Fluid overpressure in layered intrusions: formation of a breccia pipe in the Eastern Bushveld Complex, Republic of South Africa

Abstract

Fluids and volatile fluid overpressures in layered intrusions are becoming recognized as having important functions in magma chamber processes for the formation of magmatic structures and for the movement and concentration of economically important ore elements. We re-examine a breccia pipe in the Bushveld Complex to investigate the role of volatile fluid overpressure in its emplacement. This 10-m-diameter breccia pipe was emplaced vertically in the anorthosite of the Upper Critical Zone of the Bushveld Complex and contains blocks of Bushveld anorthosite, norite, and pyroxenite, apparently derived from lower in the stratigraphy. The blocks are commonly elongate, and are tightly packed within an ultramafic pegmatite groundmass. Ultramafic pegmatitic dikes radiate bilaterally from the pipe. Microprobe analyses of plagioclase in the breccia pipe blocks reveal that these grains are unzoned and have higher anorthosite content (~An85) than host rock plagioclase (~An75), indicating that the breccia pipe blocks were transported considerable vertical distances. Whole-rock major- and trace-element data show enrichment in fluid-soluble incompatible elements, supporting the role of volatile fluids in the formation of the pipe. A model is examined in which the segregation of a fluid phase from interstitial liquids in the cumulate section below the breccia pipe leads to the development of sufficient overpressure for its explosive emplacement. Fluid overpressure can develop by compaction and during crystallization of plagioclase-saturated silicate liquids as fluids exsolve from the solidifying interstitial liquid. If fluids separate faster than they can escape from the crystal pile, significant overpressures can develop. The minimum fluidization velocity which would be required to emplace blocks of the size and density found within the breccia pipe is found to be 10–110 m/s. The crystallization model predicts the development of about 400 bar of overpressure, an amount calculated to be sufficient for pipe emplacement.