BASALTIC VOLCANISM IN THE SOUTHERN BASIN AND RANGE - NO ROLE FOR A MANTLE PLUME

The Colorado River Trough (CRT), in the Southern Basin and Range, is a corridor of high extension (greater-than-or-equal-to 100%) associated with basaltic volcanism that took place during the Miocene. Two main geochemical groups of basalts are recognised: Group 1 has high Nb/La and Nb/Y, smooth patterns on mantle-normalised multi-element diagrams, and Sr and Pb isotope ratios similar to those in oceanic basalts. These are small volume post-extensional flows and they are thought to have been derived from the asthenospheric upper mantle. The bulk of the CRT basalts (> 95% by volume) fall into Group 2. These basalts have low Nb/La, low HREE contents, irregular minor and trace element patterns, low Nd-143/Nd-144 and elevated Sr and Pb isotope ratios. Such features suggest that these basalts were derived from the subcontinental lithospheric mantle and within Group 2 large variations in Ce/Yb, Nb/Y and Th/Zr can be modeled by an increase in the degree of partial melting with time as extension reaches its maximum. Most of the basalts in the CRT were derived from the continental mantle lithosphere, although there was a change from lithosphere- to asthenosphere-derived magmatism following the main period of extension. Model calculations indicate that at 100% extension, and in the presence of small amounts of H2O and CO2, partial melting takes place in the mantle lithosphere without any increase in the mantle potential temperature (T(p)). Moreover, the presence of elevated T(p) values would result in much greater proportions of asthenosphere to lithosphere derived melts than those observed, and much greater eruption rates than those estimated for the CRT. It is concluded that the chemical changes with time in the basalts of the CRT, the eruption rates, and inferred T(p) indicate that Miocene magmatism in the Southern Basin and Range was triggered by lithospheric extension, and not by the emplacement of a mantle plume.