Petrogenesis of the Mount Taylor volcanic field and comparison to the Jemez Mountains volcanic field, New Mexico

The Mt. Taylor volcanic field (MTVF) is an eroded composite volcano largely composed of trachyandesite to tracydacite lavas erupted between 3.6 and 1.5 Ma. Recent detailed mapping augmented with new dates and major and trace element analyses provides new insights on the evolution of the MTVF. Mount Taylor shares petrologic similarities with the Jemez Mountains volcanic field (JMVF), located ~100 km to the northeast on thinner lithosphere associated with the Rio Grande rift. Both volcanic fields lie on the Jemez Lineament, an alignment of Neogene volcanic fields that sits atop a Precambrian suture zone and coincides with the southeastern margin of the Colorado Plateau. Comparison of the two volcanic fields will provide an understanding of the lineament-influenced vs. extension-influenced factors in petrogenesis. Rocks from the MTVF range from 42 to 76% SiO2, with the basalts having MgO contents between 4 and 15 wt%. Lavas at the MTVF are more alkaline than those of the JMVF. The most primitive lavas are basanites that are depleted in K with respect to other incompatible trace elements, and are thought to result from low-degree partial melts of lithospheric mantle with residual amphibole, similar to primitive JMVF nephelinites and basanites. Major, trace element and isotopic data are consistent with initial deep-crustal storage and contamination of primitive magmas to produce basalts with 4% - 8% MgO, that subsequently evolve largely by fractional crystallization to produce the rest of the MTVF suite. Mildly increasing K/Nb with SiO2 indicates that minor assimilation of crust was also involved during the second stage. These findings are consistent with the conclusions of Perry et al. (1990) that the MTVF suite is the product of polybric assimilation-fractional crystallization processes. Volcanic fields along the Jemez lineament show a tendency to higher alkalinity and lower silica saturation away from the intersection of the lineament with the Rio Grande rift, consistent with greater depths, and/or lower degrees of, mantle partial melting. The smaller role for crustal contamination at MTVF compared to the JMVF (Rowe et al., 2007) is consistent with a smaller magma flux (MTVF, <0.1 km3/ka; JMVF, ~0.15 km3/ka) and consequent lower degree of crustal heating. References: Perry, F.V., Baldridge, W.S., DePaolo, D.J., and Shafiqullh, M., 1990, Evolution of a magmatic system during continental extension: The Mount Taylor volcanic field, New Mexico: Journal of Geophysical Research, vol. 95. Rowe, M.C., Wolff, J.A., Gardner, J.N., Ramos, F.C., Teasdale, R., and Heikoop, C.E., 2007, Development of a Continental Volcanic Field: Petrogenesis of Pre-Caldera intermediate and Silicic rocks and origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA): Journal of Petrology, vol. 48, p. 2063-2091.