GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE MOUNT TAYLOR VOLCANIC FIELD, AND COMPARISON WITH THE NEARBY JEMEZ MOUNTAINS

The Plio-Pleistocene Mount Taylor volcanic field (MTVF, volume 20 km³) in northwest New Mexico forms part of the Jemez lineament, a northeast-southwest trending alignment of late Cenozoic volcanic centers that coincides with a major Proterozoic lithospheric suture zone containing fertile mantle. The MTVF bears some petrologic similarities to the much larger Jemez Mountains volcanic field (JMVF, 2,000 km³) to the northeast, which also lies on the lineament and, additionally, on the locus of regional lithospheric extension marked by the Rio Grande rift. Comparison of the two volcanic fields will therefore allow separation of lineament-influenced vs. extension-influenced factors in petrogenesis, with potential broad applicability to Cenozoic magmatism in the southwestern U.S. The MTVF suite ranges from primitive basalts (15% MgO) to high-silica rhyolite, similar to the JMVF. Overall, the MTVF suite is more alkaline than the JMVF. The most primitive lavas are basanites that are depleted in K with respect to other incompatible trace elements, and are thought to be low-degree partial melts of lithospheric mantle with residual amphibole, similar to primitive JMVF nephelinites and basanites [1]. Trace-element indices of a continental crustal component, such as Pb/Ce and K/Nb, increase overall with silica content but with a large degree of scatter. This is consistent with the conclusions of earlier workers [2] that the MTVF suite is the product of variable polybaric AFC processes. The increase in crustal indices with degree of differentiation is less than in the Jemez suite. This observation, together with the more alkaline character of the MTVF and its smaller volume, is consistent with a lower magma flux from the mantle and less heating and melting of the crust than occurred during peak periods of magmatism in the Jemez Mountains. Ultimately, this may be due to a lower degree of lithospheric extension at Mt. Taylor compared to the Jemez Mountains.

[1] Wolff et al., J. Petrol. 46, 407-439 (2005); [2] Perry et al., JGR 95, 19,327-19,348 (1990).