PALEOMAGNETISM AND GEOCHRONOLOGY OF MESOPROTEROZOIC DIABASE SILLS FROM DEATH VALLEY, CALIFORNIA: EVALUATING THE BIFURCATING PLUME HYPOTHESIS FOR THE SOUTHWESTERN LAURENTIA LARGE IGNEOUS PROVINCE

Late Mesoproterozoic mafic magmatism expressed as diabase sills, dikes, and rarely exposed flows between southeastern California and New Mexico is grouped as the Southwestern Laurentia large igneous province (SWLLIP). SWLLIP magmatism has been hypothesized to be the result of 1) lithospheric delamination after ca. 1140 Ma orogenesis; or 2) decompression melting of an upwelling mantle plume beneath Laurentia. One possible scenario is that an upwelling plume encountered topography at the base of Laurentian lithosphere bifurcating the plume and resulting in voluminous magmatic activity both in the SWLLIP and in the ca. 1108 to 1084 Ma Midcontinent Rift, ~2000 km to the northeast. Recent zircon U-Pb CA-ID-TIMS dates from the Midcontinent Rift in the Lake Superior region constrain the timing of voluminous pulses of magmatism, the largest being associated with rapid (<1 Ma) emplacement of the Duluth complex at ca. 1096 Ma. Modeling shows that lateral plume migration between the localities is feasible on the timescale of a couple million years. In this work, we present new data on diabase sills within the Crystal Springs Formation of the Death Valley region. Existing ages of these sills are based on U-Pb baddeleyite dates that are discordant and lack sufficient precision and accuracy to connect to the Midcontinent Rift record and confidently evaluate SWLLIP origin hypotheses. Felsic segregations within sills host zircon which presents the opportunity to develop new high-precision CA-ID-TIMS ²⁰⁶Pb/²³⁸U dates. Our new paleomagnetic data from Crystal Spring diabase sills can be compared to well-dated paleomagnetic poles along the Keweenawan track of Laurentia’s apparent polar wander path. Our data constrain a pulse of sill emplacement in the Death Valley region slightly prior to the main stage of Midcontinent Rift magmatism and could support a bifurcating plume model, where a plume encountered western Laurentia lithosphere and laterally migrated.