Palaeomagnetism of iron-formation from the late Palaeoproterozoic Frere Formation, Earaheedy Basin, Western Australia: palaeogeographic and tectonic implications

Abstract

Abstract Oriented cores of unweathered Frere Formation (1.9–1.8 Ga), including shaly and peloidal iron-formation, were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern Earaheedy Basin at the northeastern margin of the Yilgarn Craton. Anisotropy of magnetic susceptibility measurements indicated that the Frere Formation in this area was unaffected by regional metamorphism. The predominant magnetic mineral in the iron-formation is hematite but a small amount of magnetite also is present in some samples. Stepwise thermal demagnetisation was conducted for 263 specimens. Many specimens acquired spurious components of magnetisation after treatment to 300–500 °C, ascribable to the oxidation of siderite to magnetite on heating. However, other specimens displayed a stable, high-temperature (680 °C), dual-polarity component A carried by hematite that gave a bedding dip-corrected direction of D=314.3°, I=−24.8° (K=38.5, α95=2.6°, n=78). A fold test was positive at >99% level of confidence, indicating acquisition of remanence prior to Stanley folding in the Earaheedy Basin that occurred at ∼1.78 Ga near the end of Capricorn orogenesis. The presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the Frere Formation, located at 45.2°S, 220.0°E (dp=1.3°, dm=2.4°). Component A is ascribed to a chemical remanent magnetisation (CRM) acquired soon after deposition, and indicates a palaeolatitude of 13.0±1.5° for the Frere Formation. The Frere pole plots adjacent to, but is still significantly different from, late to post-Capricorn Orogeny overprint poles from the Hamersley Province of the southern Pilbara. However, vertical-axis relative anticlockwise rotation of 10–15° for the Frere pole brings it close to these Pilbara overprint poles. The data imply that the Earaheedy Basin and possibly the Yilgarn Craton underwent clockwise rotation of 10–15° relative to the Pilbara Craton during Stanley folding. This finding accords with evidence for tectonic settings marked by sinistral strike-slip deformation adjacent to the Yilgarn–Pilbara cratonic boundary during deposition and subsequent folding in the Earaheedy Basin. The data also imply that the West Australian and North Australian cratons were in close proximity by ∼1.8 Ga.