The Residual or Roof Zone of the Bushveld Complex, South Africa

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Abstract

The geological succession of an area in the southeastern Bushveld Complex, South Africa is described here, in which up to 700 m of monzonitic rocks, more evolved than in currently proposed stratigraphic sections of the Bushveld, have been identified beneath the Rooiberg Group felsites, the roof rocks to the intrusion. A previous mass-balance calculation for incompatible elements from the upper part of the Bushveld Complex indicated that some evolved rocks are missing from currently accepted sections through the stratigraphy. Most previous attempts to constrain the location of such evolved rocks have been based on one-dimensional (vertical) modelling, and the possibility of lateral magma migration within the complex has mainly been ignored by most researchers. Characterization of the currently accepted most evolved rocks is based on two challengeable assumptions: that the crystallization surface was exactly parallel to the roof, and that the Bushveld Granite was emplaced exactly at the contact between the uppermost rocks of the complex (Upper Zone) and the felsic volcanic roof rocks (Rooiberg Group). Both assumptions are shown here to be questionable. It is inferred that this body of monzonite represents this more evolved magma that migrated laterally, not vertically. These observations show that vertical summation of the compositions in the intrusion cannot give the correct answer for the original composition of the magma. These evolved rocks contain abundant plagioclase, up to 20% in total of quartz, alkali feldspar plus hornblende, and very minor olivine, clinopyroxene and an oxide phase. They contain up to 65% SiO2 and can be called hornblende quartz monzonites. They do not display good cumulate textures. Such rocks have been reported in the older (pre-1970) literature (but with different names) from the eastern, western and northern limbs of the Bushveld, and so potentially are volumetrically significant. The more recent publications have focused on vertical sections where fairly melanocratic and oxide-rich rocks form the top contact. That contact is interpreted here to have been truncated by emplacement of the Bushveld Granite. To emphasize that the present studied rocks are mineralogically and texturally distinctly different from those reported in previous sections, it is proposed that they be designated as the Residual or Roof Zone of the Bushveld Complex.

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