GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF THE 780 MA GUNBARREL LIP OF WESTERN NORTH AMERICA, WITH EXAMINATIONS INTO NI-CU-PGE POTENTIAL AND RODINIA RECONSTRUCTION MODELS

Along the western margin of North America lie dykes, sills and volcanic rocks that have been dated at 780 Ma (U-Pb and Ar-Ar methods). The intrusions span over 2500 km, striking NE in the Yukon and Northwest Territories, Canada and striking NW in Montana and Wyoming, United States and have been identified as a radiating dyke swarm termed the Gunbarrel Large Igneous Province (LIP). Despite the vast distance between units they share remarkably similar geochemical signatures. The Gunbarrel intrusions are tholeiitic basalts to basaltic andesites with Mg#’s between 0.27-0.46. They have moderate to large negative Nb-Ta anomalies, high Pb spikes and a moderate MREE-HREE slope on primitive mantle normalized multi-element plots. These data suggests that Gunbarrel magmas derived from low degrees of partial melting of a deep source (>90 km) and experienced lithospheric interaction during ascent and emplacement. Isotopic data (ɛNd vs. ⁸⁷Sr/⁸⁶Sr) places the majority of Gunbarrel samples within the enriched OIB field consistant with a plume/deep mantle source.

Gunbarrel intrusions consist of both chalcophile-element depleted and undepleted magmas based on a (Pd/Yb)PM vs. (Th/Yb)PM plot which suggests occurrence of S-saturation and possible sulphide immiscibility with the potential to produce a Ni-Cu-PGE deposit somewhere in the plumbing system for the Gunbarrel event. LIPs have also been linked with the breakup of continents through time, and the timing of the Gunbarrel event coincides with the breakup of the Proterozoic supercontinent Rodinia. Both South China and central Australia have intraplate magmatism of similar age that is also likely part of the Gunbarrel LIP, although the exact Rodinia reconstruction of these two blocks against western Laurentia remains debated. Our geochemistry data will be compared with published data from ca. 780 Ma units in South China and Australia to assess any petrogenetic linkages between these different parts of the Gunbarrel LIP.