Paleomagnetism of late Archean rocks of Hamersley basin, Western Australia and the paleointensity at early Proterozoic

We report the results of paleomagnetic and rock magnetic measurements of late Archean rocks from Hamersley basin, Western Australia, and the paleointensity determination for early Proterozoic. Basalts and banded iron formations have two to four components of remanent magnetization, and have consistent directions for different localities, indicative of geomagnetic origin. Rock magnetic measurements of basalts reveal that the main magnetic mineral is a fine-grained magnetite present in similar to 10 ppm in mass, which do not alter when heated up to its blocking temperature. We interpret the stable component up to similar to 390 degreesC as the post-tilting thermal overprint from uplift at similar or equal to2.0 Ga, and the higher temperature component as the pre-tilting thermoviscous remanence during burial metamorphism. From the Thellier type paleointensity experiments using the thermal overprint component, we obtain a mean virtual dipole moment (VDM) estimate of (1.8-3.6) x 10(22) A m(2). This suggests that early Proterozoic was characterized by a 1 weak geomagnetic field of less than one-half of the present. (C) 2001 Elsevier Science B.V. All rights reserved.