HR: 1340h
AN: V43B-1374    [Abstracts]
TI: Rebuilding the Mojave Mantle Lithosphere by Underplating of Farallon Slab
AU: * Luffi, P I
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Saleeby, J B
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Ducea, M N
AF: University of Arizona, 1040 E. 4th St., Tucson, AZ 85721, United States
AU: Lee, C A
AF: Rice University, 6100 Main St., Houston, TX 77005, United States
AB: Several lines of evidence indicate that much of the Laramide and post-Laramie extension of the Mojave-Salinia lithosphere occurred atop the flat Farallon oceanic slab, possibly as it began to rollback to a steeper subduction trajectory. Simple mass balance that ignores erosion suggests that the ~200% extension should have been accompanied by about the same amplitude of thinning. That is, for a pre-extension lithosphere thickness of 120-150 km, similar to that of the neighboring stable Colorado Plateau, the average thickness of the Mojave- Salinia lithosphere resulted by extension should not exceed 40-50 km. Thus, the present day 70¬-90 km thickness of the lithosphere beneath the Mojave Desert cannot be explained by extension alone. Using spinel peridotite xenoliths from Dish Hill, here we test two potential causes that could be responsible for the thickness of the Mojave mantle: a) preservation of a large segment of Farallon slab attached to the bottom of the attenuated continental lithosphere, and b) upwelling and melt depletion of asthenosphere as response to removal of the Farallon slab. Xenolith whole rock chemistry, clinopyroxene trace element and Nd isotope data in combination with two-pyroxene geothermometry indicate important compositional layering of the mantle beneath Dish Hill. The shallowest levels represented by harzburgites with ε_Nd = -13 to -6.4 are interpreted as relics of the attenuated continetal lithospheric mantle. In contrast, the deeper xenoliths range from relatively fertile lherzolites to harzburgites, are moderately LREE depleted, consistently enriched in radiogenic Nd (ε¬_Nd = +5.7 to +16.1), and define two superposed, downwards depleted mantle slices. Mineral compositions of harzburgites indicate 12-15% depletion that are consistent with melting at mid-ocean ridges, but preclude deep in-situ melting of decompressing asthenosphere beneath the Mojave. The composition of the superposed mantle slices strongly suggest the presence of Farallon mantle lithosphere in which duplex structures developed during its underthrusting. Altogether, xenolith data support the idea that much of the Mojave lithosphere consists of underplated oceanic slab. If so, this may explain the relative strength of the Mojave lithosphere in comparison with the Basin and Range Province that was strongly thinned during most of the Tertiary, after it has lost the underlying flat slab, if it ever existed in that region.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3651 Thermobarometry
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting