THE PALEOPROTEROZOIC LEOPARD DIKES OF MONTANA AND WYOMING: A DISMEMBERED FRAGMENT OF THE 2.45 GA HEARST-MATACHEWAN GIANT RADIATING DIKE SWARM OF THE SUPERIOR CRATON?

A unique Paleoproterozoic WNW-trending dike swarm is found in the northern Bighorn Mountains of Wyoming and Beartooth Mountains of northwestern Wyoming/Montana. The dikes are porphyritic diabase with dyke parallel plagioclase rich interior zones. The plagioclase in these zones are subhedral to euhedral and are up to 8 cm in diameter, consisting of up to 60% of the volume of the rock. The concentration of the white saussuritized plagioclase in these zones imparts a spotted appearance to the dikes, thus prompting the name “leopard dikes”. The concentration of plagioclase into the central zones is probably due to flow differentiation during a single episode of dike emplacement. Although there have been attempts to date the dikes by Rb-Sr and K-Ar methods, their age has not been reliably determined but they are probably Paleoproterozoic in age. The overall appearance and characteristics of the dikes are similar to dikes of the 2.48-2.45 Ga Matachewan-Hearst dike swarm of the Superior craton. Recently, Heaman (1997) suggested the Wyoming craton leopard dikes may have been emplaced during a major mafic magmatic that included emplacement of layered mafic complexes, giant radial dike swarms, and flood basalts within the Superior and Karelia cratons, which together were part of the supercontinent Kenorland. Recently published isotopic dates of ca. 2.48 Ga from the Blue Draw metagabbro form the Black Hills of the Wyoming craton (Dahl et al., 2003) may also be related to this event. Limited paleomagnetic data from the Wyoming craton have been argued to be consistent with this model (Heaman, 1997), but the data must be considered to be unreliable. However, recently published paleomagnetic and geochronologic data suggest that the Wyoming and Superior cratons were adjacent at about 2.2 Ga. A paleogeographic reconstruction following Harlan et al. (2003) is consistent with the idea that the Wyoming craton leopard dikes could have been part of the giant radial Matachewan-Hearst dike swarm and place the Blue Draw metagabbro near the plume center. Subsequent breakup and rifting at about 2.15 Ga has since dispersed these continents and the records of this mafic magmatic event. New paleomagnetic and geochronologic data from the Wyoming craton leopard dikes should allow test of such models and thus could prove valuable in assessing the viability of the Kenorland plate reconstruction.