Paleointensity determination on the Late Precambrian Tudor Gabbro, Ontario

Abstract

Paleointensity determination by the Coe‐modified Thellier method was carried out on the ≈1100 Ma Tudor Gabbro of southern Ontario. The Tudor has a bivectorial natural remanent magnetization (NRM). The C NRM is a Grenvillian uplift thermoviscous remagnetization which overprints the A NRM, the surviving primary thermoremanent magnetization (TRM) of the Tudor intrusion. A sharp junction at ≈530°C between the A and C vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480–500°C in nature when the difference in time scales is taken into account. The unblocking temperature distributions of the A NRM and of laboratory TRM also match. A is carried by single‐domain or small pseudo‐single‐domain grains, probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580°C. Arai plots of NRM versus partial TRM are linear from 520°C or 530°C to 580°C for 45 specimens of 19 samples from 9 sites. The 45 reliable paleointensity values from these specimens give a virtual axial dipole moment (VADM) of 4.6±0.8 × 1022 A m2 around 1100 Ma. This is about one half of the modern field intensity but is similar to the mean Cretaceous‐Cenozoic intensity and to most Archean and Proterozoic VADMs. Keweenawan rocks, whose ages are within ≈20 Myr of the Tudor Gabbro, have a VADM of 11.4×1022 A m2. The difference between the Tudor and Keweenawan VADMs is similar to differences recorded over similar time intervals in the Cretaceous and seems to represent normal variation in geomagnetic field strength throughout Earth's history.