Unraveling Archean magnetic histories by a comparison of whole rock and single crystal analyses of granites from Western Australia

Obtaining directional and paleointensity data from Precambrian age igneous rocks is often difficult because of unfavorable magnetic mineralogy - including multidomain (MD) magnetite- and the propensity of these grains to record thermoviscous overprints. Bono and Tarduno (2015) applied the method of single silicate crystal analysis to the study of the 565 Ma Sept-Îles Complex to isolate magnetization directions carried by single domain needles hosted in plagioclase feldspar (identified by high resolution scanning electron microscopy) whereas Cottrell et al. (2016) reported paleomagnetic directions carried by Fe-Cr grains in fuchsite of the Jack Hills metaconglomerate (also identified by scanning electron microscopy). Here we apply these techniques to the study of Late Archean and Paleoarchean granites of the Yilgarn and Pilbara cratons, respectively. Whole rock data from ca. 2.6 Ga granite from the Jack Hills show significant scatter, as expected given potential MD carriers. Nevertheless, there we find no evidence for a ca. 1 Ga overprint claimed by Weiss et al. (2015). Scanning electron microscopy reveals that feldspar and quartz from this granite contain ubiquitous opaque Fe-Ti oxide inclusions. Select isolated grains (approximately 0.5 to 1 mm in size), chosen to avoid large, potentially MD inclusions, have pseudo-single domain like magnetic hysteresis properties, and have natural remanent magnetization (NRM) intensities of 1 x 10^-8 emu to 1 x 10^-9 emu which are measurable with the University of Rochester ultra high resolution small bore 3 component DC SQUID magnetometer. Whole rock study of Paleoarchean granites from the Pilbara region reveal the previously undocumented influence of 2.7 Ga overprints most likely related to the emplacement of the Fortescue basalts. We find that feldspars and quartz from these granites also yield pseudo-single domain magnetic hysteresis behavior. Selected crystals from these granites yield NRM intensities of 1 x 10^-8 to 1 x 10^-7 emu. CO₂ laser demagnetization results from single silicate samples from both of these granites will be discussed.