THE ROLE OF THE MANTLE DURING CRUSTAL EXTENSION - CONSTRAINTS FROM GEOCHEMISTRY OF VOLCANIC-ROCKS IN THE LAKE MEAD AREA, NEVADA AND ARIZONA

One of the fundamental questions in areas of large-magnitude extension and magmatism is the role of the mantle in the extension process. The Lake Mead area is ideally suited for developing models that link crus tal and mantle processes because it contains both mantle and crustal b oundaries and it was the site of large-magnitude crustal extension and magmatism during Miocene time. In the Lake Mead area, the boundary be tween the amagmatic zone and the northern Colorado River extensional c orridor parallels the Lake Mead fault system and is situated just to t he north of Lake Mead. This boundary formed between 11 and 6 Ma during , and just following, the peak of extension and corresponds to a conta ct between two mantle domains. During thinning and replacement of the lithospheric mantle in the northern Colorado River extensional corrido r, the lithospheric mantle in the amagmatic zone remained intact. Cont rasting behavior to the north and south of this boundary may have prod uced the mantle domain boundary. The domain to the north of the bounda ry is characterized by mafic lavas with a lithospheric mantle isotopic and geochemical signature (epsilon(Nd) = -3 to -9; Sr-87/Sr-86 = 0.70 6-0.707). To the south of the boundary in the northern Colorado River extensional corridor, lavas have an ocean island basalt (OIB)-mantle s ignature and appear to have only a minor lithospheric mantle component in their source (epsilon(Nd) = 0 to +4; Sr-87/Sr-86 = 0.703-0.705). M afic lavas of the northern Colorado River extensional corridor represe nt the melting of a complex and variable mixture of asthenospheric man tle, lithospheric mantle, and crust. Pliocene alkali basalt magmas of the Fortification Hill field represent the melting of a source compose d of a mixture of asthenospheric mantle, high U/Pb (HIMU)-like mantle, and lithospheric mantle. Depth of melting of alkali basalt magmas rem ained relatively constant from 12 to 6 Ma during, and just after, the peak of extension but probably increased between 6 and 4.3 Ma followin g extension. Miocene and Pliocene low epsilon(Nd) and high Sr-87/Sr-86 magmas and thileiites at Malpais Flattop were derived from a lithosph eric mantle source and were contaminated as they passed through the cr ust. The shift in isotopic values due to crustal interaction is no mor e than 4 units in epsilon(Nd) and 0.002 in Sr-87/Sr-86 and does not ma sk the character of the mantle source. The change in source of basalts from lithospheric mantle to asthenospheric mantle with time, the OIB character of the mafic lavas, and the HIMU-like mantle component in th e source are compatible with the presence of rising asthenosphere, as an upwelling convective cell, or plume beneath the northern Colorado R iver extensional corridor during extension. The Lake Mead fault system , a major crustal shear zone, parallels the mantle domain boundary. Th e Lake Mead fault system may locally represent the crustal manifestati on of differential thinning of the lithospheric mantle.