Emplacement of metabreccia along the Whistle offset dike, Sudbury: implications for post-impact modification of the Sudbury impact structure

Abstract

The Whistle offset dike is a radial dike linked to the ca. 1850 Ma Sudbury Igneous Complex (SIC) in Sudbury, Ontario. It is linked through the funnel-shaped Whistle embayment which formed as a depression along the floor of a superheated impact melt sheet represented by the present-day SIC. The dike consists of metabreccia, quartz diorite, and inclusion-bearing quartz diorite, emplaced in older Archean gneisses and intrusions. Metabreccia has a fine-grained, igneous-textured matrix surrounding Archean basement clasts. It overlaps in major-element compositions (MgO, CaO, SiO 2 ) with the two quartz diorites. The three have similar chondrite-normalized rare-earth element patterns characterized by pronounced negative slopes, i.e., (La/Yb) N = 17-29, and a weak negative Eu anomaly. This suggests that they originated from the same source melts. Metabreccia has higher high-field-strength element (HFSE) ratios, i.e., Gd/Yb, Ce/Yb, Zr/Y, and lower model initial 207 Pb/ 204 Pb isotopic ratios than the two quartz diorites, suggesting that it assimilated more Archean rocks with higher HFSE ratios and lower 207 Pb/ 204 Pb ratios. It formed as SIC melts mixed with melted basement rocks at the SIC-basement contact. This produced a strongly contaminated, inclusion-bearing melt, which drained downward into an offset structure or fault, entrained more fragments plucked from the walls of the fault, and crystallized as metabreccia. Less contaminated quartz diorite was then emplaced in the still-viscous metabreccia during reactivation of the fault caused by isostatic readjustment of the gravitationally unstable crater. More melting, assimilation of clasts, and mixing of SIC melts at the SIC-basement contact produced inclusion-bearing quartz dioritic melts that were emplaced last along the Whistle offset fault.Résumé : Le dyke décalé de Whistle est un dyke radial relié au complexe igné de Sudbury (CIS), Ontario,~1850 Ma. Il y est relié par l'échancrure de Whistle, en forme d'entonnoir, qui s'est formée comme une dépression le long du plancher d'une couche surchauffée de fusion d'impact représentée par le CIS actuel. Le dyke comprend une métabrèche, une diorite quartzique et une diorite quartzique avec des inclusions, mises en place dans d'anciens gneiss et intrusions de l'Archéen. La métabrèche a une matrice à grains fins à texture ignée entourant des clastes du socle archéen. Sa composition d'éléments majeurs (MgO, CaO, SiO 2 ) chevauche celles des deux diorites quartzitiques. Les trois ont des patrons similaires d'éléments de terres rares, normalisés pour la chondrite, caractérisés par des pentes négatives prononcées, soit (La/Yb) N = 17 à 29, et une faible anomalie négative Eu. Cela suggère qu'ils proviennent des mêmes sources de roches en fusion. Les rapports des éléments à forte liaison atomique (HSFE), soit Gd/Yb, Ce/Yb, Zr/Y, de la métabrèche sont plus élevés et ses rapports initiaux modèles 207 Pb/ 204 Pb sont moins élevés que ceux des deux diorites quartziques, suggérant qu'elle a assimilé plus de roches archéennes avec des r...

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

“…; Lafrance et al. ). In the earliest studies, the texture of the Breccia was described as metamorphic, with some evidence of partial melting (Greenman ; Dressler ).…”

“…Petrographic and petrological analyses of the morphologically similar Metabreccia within the Whistle Offset Dike by Lafrance et al. () demonstrate a co‐genetic origin of the breccias as reworked and metamorphosed basement rocks.…”

“…, ; Lafrance et al. ). Of the three breccia types, the Footwall Breccia is of particular interest in this study and is, thus, described in more detail.…”

“…; Lakomy ; Lafrance et al. ) emphasized the importance of partial melting in the formation of the breccia (Lakomy ; McCormick et al. ; Lafrance et al.…”

“…), and (3) partial melting of target rocks in contact with the cooling impact melt sheet (Coats and Snajdr ; Lafrance et al. ). Although these hypotheses are based in part on microstructural analysis, none of the studies systematically investigated the microstructure of the thermal aureole including the Footwall Breccia to elucidate its origin.…”