Paleomagnetism of the Central Metasedimentary Belt, Grenville Province, Ontario: Differential uplift or collision of terranes?

Abstract

A paleomagnetic traverse across the Central Metasedimentary Belt of the Precambrian Grenville Province in southern Ontario, linked to an earlier study of the adjoining Central Gneiss Belt, sampled 54 sites in the Frontenac, Sharbot Lake, Mazinaw, Elsevir and Bancroft terranes. 40Ar/ 39Ar thermochronometry indicates differential unroofing of these terranes, with slow cooling (1.0–1.5 °C/Ma) along parallel temperature–time paths. Presently exposed rocks cooled through ≈ 500 °C around 1100 Ma (Frontenac), 1010 Ma (Elsevir), 960 Ma (Central Metasedimentary Belt boundary zone) and 930 Ma (Mazinaw) [Cosca, M.A., Sutter, J.F., Essene, E.J., 1991. Cooling and inferred uplift/erosion history of the Grenville orogen , Ontario: constraints from 40Ar/ 39Ar thermochronology, Tectonics 10 (1991) 959–977]. 1100–930 Ma was a time of rapid paleolatitude shift but the various terranes do not have distinct paleomagnetic signatures. Frontenac, Sharbot Lake and Mazinaw terranes have similar reverse-polarity paleopoles in the central part of the Grenville polar wander track. Frontenac and Sharbot Lake normal poles are typical ≈ 1000 Ma Grenville A poles, while Mazinaw normal poles seem to be somewhat older. Frontenac poles bear no resemblance to 1100 Ma Keweenawan poles. The Elsevir pole and some Frontenac poles resemble Grenville B poles (600–800 Ma). Although 40Ar/ 39Ar hornblende ages were reset thermally between 1100 and 930 Ma during differential uplift of terranes, the terranes were not remagnetized at these same times. Frontenac, Sharbot Lake and Mazinaw terranes (like much of the boundary zone and the Central Gneiss Belt) seemingly were all remagnetized between ≈ 1000 and ≈ 900 Ma, perhaps by a widespread thermochemical event. A 600–800 Ma event affected Elsevir and part of the Frontenac, at the western and eastern extremes of the traverse, but not areas between. The lack of correlation between 40Ar/ 39Ar ages and paleomagnetic results for the Frontenac, Sharbot Lake, Mazinaw and Elsevir casts doubt on the thermochronometric age calibration of the Grenville apparent polar wander path, which is based on the assumption that the magnetic and isotopic systems respond in similar ways to slow cooling during uplift so that their blocking temperatures can be directly compared. The most discordant results – those of the Frontenac – could be reconciled if the Frontenac terrane acquired its A magnetization around 1100 Ma before it docked with Laurentia. This is geologically plausible but the match between the Frontenac poles and 900–1000 Ma Grenvillian poles seems too close to be coincidental.