A MANTLE MELTING PROFILE ACROSS THE BASIN AND RANGE, SW USA

The major and trace element composition of late Cenozoic basalts (0-10 Ma) across the Basin and Range province (B&R) preserve a clear signal of mantle melting depth variations. FeO, Fe _8.0 and Tb/Yb increase, whereas Si_8.0 and A_8.0 decrease, from west to east across the B&R along a profile at 36-37°N. These variations are qualitatively consistent with shallower melting beneath the Western Great Basin (WGB) than in the central B&R. In order to quantify the depth range and percent of decompression melting, we invert primary Na₂O and FeO contents of basalts using a melting model based on the partitioning of FeO and MgO in olivine, and Na2O in clinopyroxene. An independent inversion, using the rare earth elements, corroborates the melting depths obtained from the major element model, and places most of the melting beneath the central B&R in the garnet-peridotite stability field. We find that the shape of the melting region across the B&R closely mimics the shape of the mantle lithosphere, as inferred from geological and geophysical observations. Melting across the study area occurs largely within the asthenosphere and generally stops at the base of the mantle lithosphere. In the WGB, melting paths are shallow, from 75-50 km, and in some cases extend almost to the base of the crust. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned (or absent) mantle lithosphere. Shallow melting is consistent with geobarometry and isotopic compositions of local mantle xenoliths. Lithospheric thinning was caused by thermal erosion during Mesozoic subduction, and/or simple shear or foundering during Cenozoic extension. In contrast, melting beneath the central B&R occurs beneath thick mantle lithosphere, and requires mantle potential temperatures 200°C hotter than normal (melting paths from 140-100 km). The excess temperature beneath the central B&R is consistent with active upwelling of hot mantle in this region.