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

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Abstract The Kidd Creek copper-zinc-silver deposit, situated in the Late Archean Abitibi greenstone belt of northeastern Ontario, represents one of the world’s largest and highest grade volcanogenic massive sul-fide deposits. Its discovery, in late 1963, not only marks a major event in Canadian exploration history, but is also intimately linked to the ascendence and successful application of the syngenetic volcanogenic concept in Canada. Events leading up to and surrounding the discovery are documented and provide a historical background to other contributions in this volume on the geology of the Kidd Creek deposit.

Abstract Results from a comprehensive U-Pb geochronology study of the Kidd Creek deposit and the surrounding Kidd Volcanic Complex are presented. Eleven new zircon and two titanite ages are reported and integrated with U-Pb age results on five related samples, which were published in a previous study. Zircon ages for rhyolite volcanism of the Kidd Volcanic Complex range from 2717.0 + –2 2 . . 5 6 to 2711.5 ± 1.2 Ma. This age range is established on immediate footwall and hanging-wall rhyolites of the Kidd Creek orebody. Since both footwall and hanging-wall rhyolites can be linked to ore formation, the conclusion must be that the giant Kidd Creek deposit is the product of unusually long-lived, albeit episodic, sea-floor hydrothermal activity. Our most precise estimate for the age of footwall rhyolites at Kidd Creek is provided by results on two nearby rhyolites that have been dated at 2716.1 ± 0.6 and 2716.0 ± 0.5 Ma, respectively. The age of at least one large massive sulfide lens at Kidd Creek, the North orebody, has been tightly bracketed between the age of footwall rhyolites and the age of an overlying rhyolite lapillistone horizon dated at 2715.8 ± 1.2 Ma. Hence, ore formation of individual massive sulfide lenses appears to have been rapid and well within the resolution limits of current U-Pb dating techniques. Based on the large number of ages, it appears that volcanic activity of the Kidd Volcanic Complex can be divided into four general phases, each of which is supported by at least one or more high-precision ages: phase I, onset of bimodal komatiite and rhyolite volcanism, probably as early as 2717.7 ± 1.1 Ma, and extrusion of the footwall assemblage at Kidd Creek at ca. 2716 Ma; phase II extrusion of ca. 2714 Ma rhyolites; phase III, extrusion of ca. 2711 Ma rhyolite, including the quartz porphyritic hanging-wall rhyolite at Kidd Creek; and phase IV, extrusion of the hanging-wall basalt sequence and intrusion of subvolcanic gabbro sills sometime after 2711 Ma. The Kidd Creek deposit probably formed along the axial zone of a slow-spreading rift basin that developed during extension of an older volcanic-arc assemblage. This older arc assemblage, which included ca. 2723 and 2735 Ma components, is probably represented by the Deloro Group and correlative assemblages exposed to the south of Timmins. Rifting of the older volcanic substrate, and partial melting to produce the rhyolites, was induced by the arrival of a hot mantle plume that gave rise to the komatiites of the Kidd-Munro assemblage. Graywacke turbidites in the Kidd Creek area are all younger than ca. 2699 Ma and do not form the deeper stratigraphic footwall to the deposit. Instead, the graywackes probably overlie, unconformably to disconformably, all the volcanic assemblages in the region. From the sub -sequent protracted structural-metamorphic evolution in the area, two discrete events have been dated: the 2663.3 ± 3.3 Ma intrusion of the Prosser porphyry granitoid stock and a discrete 2639.1 ± 7.2 Ma metamorphic-hydrothermal event. The timing of both events corresponds closely to ages for granulite facies metamorphic events in lower crustal rocks of the nearby Kapuskasing structural zone.