Discordant ultramafic pegmatoidal pipes in the Bushveld Complex

Discordant ultramafic pipes cut most of the layered sequence of the Bushvel d Complex. We have studied one pipe in detail, the Tweefontein pipe, which cuts the Critical Zone, eastern Bushveld Complex, because it is well-expose d in a new road cutting. Field relations suggest that these pipes were empl aced while the layered rocks were extremely hot and incapable of brittle fa ilure. The existence of displaced chromitite and anorthosite fragments in t his discordant body is suggestive of an intrusive magmatic, rather than met asomatic, mode of emplacement. Initial Sr isotopic ratios of plagioclase fr om the pipe are in the range 0.7073 to 0.7079, which contrast with typical ratios of 0.7055 to 0.7065 for the Critical Zone, and >0.708 for Main Zone. These data preclude an origin for the pipe as residual magmas from the adj acent layered rocks. The compositions of, and extensive exsolution in, pyro xenes in the pipe indicate temperatures of formation comparable to those of the layered sequence itself, and that they underwent slow cooling comparab le to the surrounding layered rocks, such that they both have similar closu re temperatures. Preferential replacement of leuconoritic layers suggests a temperature of emplacement in excess of the plagioclase-pyroxene cotectic temperature. The per mil delta O-18 difference between plagioclase and pyro xene (Delta (plag-px)) for samples from within the pipes ranges from 0.4 to 1.0, and averages 0.7 (for nine pairs), compared to Delta (plag-px) of 0.4 to 0.6 for host rocks, again consistent with magmatic temperatures of form ation. Oxygen isotope ratios for plagioclase and pyroxene in the pipes and layered host rocks are comparable, and preclude a significant fluid contrib ution from metamorphosed sediments in the floor of the Bushveld Complex in the formation of the primary mineralogy. The presence of hornblende, and oc casional higher Delta (plag-px) values than in the normal layered sequence rocks suggest lower temperature equilibration in the pipe, probably in the presence of a fluid. Higher absolute delta O-18 values for both minerals in a few of the pipe and host samples suggest reaction with a later fluid. Th ese discordant ultramafic pipes are considered to form by emplacement of ma gma batches, which are Sr-isotopically distinct from those which produced t he adjacent layered rocks of the Bushveld Complex, but were nevertheless ex tremely closely related in time to the main intrusive events. Dissolution o f host rocks, rather than purely mechanical dilation, provided the space fo r pipe emplacement. However, the pipe may have acted ultimately as a channe lway for low-temperature hydrothermal fluids related to later faulting in t he immediate vicinity.