Constraints on the magmatic evolution of the Lower Main Zone and Platreef on the northern limb of the Bushveld Complex as inferred from the Moordkopje drill core

[摘要] This thesis reports the results of a detailed petrographic, geochemical, geophysical andisotopic study of the lower part of the Main Zone, the Platreef and its footwall as exposedby the 1563.02 m deep Moordkopje (MO-1) drill hole situated on the Northern Limb ofthe Bushveld Complex, where the footwall to the Platreef is granitic. A total of 179samples were collected as part of the study, with the Main Zone being represented by 156samples. The Main Zone of the Northern Limb shares a broad similarity with that asknown from the Eastern and Western limbs, but differs from the latter in a number ofpertinent points. Well-known marker horizons of the Eastern and Western limbs arelacking in the Northern Limb, which hosts an anomalous troctolite layer not known fromthe other limbs. The lower Main Zone of the Northern Limb shows very limiteddifferentiation as exemplified by paramaters such as the An% of plagioclase, the Mg# ofmafic silicates and the modified differentiation index and also lacks the presence ofinverted pigeonite, which is locally a major constituent of the Main Zone in the Easternand Western limbs. The lower Main Zone of the Northern Limb also shows abundantevidence for the decoupling of differentiation trends of plagioclase and pyroxene, afeature that is only locally observed within the main body of the intrusion. Veryimportantly, the lower Main Zone also exhibits significant isotopic disequilibriumbetween co-existing plagioclase (Sri ~0.708) and orthopyroxene (Sri up to ~0.711) incertain samples, a feature that has not been described for the Main Zone as present in themain body of the intrusion.Many of the features of the lower Main Zone in the Northern Limb may be explained if itwas intruded not as repeated influxes of magmas, but instead by the repeated intrusion ofcrystal mushes from a deep-seated staging chamber. The staging chamber, situatedroughly at the boundary between the upper and lower crust, is proposed to have consistedof two sub-compartments connected at depth, with the wall rocks of the subcompartmentsdiffering in terms of their isotopic compositions. Plagioclase crystallizedwithin the staging chamber is deemed to have floated to the tops of the sub-compartmentswith mafic minerals settling to their bases. As such, magmas entering the BushveldComplex may host plagioclase derived from the one sub-compartment and mafic silicates derived from the other. Isotopic models using Vredefort crust suggest that the subcompartmentthat contributed mostly plagioclase to the Bushveld Complex may havebeen contaminated by ~25% of a melt dominated by lower crustal material (70-80%)with a smaller contribution from upper crustal material (20-30%), whereas the subcompartmentfrom which the mafic silicates were derived, was contaminated by similaramounts of a melt richer in upper crustal material (30-40%) and poorer in lower crustalmaterial (60-70%).In support of a model whereby the lower Main Zone was formed by the intrusion ofcrystal mushes rather than by in-situ fractional crystallization, is the fact that the bulkcompositions of the cumulates of the Main and Upper zones of the Northern Limb do notappear to be of a suitable composition to allow for the co-crystallization of plagioclase +2 pyroxenes.The Platreef as present in the MO-1 drill core consists from top to bottom of about 140 mof orthopyroxene-rich cumulates, followed by a package of hybridized rocks in whichinterstitial quartz-feldspar intergrowths abound, which is in turn followed by what wastermed 'pyroxene hornfels’ and then the granitic footwall. Comparison of the REE datafor Platreef samples along the strike of the Northern Limb allowed for the identificationof those Platreef cumulates least affected by footwall interaction, typically being thoserocks occurring in the vicinity of the hangingwall contact of the Platreef. Incompatibletrace element abundances and ratios suggest a B-1 magmatic lineage for the Platreef,which is supported by the Sr isotopic composition of early formed orthopyroxene withinthe Platreef (Sri ~0.706). Other parameters such as the Nd isotopic composition oforthopyroxene ( Nd = -7.8), however, suggest affiliations with the Main Zone,highlighting the need for additional study.

[发布机构] University of the Witwatersrand