Abstract

A detailed investigation of approximately 600 specimens of Sudbury norite has isolated seven distinct remanence directions that coexist in this single rock unit. Complex thermal demagnetization behavior indicates that many of these directions were acquired by a chemical magnetization process. It was not possible to erect a chronology of remanence acquisition events based purely on unblocking temperature relationships. Rather a chronology was established by making use of the degree of alteration and deformation of differing specimens, the areal distribution of a remanence phase, and the relationship of the remanence direction to local geological structures.The seven distinct remanence phases (N1 to N7) suggest that at least six post-crystallization events have been of sufficient local severity to leave a lasting imprint on the magnetic record of the Sudbury norite. The absolute timing of these events is unknown. All deformation was complete prior to the intrusion of the l,250-m.y.-old Sudbury dikes.Initially the (N1) Sudbury norite occupied an open syncline, with limbs dipping approximately 20 degrees , which plunged gently to the southwest. Next, tectonism (F 1 ) produced a crudely symmetrical basin, with all limbs dipping inward between 40 degrees to 50 degrees . Completion of this tectonic event is shown by the regional metamorphic event recorded as N2 on the south, east, and west ranges. On the south range where N2 is dominant, metamorphism was nodal: however, some zones of N1 survived with no trace of the development of N2. N2 was followed by a second deformational event (F 2 ) which produced rotations of the east and south ranges. After this second deformation, N3 and N4 were imprinted on the norite. The mechanisms that gave rise to these magnetizations are uncertain. However, the dominant distribution of N3 in the uppermost parts of the norite suggests that it is somehow related to a later development of the oxide-rich gabbro, or transition zone. Further investigations of this suggestion are ongoing. After acquisition of N3 and N4, the final deformation (F 3 ) involved a block rotation of the south range relative to the north range.Two phases of remanence-carrying sulfide have been recognized as N4 and N5. The time of formation of the N5 phase is uncertain. The sulfide phase associated with N4 postdates the F 1 and F 2 tectonic events. The significance of N6 is not presently understood, but it should become clear once further investigations in the Sudbury basin have been completed. The last event, N7, records a thermal event, related to uplift along Grenville front.

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