percolates into the Bushveld sill. Modeling also suggests that the maximum extent of the metamorphic aureole was reached at about the same time that the Main zone began to crystallize. It is suggested that rapid inflation of the Bushveld sill induced the sudden and catastrophic expulsion of overpressured country fluids to both generate the diatreme and contaminate the Main zone magma, resulting in the Main zone enrichment in a crustal Sr isotopic signature. By analogy, it is also suggested that hydration melting in the mantle wedge is episodically driven by similar sudden influxes of slab fluids that are able to retain their geochemical and isotopic character by rapid channeled influx. This can be aided by flow focusing and diapir formation at irregularities at the upper slab-mantle contact.