Compositional cyclicity in a pyroxenitic layer from the Main Zone of the Western Bushveld Complex: evidence for repeated magma influx

Abstract

Lacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, 1100 m above the Main Zone -Critical Zone contact, a 10 m thick pyroxenitic layer was intersected in an exploration borehole from the southern sector of the western Bushveld.Within the pyroxenitic layer, mineral chemistry defines a series of five cycles of upward Mg and Cr enrichment in pyroxenes, and Ca enrichment in plagioclase. The mineral chemistry, supported by textural evidence, suggests the influx of successive surges of magma. Sustained streaming of magma gave rise to adcumulate textures in the central portion of each cycle, with orthocumulate textures at bases and tops of cycles representing waxing and waning stages of magma surges.

The Merensky Reef, Bushveld Complex: Mixing of Minerals Not Mixing of Magmas

“…This distinctive lithology occurs throughout the Bushveld Complex and is known as the Porphyritic Gabbro Marker horizon (e.g., von Gruenewaldt, 1973;Mitchell, 1990;Maier and Mitchell, 1995). Closely spaced sampling was undertaken of the Merensky and Bastard pyroxenites.…”

Magma replenishment, and the significance of poikilitic texlures, in the Lower Main Zone of the western Bushveld Complex, South Africa

“…Whole-rock geochemical data (Table 1) were obtained by XRF spectrometry at Rhodes University, Grahamstown and the University of Natal, Pietermaritzburg. Mineral analyses were performed using Cambridge Microscan V and Jeol CXA 733 electron microprobes at Rhodes University, analytical conditions for which are summarized by Mitchell (1990Mitchell ( , 1996. Strontium isotope determinations were performed at the Bernard Price Institute, University of the Witwatersrand, using the methods outlined by Kruger (1994).…”

“…In the three boreholes investigated, the basal portion of unit N-II is bounded by pyroxenite layers. Ultramafic layers within plagioclase-rich cumulate sequences are commonly interpreted as the products of fresh injections of magma (Scoon and De Klerk, 1987;Campbell and Turner, 1989;Mitchell, 1996). It would therefore be reasonable to assume, as a starting hypothesis, that the two pyroxenite layers in unit N-II are the basal layers of two successive cyclic units, similar in essence to the Merensky and Bastard units at the top of the Upper Critical Zone.…”

“…THE accumulation of trace element, isotopic and mineral chemical data for the Main Zone of the Bushveld Complex in recent years (Mitchell, 1990;Kruger, 1994;Mitchell, 1996) has created new perspectives on this part of the Bushveld succession. Chemical and petrographic reversals at the Pyroxenite Marker, some 2000 to 2500 m above the base of the Main Zone (MZ), are well documented (yon Gruenewaldt, 1973;Molyneux, 1974;Cawthorn et al, 1991).…”

“…Below the Pyroxenite Marker, several lesser chemical inflections have been identified, especially in the composition of low-Ca pyroxene, and consequently the LMZ has itself been subdivided into a series of units (Mitchell, 1990;1996). Of these, the lowermost two (N-I and N-II) are leuconoritic, the remainder being essentially gabbronoritic.…”