Thermal overprinting of natural remanent magnetization and K/Ar ages in metamorphic rocks.

Abstract

Burial metamorphism generates secondary components of NRM (natural remanent magnetization). Often two or more remanences are found superimposed in a single geological unit. We trace the steps necessary to resolve and date such magnetic overprints in three studies of intensely metamorphosed rocks from the Grenville Province of the Canadian Precambrian Shield. First, it is shown that the various components may be associated with different rock-forming minerals and can be separated by orienting aggregates of dark or light grains in coarse-grained samples. Next, these magnetizations are shown to have originated at least in part as TRM’s (thermoremanent magnetizations). Their relative ages can then be assigned on the basis of blocking temperatures. Finally, we estimate their absolute ages using the 40Ar/39Ar stepwise heating technique, taking into account the effects of a long heating-cooling history on both the magnetic and radiometric systems.