Magnetic properties and paleointensity of pseudotachylytes from the Sudbury structure, Canada: Petrologic control

Since a pseudotachylyte is generated through a combination of comminution and frictional melting, it may record an ambient Earth's magnetic field as a thermal remanence during the cooling from friction melt. To test the reliability of determining the paleointensity, the Coe-modified Thellier method was carried out on a small suite of pseudotachylytes in large-displacement fault zones from the North-range Sudbury impact structure. Pseudotachylytes studied here are classified into two types in their protolith lithology: gneiss-origin and granitoid-origin pseudotachylyte. Petrologic observations confirm the presence of abundant fine-grained magnetite inclusions and coarse-grained titanomagnetites with exsolved ilmenite lamella in gneiss-origin pseudotachylyte. In granitoid-origin pseudotachylyte, we found a scatter distribution of multidomain magnetite. Although the granitoid-origin pseudotachylyte fails to estimate the paleointensity, the gneiss-origin pseudotachylytes yield weak paleointensity values, which are consistent with previous paleointensity data from the norite in Sudbury Igneous Complex. These results suggest that the protolith petrology controls a magnetic mineralogy in pseudotachylytes and the reliability as recorder of ancient Earth's magnetic field.