title: U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa.
abstract: Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U-Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U-Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.
authors.affiliation: Department of Earth Sciences, University of Toronto, 22 Russell St, Toronto, Ontario, Canada M5S 3B1.
authors.forename: James E
authors.initials: JE
authors.lastname: Mungall
authors.affiliation: Department of Earth Sciences, University of Toronto, 22 Russell St, Toronto, Ontario, Canada M5S 3B1.
authors.forename: Sandra L
authors.initials: SL
authors.lastname: Kamo
authors.affiliation: Bushveld Chrome Resources, Postnet Suite 911, Private Bag x153, Bryanston, Johannesburg 2021, South Africa.
authors.forename: Stewart
authors.initials: S
authors.lastname: McQuade