Occurrences of pseudotachylyte at the East Bull Lake and Shakespeare–Dunlop intrusions, Ontario, Canada

Abstract

Pseudotachylyte bodies were recently identified within and adjacent to the Early Proterozoic East Bull Lake and Shakespeare–Dunlop intrusions, located approximately 25–40 km west-southwest of the western margin of the Sudbury Igneous Complex. These breccia-like bodies locally form extensive vein networks and are preferentially developed along the contact between the intrusions and older Archean granitoid rocks. The pseudotachylyte veins comprise variable proportions of locally derived rock fragments and an aphanitic to fine-grained crystalline matrix that commonly displays flow textures. The veins appear to have formed by intense cataclasis and (or) frictional melting. These occurrences are very similar in appearance to Sudbury Breccia dykes that are observed at a radial distance of up to 80 km from the Sudbury Igneous Complex. Sudbury Breccia is widely believed to have formed as a result of the Sudbury event—a cataclysmic explosion that occurred at 1.85 Ga. The location of the pseudotachylyte veins described herein may coincide with one of the concentric bands of relatively intense Sudbury Breccia development observed to the north of the Sudbury Igneous Complex.

"Sudbury Breccia" at Whitefish Falls, Ontario: evidence for an impact origin

“…Nipissing diabase intrusions cut all early structures, indicating termination of the Blezardian orogeny by ca. 2219 Ma (Riller et al 1999 (Peredery and Morrison 1984); 2, Lake Temagami (Simony 1964); 3-5, Superior Province northeast of the Sudbury Igneous Complex (Thompson and Spray 1994); 6 and 7, East Bull Lake and Shakespeare-Dunlop intrusions (Chubb et al 1994); 8, Emo, Rhodes, and Botha townships (Dressler 1979); 9, Whitefish Falls (Card 1984; this study) (modified from Bennett et al 1991;Spray and Thompson 1995). producing tight, doubly plunging, upright folds (Card 1984;Zolnai et al 1984;Riller et al 1999). The development of an east-trending cleavage, axial planar to these folds, is characteristic of late-stage Penokean deformation (Zolnai et al 1984) and is clearly recognized in the Whitefish Falls area (Card 1978(Card , 1984 this study).…”

“…Examples of Sudbury breccia, regionally distributed throughout the country rock, define the extent of the Sudbury Structure (Grieve et al 1991). Examples of Sudbury Breccia have been found at 25, 40, and 80 km to the northwest in the Superior Province (Thompson and Spray 1994); in Huronian-aged sediments -90 km to the northeast at Lake Temagami (Simony 1964); at the East Bull Lake and Shakespeare-Dunlop intrusions 25-40 km to the southwest (Chubb et al 1994); and at Whitefish Falls, 70 km to the southwest (Fig. 1).…”