Regional trends of chemical variation and thermal erosion in the Upper Critical Zone, Western Bushveld Complex

Abstract

A comparison of R.P.M. Union and Amandelbult Sections reveals close geochemical and stratigraphic correlations, but the sequence at the latter is more complex. Marie members at Union Section are consistently more magnesian than their equivalents at Amandelbult Section and (where available data allow comparisons) than at Rustenburg Section. This, taken together with regional patterns of progressive attenuation and elimination of leucocratic rocks beneath harzburgitic layers, identifies Union Section as proximally located with reference to an irruptive centre from which primitive liquids were injected along the floor/supernatant liquid interface. Successive injections led to thermal erosion of the floor, causing 'dimpling' and 'potholing' on a local scale, and elimination of noritic-anorthositic layers on a regional scale. Such erosion of the floors beneath new inputs of magmatic liquid is likely to be initiated by only partial remelting, mainly of lower-temperature phases (sodic rims to zoned plagioclase grains, and Fe-enriched pyroxenes) within intercumulus space. It is argued that resultant contamination of the primitive liquids may have led to direct superposition of anorthosites (accompanied by rare troctolite) upon harzburgite, without intervening pyroxenite members, as in the Pseudoreef Multicyclic Unit.

Platinum-group elements in the Pyroxenite Marker, Bushveld Complex: implications for the formation of the Main Zone

“…This suggests upward percolation of an anorthositic magma or crystal mush in response to compaction. Lobate contacts between basal felsic and overlying mafic-ultramafic layers are also observed in the Upper Critical Zone, below the Merensky Reef, where they are either interpreted as erosional features (Lee, 1981;Eales et al, 1988) reaction replacement phenomena (Nicholson and Mathez, 1991). Further, Parsons and Becker (1987) described load structures very similar in appearance to those described here from the Klokken intrusion of Greenland.…”

Concentration of Platinum Group Elements during the Early Earth Evolution: A Review*

“…Well-defined cyclicity is exhibited by a transition from pyroxenite, through melanorite, norite (35% -65% plagioclase) and leuconorite (65% -85% plagioclase) to anorthosite (≥85% plagioclase) in the Bastard and Merensky Units [196]. The authors [196] reported presence of abrupt transitions such as chromitite-anorthosite-chromitite, harzburgite-anorthosite-harzburgite, and leuconorite-anorthosite-leuconorite for the Western Bushveld Complex. Mitchell and Manthree [203] demonstrate that the uppermost unit of the Upper Critical Zone (an anorthosite with large intercumulus pyroxene mottles, designated unit HW5 at the Impala Platinum Mines), whilst petrographically part of the Giant Mottled Anorthosite, has some chemical attributes that link it to the Main Zone, and they attribute the hybrid geochemical signature of HW5 unit to partial melting of the anorthosite, due to an influx of the Main Zone magma above it.…”

“…In some places the transition between the Upper Critical and Main Zones of the Bushveld Complex is within or above the distinctive Giant Mottled Anorthosite [203]. Eales et al [196] draw the boundary between the Critical and Main Zones at the top of the Bastard Unit (at the upper contact of the Giant Mottled Anorthosite). Well-defined cyclicity is exhibited by a transition from pyroxenite, through melanorite, norite (35% -65% plagioclase) and leuconorite (65% -85% plagioclase) to anorthosite (≥85% plagioclase) in the Bastard and Merensky Units [196].…”

“…Eales et al [196] draw the boundary between the Critical and Main Zones at the top of the Bastard Unit (at the upper contact of the Giant Mottled Anorthosite). Well-defined cyclicity is exhibited by a transition from pyroxenite, through melanorite, norite (35% -65% plagioclase) and leuconorite (65% -85% plagioclase) to anorthosite (≥85% plagioclase) in the Bastard and Merensky Units [196]. The authors [196] reported presence of abrupt transitions such as chromitite-anorthosite-chromitite, harzburgite-anorthosite-harzburgite, and leuconorite-anorthosite-leuconorite for the Western Bushveld Complex.…”

“…So-called gabbroic cumulates contain plagioclase of up to: 70% in troctolites, 65% in norites, and 55% in olivine gabbro and gabbronorites [195]; 40% in chromitites [34] [161]; 65% -85% in leuconorites [196]; 80% in leucogabbronorites or mottled anorthosites [197]; 70% -80% in anorthositic gabbro [198], and 85% -95% in anorthosites [199]. It is evident that all these rocks require presence of huge amounts of plagioclase which could be provided only by anorthosites.…”