U-Pb geochronology of the sublayer environment, Sudbury Igneous Complex, Ontario

Cu-Ni sulfide deposits of the Sudbury Igneous Complex are hosted by the inclusion-rich sublayer, which occurs discontinuously between the footwall and main mass of the Sudbury Igneous Complex, by brecciated footwall in the vicinity of the Sudbury Igneous Complex, and by offset dikes protruding for tens of kilometers into the country rocks. The genesis of mafic-ultramafic inclusions in the sublayer and the relationships between the sublayer, main mass, and offset dikes have long been at the center of debate on Sudbury geology. This paper reports U-Pb isotope data on zircon and baddeleyite from samples representing norite matrix, melanorite and olivine melanorite pods, a metapyroxenite inclusion, and diabase from the sublayer at the Whistle embayment. These samples yield ages ranging from 1848.1 +/- 1.8 to 1849.1 +/- 1.1 Ma, similar to a U-Pb ape of 1849.8 +/- 2.0 Ma obtained for zircons in quartz diorite from the Copper Cliff offset, and indistinguishable from the previously published average age of 1850 +/- 1 Ma for the main mass of the Sudbury Igneous Complex. None of the samples yield evidence for inherited zircon components. The isotopic data prove that the exotic mafic-ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive, moderately Mg-rich magmas that crystallized in the early stages of development of the Sudbury Igneous Complex. Such Mg-rich cumulates are unlikely to have crystallized from melted average upper crust.