Previous studies have placed the transition between the Upper Critical and Main Zones of the Bushveld Complex somewhere within or above the distinctive Giant Mottled Anorthosite, but have lacked the resolution to precisely pinpoint the level at which Main Zone geochemical signatures predominate over Critical Zone signatures. Using a sample spacing of 5m or less, we demonstrate that the uppermost unit of the Upper Critical Zone (an anorthosite with large intercumulus pyroxene mottles, designated unit HW5 at Impala Platinum Mines), whilst petrographically part of the Giant Mottled Anorthosite, has some chemical attributes that link it to the Main Zone. We attribute the hybrid geochemical signature of unit HW5 to partial melting of the anorthosite, due to an influx of Main Zone magma above it, followed by hybridization of the remelted intercumulus material with evolved intercumulus liquids from the overlying Main Zone.

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