Impact melt dikes in the Sudbury multi‐ring basin (Canada): Implications from uranium‐lead geochronology on the Foy Offset Dike

Abstract

Abstract-To investigate the origin of Offset Dikes and their age relationships to major impact generated lithologies in the Sudbury multi-ring impact structure, such as the Main Mass of the Sudbury "Igneous" Complex, zircon and baddeleyite were dated by the V-Pb chronometer. The rocks analysed are one diorite and two quartz diorites from inside the Foy Offset, one quartz diorite from the contact zone, and two country rock samples collected at 10 and 30 m distances from the contact within the Levack Gneiss Complex. The 21 analysed zircon and baddeleyite fractions yield a crystallization age of 1852 +4/-3 (20-) Ma for the accessory minerals in the Foy Offset Dike and an age of2635 ± 5 Ma for the shocked Levack country rock, in which zircons show significant shock effects (multiple sets of planar fractures), in contrast to the totally unshocked zircons of the Offset Dike. Within given errors, the new age of 1852 Ma is identical to the pooled 1850 ± I Ma V-Pb age determined by Krogh et al. (1984) as the crystallization age of accessory phases in different lithologies of the Sudbury "Igneous" Complex, which has been interpreted to represent the coherent impact melt sheet of the Sudbury Structure. This excellent agreement of the ages substantiates that emplacement of the Offset Dikes occurred coevally with the formation of the impact melt sheet. Total absence of inherited zircons in the central part of the Foy Offset indicates melting of the precursor material at temperatures well above 1700 DC, which emphasizes the origin of the dike lithologies by impact melting.

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

“…Accordingly, a minimum initial temperature of 1800°C is assumed for the melt sheet, which allows for at least 400°of cooling to account for initial thermal equilibration. This is consistent with temperature estimates in excess of 1700°C, based on the absence and presumed absorption of inherited target rock zircons in fragments carried in the so-called Foy Offset [Ostermann et al, 1996].…”

Shallow impact: Isotopic insights into crustal contributions to the Sudbury impact melt sheet

“…The age-corrected Pb isotope ratios of these samples are distinct from all other samples, with very unradiogenic initial values. Uncertainty in the initial ratios includes propagated uncertainty in the measured Pb isotope ratios, l, decay constants (Jaffey et al, 1971) and crystallisation age (Krogh et al, 1984;Ostermann et al, 1996). In order to evaluate the contributions of target rocks to the melt sheet, it is necessary to constrain the isotopic composition of target rock groups at the time of impact, 1850 Ma.…”

Preservation of detrital shocked minerals derived from the 1.85 Ga Sudbury impact structure in modern alluvium and Holocene glacial deposits

“…Shocked zircons have since been reported in rocks from both the North and South Ranges; grains in the Levack gneiss, Murray granite, and the Copper Cliff rhyolite display multiple sets of crystallographically oriented fractures visible on external grain surfaces (Krogh et al, 1984Ostermann et al, 1996;Corfu et al, 2003). In addition to bedrock and bedrock clasts, shocked zircons with PFs and granular texture have also been documented in the matrix of samples from the Onaping Formation (Bohor and Betterton, 1992;Bohor et al, 1993).…”