Lateral uniformity of the Pyroxenite Marker Transition in the western Bushveld Complex, South Africa

Abstract

Mineralogically distinctive layers in the Bushveld Complex, South Africa, can usually be traced for hundreds of km in both the eastern and western limbs. They are remarkably uniform laterally in mineral chemical composition. There is one notable exception, namely the Pyroxenite Marker in the middle of the Main Zone in the eastern limb. It defines the boundary between the Lower Main Zone and Upper Main Zone. Toward the south in the eastern limb mineral compositions become more evolved, and ultimately the Pyroxenite Marker layer itself disappears and is replaced by magnetite-bearing rocks. In all previously published profiles through the eastern Bushveld, through a 400 m interval with the Pyroxenite Marker in the middle, there is a regular prolonged reversal in the anorthite content of plagioclase and mg# of pyroxene of about ten units, attributable to magma addition. In contrast, in the western limb there is no actual outcrop of this layer, but it has been located in the BK borehole from the centre of the limb. A similar magnitude of reversals in mineral compositions as in the east was reported in a previous study. In this study, a second profile was taken close to the eastern limit of the western limb to test if there was lateral variation comparable to that observed in the eastern limb. More mineralogical data are also reported from the borehole intersection. These two sections from the western limb show extremely similar changes in mineral compositions. In the eastern limb the location of the Pyroxenite Marker also shows an upward, abrupt change from primary pigeonite (below) to primary orthopyroxene (above). The same change occurs in the west, and at the same mineral compositions as in the east, and so this boundary can be defined by the pigeonite to orthopyroxene transition even in the absence of an actual pyroxenite layer. Hence, the term Pyroxenite Marker Transition is more applicable. Both in the east and west more primitive plagioclase compositions occur well above this boundary, and so magma addition and/or mixing continued well into the Upper Main Zone.