Hybrid norite and the fate of argillaceous to anhydritic shales assimilated by Bushveld melts

Abstract

Mafic sills of the Marginal Zone are widespread within the footwall of the Bushveld Complex with some of them being enclosed in later Bushveld intrusions. In the central sector of the northern limb of the Bushveld Complex on the Turfspruit farm, a sill of norite enclosed within the Lower Zone cumulates contains numerous semi-assimilated fragments of anorthositic and magnetite-orthopyroxenitic lithologies that are interpreted to be the partially melted xenoliths of local argillaceous, magnetite-bearing, and anhydritic shales. Almost pure anorthite with eutectic inclusions of microgranular Al-rich orthopyroxene composes the interstitial material in the Lower Zone chromite-olivine cumulates at the contact with the magnetite-bearing norite. An identical association of anorthite+Al-rich orthopyroxene occurs in the overlying Platreef pyroxenite at the contact with anhydritic hornfels suggesting that an exchange reaction has likely involved anhydrite in both cases. Strontium isotope signatures of plagioclase from the norite support its hybrid origin, whereas Sr isotopes in anorthite from harzburgite is within the range of the most primitive Bushveld rocks indicating a predominant contribution of magmatic Sr. We suggest that the observed mineral assemblages of Al-rich and Cr-poor orthopyroxene, Fe-rich anorthite with elevated Ga, and Al-rich, Cr-depleted spinel, as well as the changes in mineral chemistries towards those compositions, are indicative of assimilation of evaporites and argillaceous shales by Bushveld melts on Turfspruit and elsewhere in the northern limb.