Intrusive and tectonic history of the Sudbury micropegmatite; the evidence from paleomagnetism

A paleomagnetic analysis of over 600 specimens of Sudbury micropegmatite indicates the presence of five distinct remanence directions. Four of these directions are similar to directions previously reported from the Sudbury norite. A chronology of remanence acquisition events was erected on the basis of one positive contact test and the relationship of the remanence direction to regional geological structures, with additional information from blocking temperature spectra. Integrating this information with that already reported from the Sudbury norite makes it possible to construct a more complete model for the tectono-metamorphic evolution of the Sudbury irruptive.The Sudbury irruptive magma was intruded into an open syncline. As the lower portions of the magma chamber cooled below the Curie point, the noritic unit retained an N1 remanence direction. A basinwide subsidence (F 1 ) of the magma body, which produced a symmetrical basin in the norite, was followed by the intrusion of a differentiate of the original irruptive magma--a differentiate micropegmatite. This is recorded as M2 in the micropegmatite and as the metamorphic overprint N2 in the norite. As might be expected, the deeper levels (south range) suffered the most extensive alteration during this event. During a second folding event (F 2 ) which followed, the south range was rotated by 32 degrees , the east range by 23 degrees . This new data from the micropegmatite shows that the north range was also rotated (12 degrees ) during this event (this was not detected in the norite data). All rotations are toward the center of the basin. A second phase of micropegmatite (MPG2) was later intruded along the boundary between the norite and the overlying Onaping, thus disrupting the initial micropegmatite phase. These two phases of micropegmatite probably correspond to the granophyric and plagioclase-rich phases recognized by Peredery and Naldrett (1975). A later remanence event (M6 + N6) which is overprinted on both the norite and the micropegmatite may have originated from a separate intrusion in the transition zone.The exact timing of these events is uncertain. The initial norite intrusion, differentiation, and subsidence and the first micropegmatite intrusion probably occurred close to the radiometric age for the norite ([asymp] 1,850 to 1,950 m.y.). The age of the second intrusive event is less certain. It may postdate some of the transbasin faulting. Gibbins and McNutt (1975) have argued that the 1,680-m.y. age for the micropegmatite dates a regional metamorphic event. If valid, this places the intrusion of the second micropegmatite phase at some point in the interval of 1,680 to 1,850 m.y. ago.