MULTIPLE PHASES OF WESTERN LAURENTIAN RIFTING: ABORTED EARLY EDIACARAN-CAMBRIAN VOLCANIC EVENTS?

Western Laurentian magmatic rocks contain geochemical and petrographic signatures that suggest that one or more rifting processes developed during the Ediacaran and early Cambrian and were aborted shortly afterwards. Depositional remnants of these volcanic events are preserved in Ediacaran-Cambrian successions in the Western Cordillera in Sonora, Mexico (El Arpa, La Cienega and Cerro Rajón fms), California (Wyman, Reed fms), Nevada (Stirling Quartzite, Wood Canyon Formation), Utah (Browns Hole, Prospect Mountain, and Tintic fms), and British Columbia (Eagle Bay Assemblage, Fish Lake Volcanics). Although strict age control on these units is poor, they are of interest because they contain similar mafic-ultramafic rocks including picrobasalts, basalts, basanites, trachy-basalts and andesitic basalts. Petrography and field relations indicate these rocks experienced variable alteration?, resulting in a range from slightly altered basalts to greenschist facies actinolite granofels. These volcanics have generally high TiO₂ values and low SiO₂ concentrations. Whole-rock Sr-Nd-Pb-Hf isotope values suggest a genetic relationship between samples, including an Enriched Mantle I reservoir for the magmatism. Even though these rocks span a range of ages, from Ediacaran to early Cambrian, if these suites of rocks appear to be geodynamically related based on tectonic discriminant analysis of immobile trace elements that suggest a rifting process. That process was less developed in Sonora and in British Columbia, where OIB alkaline volcanism appears to have occurred and slightly more developed volcanism in Nevada and Utah, where E-MORB and tholeiitic characteristics are present. These results lead us to hypothesize that multiple rifting processes in western Laurentia may have been catalyzed by 250 Ma of continental margin extension and/or a plume-ridge interaction during Late Neoproterozoic to Cambrian time. Furthermore, these volcanic vestiges were found in mature platform deposits and there is no trace of further development of the rifting process, suggesting an aulacogen or aborted rift.