NRM-IRM(S) DEMAGNETIZATION PLOTS OF INTRUSIVE ROCKS AND THE ORIGIN OF THEIR NRM

Alternating field demagnetization characteristics of laboratory-induced thermal remanence (TRM) and saturation isothermal remanence (IRM(s)), have been determined for a collection of samples representing the major intrusive rock types. These results are compared to NRM: IRM(s) curves for the same intrusive specimens, and for a smaller number of extrusive and high-grade metamorphic rocks samples. A number of the intrusive rock specimens which contained primary magnetic mineralogies display NRM: IRM(s) curves which are similar in character to their TRM: IRM(s) counterparts. The NRMs of these samples are generally unidirectional and resistant to thermal demagnetization, suggesting their thermal origin. Intrusive samples with dissimilar NRM: IRM(s) and TRM: IRM(s) curves are often directionally unstable, or contain secondary or altered magnetic mineralogies. In contrast to the mixed behavior of the intrusive samples, the NRMs for a collection of extrusive rock samples are generally TRM-like, whereas the NRMs of high-grade metamorphic rock samples more often display characteristics which are inconsistent with a simple thermal origin of NRM. This study suggests that NRM: IRM(s) curves may be useful in evaluating whether the rémanent magnetization of crystalline rocks is predominantly of primary or secondary origin. © 1990.