GEOGRAPHIC EXTENT OF CONTINENTAL LITHOSPHERIC MANTLE METASOMATISM DURING THE LARAMIDE OROGENY BASED ON VOLCANIC ROCK GEOCHEMISTRY, SOUTHWESTERN NORTH AMERICA

Although geochronologic studies of mantle and crustal xenoliths entrained in Cenozoic volcanic rocks from the southern Colorado Plateau reveal that aqueous fluid metasomatism affected the deep continental lithosphere beneath this region during the Late Cretaceous to Paleogene Laramide Orogeny, the paucity of xenoliths elsewhere in SW North America limits efforts to determine the lateral extent of metasomatized lithosphere in this fashion. However, existing data from nearly 6,000 Cenozoic volcanic rock samples from SW North America suggests that basalts and basaltic andesites with uncorrelated Ta and Th abundances and “intermediate” Ta/Th values (0.1 to 0.6) are the products of melting of rutile +/- apatite-bearing metasomatized lithospheric mantle (CLM). These rocks also have uniformly high initial ⁸⁷Sr/⁸⁶Sr (0.704 to 0.708) but Nd isotopic compositions that vary consistently with lithospheric age. The decoupling of the volcanic rock Nd and Sr isotopic compositions suggests that the metasomatizing fluids carried Sr, with high initial ⁸⁷Sr/⁸⁶Sr, but not light rare earth elements. A potential source of the relatively radiogenic Sr in these fluids is tectonic melange that accompanied subduction of the Farallon Plate, given recent field evidence that underplating of SW North American during the Late Cretaceous to Paleogene by tectonic melange occurred as far inboard as west-central Arizona (Strickland and Singleton, 2018, GSA Abstr. w/Prog.). Isotopic data from the Catalina Schist at the continental margin (King et al, 2006, EPSL) reveals that aqueous fluids generated in the tectonic melange mobilized strontium with isotopic compositions that overlapped those observed for the intermediate Ta/Th volcanic rocks, but did not mobilize the LREE. As a result, we consider the distribution of intermediate Ta/Th volcanic rocks to be a measure of the geographic distribution of CLM affected by slab- and tectonic melange- derived fluids during the Laramide Orogeny. Using the palinspastic reconstruction of McQuarrie and Wernicke (2005, Geosphere) and assuming no relative movement of CLM and surface since eruption of a given volcanic rock, then the distribution of metasomatized CLM likely extended inboard as far as the southern Rocky Mountains and to the north as far as the southern margin of the Archean Wyoming Craton.