EDIACARAN AND CAMBRIAN VOLCANICS OF NEVADA: IMPLICATIONS FOR THE LATE NEOPROTEROZOIC LAURENTIAN MARGIN”

The Ediacaran Stirling Quartzite and Ediacaran to Cambrian Wood Canyon Formation of Nevada are predominantly continental margin to shallow marine successions that contain intrusive and extrusive mafic rocks, here interpreted as flows and sills. At some localities, up to four discrete flows occur, with total remnant thickness ranging from ~20-47 m; this basaltic succession is bounded by siltstones and quartzites that have bedforms, fossils and structures that are consistent with deposition in fluvial to subtidal settings. The combined thickness of remnant flows ranges from ~20 to ~47 m and they tend to have loaded and altered bases with tops that locally are eroded, whereas sills are less than 1 m thick and obliquely cross-cut bedding and bear altered tops and bases. Peperitic textures are locally preserved in the upper to middle part of the flow succession, suggesting emplacement occurred in shallow water. Although the metabasalts locally experienced low-grade metamorphism and hydrothermal alteration, porphyritic textures are common, with abundant plagioclase and pyroxene pseudomorphs. Diopsides occur in less altered samples. Major elements in these volcanics suggest a mildly alkaline to subalkaline magmatism with low SiO₂ (45.2% - 51.4%), and alkali (Na2O+K2O = 4.13-3.61) concentrations. Immobile trace elements (Zr/Ti vs Nb/Y) characterize these volcanics as basalts, and normative mineralogy diagrams suggest they were sourced from transitional to tholeiitic volcanism with E-MORB signatures and moderately enriched sources, possibly associated with a continental rift. Similar volcanics are intercalated in other Ediacaran-Cambrian strata of the North American Cordillera, suggesting a related, possibly coeval series of continental rift events may have occurred among the western margin of Laurentia.