Impact melting and 1850-Ma offset dikes emplacement in the Sudbury impact structure: Constraints from zircon and baddeleyite U-Pb ages

Details Impact melting and 1850-Ma offset dikes emplacement in the Sudbury impact structure: Constraints from zircon and baddeleyite U-Pb ages Impact melting and 1850-Ma offset dikes emplacement in the Sudbury impact structure: Constraints from zircon and baddeleyite U-Pb ages

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.513o&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 513

Computer Science

1

Impact Damage, Lead Isotopes, Meteorite Collisions, Radioactive Age Determination, Rock Intrusions, Uranium Isotopes, Zirconium, Breccia, Cooling, Lithology, Temperature Effects

Scientific paper

The main mass of the Sudbury Igneous Complex is interpreted to represent the differentiated central part of a coherent impact melt sheet. At its base, a crystalline melt breccia forms a discontinuous thin layer (sublayer). An extensive dike system occurs along the boundary of this melt sheet reaching outward from the main mass of the Sudbury Igneous Complex into the country gneisses. These 'offset dikes' can be distinguished in three types: radial dikes, dikes occurring parallel and concentric to the Igneous Complex, or melt pockets of discontinuous occurrences. The dominant lithology of these 'offset dikes' is of quartz dioritic composition, with either hypersthene, clinopyroxene, or amphibole as the dominant mafic mineral. Proximal to the Igneous Complex, the offset dikes contain a variety of crystalline melt breccia from the sublayer, and they show sharp contacts with the country rocks. Although several models describe the genetic relationship between the Igneous Complex, the sublayer, and the offset dikes, exact age relationships between them are still unclear. To improve these models, we have analyzed zircon and baddeleyite from four different rocks, collected from the Foy offset dike. This 1849-Ma zircon-baddeleyite age of the Foy offset dike is in excellent agreement with earlier U-Pb zircon and baddeleyite ages from the Sudbury Igneous Complex, giving values of 1850.0 +/- 1.3 Ma for a black norite, and 1850.0 +/- 3.4/-2.4 Ma for mafic nortie. This coincidence in ages substantiates that the offset dikes are an impact-melt product that invades the unmelted country gneisses and their replacement occurred coevally with formation of the coherent impact-melt layer. The absence of older, inherited zircon components requires total melting of the Archean gneiss target rocks at temperatures in excess to 1700 C, and indicates that cooling occurred on a timescale that was too short to allow country rock assimilation even in the contact zones.

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