EDIACARAN-CAMBRIAN RIFT RELATED VOLCANISM IN WESTERN LAURENTIA: ABORTED RIFT EVENT(S)?

The Western Cordillera contains magmatic remnants of Ediacaran and Cambrian rift-related volcanism. Such rocks include mafic to mafic-intermediate picrobasalts, basalts, basanites, trachy-basalts and andesitic basalt. Although the rocks that record this volcanism are geographically close to the hypothesized rift margin of Rodinia, they post-date the main rifting phase by ~250 Ma, and likely record at least three episodes of subsequent rifting in this region. For this study, a suite of extrusive and intrusive volcanics from the El Arpa, La Cienega and Rajón Fms (SON), Reed and Wyman Fms (CA), Stirling Quartzite and Wood Canyon Fms (NV), Prospect Mountain, Tintic, and Browns Hole Fms (UT), and Eagle Bay Assemblage and Fish Lake Volcanics (BC) were analyzed. Rocks locally exhibit variable metamorphism, ranging from slightly altered picrobasalts to greenschist facies actinolite granofels. These rocks are high in TiO2 and exhibit small variations in SiO2 in the basic-ultrabasic fields. Sr-Nd-Pb-Hf isotopes suggest a genetic relationship between samples, including an EMI source for the magmatism. If this suite of rocks is geodynamically related, tectonic discriminant analysis of immobile trace elements suggests a rifting process that is less developed in Sonora, where Cambrian volcanics exhibit primary signatures (Cr < 1520ppm, Ni < 882 ppm) and plot in the anorogenic enriched mantle reservoir. Slightly more developed rifting and fusion rates are recorded by Ediacaran volcanics in Nevada and by Ediacaran and Cambrian volcanics in Utah, where E-MORB characteristics and a decrease in trace element concentrations occur, suggesting a small evolution in the rifting process. Considered in light of other Ediacaran and Cambrian volcanics in Idaho, Colorado, and Oklahoma, our data are consistent with a genetically related, but diachronous series of western Laurentian rifting events that developed along a passive margin, which we hypothesize were catalyzed by 250 Ma of continental margin extension and/or a plume-ridge interaction. To refine this hypothesis, better geochronologic control on these magmatic remnants is needed, together with independent (e.g., stratigraphic, bedform) evidence of tectonism from coeval strata.