Paleomagnetism and U–Pb geochronology of the Black Range dykes, Pilbara Craton, Western Australia: a Neoarchean crossing of the polar circle

ABSTRACT

We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS ²⁰⁷Pb/²⁰⁶Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP ²⁰⁷Pb/²⁰⁶Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α₉₅ = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A₉₅ = 15.0°. The pole's reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.

KEYWORDS:

• paleomagnetism
• U–Pb geochronology
• Black Range Dolerite Suite
• Pilbara craton
• Western Australia
• Neoarchean

Acknowledgements

We thank D. Chad Moore, Ian Rose and Taylor Kilian for field assistance; Martin Van Kranendonk for logistical advice; and Jennifer Kasbohm, Blair Schoene and Michael Wingate for discussions during preparation of the manuscript. Michael Wingate and Sergei Pisarevsky are also thanked for their constructive reviews of the manuscript. Funding was provided by the U.S. National Science Foundation, Swedish Research Council, Royal Physiographic Society in Lund, and Yale University.

Disclosure statement

No potential conflict of interest was reported by the authors.