Back-thrusting and overturning of the southern margin of the 1.85 Ga Sudbury Igneous Complex at the Garson mine, Sudbury, Ontario

Abstract

The Garson deposit is one of several deformed magmatic Ni–Cu–platinum group element (PGE) deposits in the South Range of the 1.85Ga Sudbury structure. The deposits occur along the southeast limb of the folded Sudbury Igneous Complex (SIC), at the contact between the SIC basal norite and underlying Paleoproterozoic metabasalt and metasedimentary rocks of the Huronian Supergroup. At the Garson deposit inclusion-rich breccia and disseminated Ni–Cu–PGE sulfide ores are hosted by steeply south-dipping shear zones and splays that underwent two major ductile deformation events (D1 and D2). D1 is characterized by a steeply, south-dipping, S1 foliation and a down-dip L1 mineral stretching lineation defined by ferrotschermakite in metabasalt and by magnesiohornblende in norite. Coexisting ferrotschermakite and oligoclase in metabasalt indicate amphibolite facies conditions during D1. The shear zones formed along or near the SIC-Huronian contact during the 1.7–1.6Ga Mazatzal–Labradorian Orogeny. They formed as layer-parallel, north-dipping, north-over-south thrusts in response to flexural slip during buckling of the SIC. As the general transport direction was from south to north during the Mazatzal–Labradorian Orogeny, the D1 shear zones are back-thrusts with opposite transport direction. The thrusts imbricated the SIC, underlying Huronian rocks, and ore zones, and emplaced slivers of Huronian rocks and metabreccia into the overlying norite. In contrast to the D1 shear zones at Garson, the Thayer Lindsley and the regional South Range shear zones, which transgress the SIC-Huronian contact at a high angle, formed as moderately SE-dipping reverse shear zones as a result of localization of folding-induced strain near the hinge zone in order to accommodate further flattening and tightening of the SIC with progressive D1 shortening. Together with the southeast limb of the SIC, the D1 shear zones were overturned into their present steep southerly dips and were reactivated as south-over-north shear zones either later during the Mazatzal–Labradorian Orogeny or during the 1.5–1.45Ga Chieflakian event, which was coeval with accretion of juvenile Andean-style, calc–alkaline magmatic arcs along the entire southeastern margin of Laurentia. During this D2 reactivation event, S1 was transposed and locally preserved in crenulations bounded by a S2 chloritic shear foliation, which has a strong down-dip L2 mineral chlorite lineation and parallel L2 ductile slickenlines.