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Abstract

It is likely that Archaean cratons of Laurentia had different palaeogeographic histories prior to their amalgamation. New palaeomagnetic, geochronological and geochemical evidence supports a reconstruction of the Wyoming craton adjacent to the southern margin of the Superior craton at 2.16 Ga, before rifting (c. 2.1–2.0 Ga) and eventual reamalgamation after the Hudsonian Orogeny (c. 1.8 Ga). U–Pb ages (TIMS on baddeleyite) from five dykes yield two groups of ages at c. 2164 and 2155 Ma. The younger group of ages defines the Rabbit Creek swarm at 2161–2152 Ma and precisely dates its palaeomagnetic pole. Two large and differentiated dykes (>100 m) in the Bighorn and Wind River uplifts are geographically related to the Rabbit Creek swarm but have slightly different orientations and yield slightly older ages at 2171–2157 Ma. These dykes may be parts of a single intrusion (the ‘Great Dyke of Wyoming’) that spans over 200 km between uplifts, possibly representing a different magmatic event. This older event does not have enough distinct intrusions to provide a correctly averaged palaeomagnetic pole, but correlates with magmatism in the Superior craton and has a palaeomagnetic remanence comparable to the Rabbit Creek dykes. With minor tilt corrections, the palaeomagnetic data from the Rabbit Creek swarm and Powder River–South Pass dykes support a reconstruction of the southeastern Wyoming craton against the southern Superior craton. This fit juxtaposes the Palaeoproterozoic Huronian and Snowy Pass Supergroups along two passive margins that experienced a prolonged period of mafic magmatism (>100 myr) and rift basin development. Although there are slight geochemical variations across the Rabbit Creek swarm, all dykes fit into two distinct groups that are independently dated and internally consistent.

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Supporting figures and locality tables are available at www.geolsoc.org.uk/SUP18824

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