GEODYNAMIC MODELING OF PLUME-LITHOSPHERE INTERACTION BENEATH THE YELLOWSTONE HOTSPOT TRACK

Although commonly attributed to a mantle plume, time-transgressive magmatism of the Snake River Plain-Yellowstone (SRPY) province differs in important ways from that associated with typical oceanic hotspots. A fundamental question concerns the relative contributions of lithosphere vs. upwelling sub-lithospheric mantle to formation of SRPY basaltic magmas. Specifically, association of this province with initially thick and cold Archean lithosphere (Wyoming craton) poses a problem in that this lid will hinder and possibly prevent melting of rising plume material. Melting can only occur if (1) the lid can be substantially thinned over geologically reasonable time and/or (2) the upwelling material is exceptionally warm. Petrologic modeling indicates that SRPY primitive basalts last segregated from mantle at ~1450°C and ~100 km depth, suggesting that their source is only slightly warmer than MORB-source mantle and significantly cooler than sources of oceanic hotspot magmas.

Geodynamic models were developed to evaluate lithospheric thinning processes. If the lithosphere is initially more than 200 km thick, attenuation by at least a factor of two is required to allow decompression melting of an ascending plume, assuming low volatile content and high excess temperature (Tp >1500°C). Fully dynamic models were applied to investigate the extent and rate of lithosphere thinning assuming an initial structure representative of the Wyoming craton. Thermal erosion by plume impingement alone appears incapable of providing the required lithospheric thinning. Alternative models (e.g., low-angle Laramide subduction, delamination) also conflict with geochemical evidence that SRPY basalts contain a dominant contribution of old, isotopically evolved mantle material - presumably derived from subcontinental lithospheric mantle (SCLM). Our tentative conclusions are that SCLM is likely to be preserved, that the thick SCLM lid prevents substantial melting of rising plume material (tomographically imaged), and SRPY basalts are predominantly derived by melting of lithospheric mantle.