The South Range Breccia Belt of the Sudbury Impact Structure: A possible terrace collapse feature

The South Range Breccia Belt (SRBB) is an arcuate, 45 km long zone of Sudbu ry Breccia in the South Range of the 1.85 Ga Sudbury Impact Structure. The belt varies in thickness between tens of meters to hundreds of meters and i s composed of a polymict assemblage of Huronian Supergroup (2.49-2.20 Ca), Nipissing Diabase (2.2 Ga), and Proterozoic granitoid breccia fragments ran ging in size from a few millimeters to tens of meters. The SRBB matrix is c omposed of a fine-grained (similar to 100 mu m) assemblage of biotite, quar tz, and ilmenite, with trace amounts of plagioclase, zircon, titanite, epid ote, pyrite, chalcopyrite, pyrrhotite, and occasionally chlorite. The SRBB hosts the Frood-Stobie, Vermilion, and Kirkwood quartz diorite offset dykes , the former being associated with one of the largest Ni-Cu-PGE sulphide de posits in the world. Optical petrography and whole-rock geochemistry concur with previous studie s that have suggested that the matrix of the SRBB is derived from comminuti on and at least partial frictional melting of the wall rock Huronian Superg roup lithologies. Rare earth element (REE) data from all sampled lithologie s associated with the SRBB exhibit crustal signatures when normalized to C1 chondrite values. Additionally, REE data from the quartz diorites, dissemi nated sulphides in Sudbury Breccia, and a sample of an aphanitic biotitehor nblende tonalite dyke exhibit flat slopes when compared to the mafic and fe lsic norites, quartz gabbro, and granophyre units of the Sudbury Igneous Co mplex (SIC), which suggests that these lithologies are representative of bu lk SIC melt. We suggest that the SRBB was formed by high strain-rate (>1 m/s), gravity-d riven seismogenic slip of the inner ring of the Sudbury Impact Structure du ring postimpact crustal readjustment (crater modification stage). Failure o f the hanging wall may have facilitated the injection of bulk SIC melt into the SRBB, along with the Ni-Cu-PGE sulphides of the Frood-Stobie deposit. Postimpact Penokean (1.9-1.7 Ga) tectonism, particularly northwest-directed shearing along the South Range Shear Zone and associated thrust faulting, could account for the present subvertical orientation of the SRBB, and the apparent lack of a connection at depth with the SIG.