Crustally Contaminated Komatiite: Primary Source of the Chromitites and Marginal,Lower, and Critical Zone Magmas in a Staging Chamber Beneath the Bushveld Complex

摘要

In the lowermost rocks of the Rustenburg Layered Suite (Bushveld Complex), levels of total Cr_2O_3 are upto ten times higher than its putative solubility (0.16% Cr_2O_3) in the magmas that many investigators haveaccepted as parental to the dunites, harzburgites, pyroxenites, and norites of the Lower, Lower Critical, andUpper Critical zones. Various existing models fail to explain this “chromium paradox” in so far as they mayexplain why chromite is precipitated but do not account for its abundance in many cumulates. Simple mixingcalculations and quantitative Adiabat_1ph runs support our model in which a heterogeneous (liquid + crystals)system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30% ofcrustal melts at depths appropriate to pressures between 10 and 4.5 kbars. We contend that crystal settling andthe effects of elutriation during magma ascent had generated a high SiO_2 cap with modal olivine, orthopyroxene,and chromite increasing with depth in the staging chamber. The magma from the high SiO_2 cap of thestaging chamber was first injected into the largely sedimentary Transvaal Supergroup, generating what is nowtermed the Marginal zone. Slurries of crystals plus liquid from the spasmodically rejuvenated main body of thestaging chamber were then injected into the shallower Bushveld chamber to form the Lower and Criticalzones. Chromite within the slurries introduced in this way could account for their high gross Cr content. Frequentreversals of normal evolutionary trends evident in the cumulates are attributable to the episodic natureof the influxes from the staging chamber. The lithologically diverse nature of the floors upon which the chromititelayers accumulated (peridotites, pyroxenites, norites, and anorthosites) was therefore more a function ofthe timing of emplacement of the Cr-rich slurries generated in the deeper reservoir than of specific evolutionaryprocesses within the Bushveld chamber itself.