PRECAMBRIAN PALEOMAGNETISM; STILL A DAUNTING, BUT OFTEN A HIGHLY REWARDING ENDEAVOR (Invited Presentation)

Major strides have been made by the paleomagnetic community in our understanding of Precambrian paleomagnetism over the past six or so decades since the early efforts by the giants who established paleomagnetic research. This year marks the 60^th anniversary of the classic 1957 papers by E. Irving “The origin of the palaeomagnetism of the Torridonian Sandstones of N.W. Scotland” (Phil. Trans. R. Astr. Soc. A, v250, p 100) and by K. Creer, E. Irving, and S.K. Runcorn “Geophysical interpretation of palaeomagnetic directions from Great Britain” (Phil. Trans. R. Astr. Soc. A, v 250 p144), where they report data from Precambrian sedimentary rocks and assess how these data may coherently link up with results from Paleozoic rocks. Paleomagnetists have much to celebrate in terms of the determination of robust, to relatively robust configurations of cratonic blocks over key time intervals in the Precambrian, and in particular the Proterozoic, as well as documenting the assembly and dispersal of large cratonic blocks (e.g., the Indian Subcontinent) to supercontinents (e.g., Columbia and Rodinia). Data from Precambrian rocks have provided us with a broader understanding of the geomagnetic field, including a reasonable estimate of when the field began to maintain an axial geocentric dipole configuration over moderate time intervals (possibly the earliest, most convincing evidence for such a configuration is from the ca. 2.473 Matachewan and ca. 2.446 Hearst dike swarms, central Canada). Some Precambrian rocks have yielded high-quality geomagnetic field paleointensity data, allowing attempts to define the generation of a stable geodynamo in the core (predicted to have begun by ~3.5 Ga, with evidence for an even older, pre-4.0 Ga age). Key advances have been made when high-precision numerical age data from igneous rocks (e.g., widespread dike swarms) are linked with high-quality paleomagnetic data from these rocks. Challenges remain, partly due to the relative dearth of Precambrian rocks capable of yielding high-quality, interpretable data with robust age assignments. There is always the question of whether a specific result has adequately averaged the geomagnetic field to provide a reliable paleomagnetic pole. Precambrian rocks are certainly not immune to remagnetization processes/events, including those in the Phanerozoic!