HR: 1340h
AN: V43E-1460    [Abstracts]
TI: Origin of a basalt-to-trachyandesite/dacite suite in the Jemez volcanic field (New Mexico, USA) by upper crustal contamination
AU: * Baldridge, W S
EM: sbaldridge@lanl.gov
AF: Los Alamos National Laboratory, Earth and Environmental Science Division, M.S. D462, Los Alamos, NM 87545 United States
AU: Warren, R G
EM: rgw@lanl.gov
AF: Los Alamos National Laboratory, Earth and Environmental Science Division, M.S. F665, Los Alamos, NM 87545 United States
AB: Magmatic compositions in large, long-lived volcanic fields typically evolve through a variety of petrogenetic processes. Determining their relative roles may be difficult because (1) multiple processes may act simultaneously at a single volcano, (2) different processes may produce similar compositional results, and (3) individual processes may differ significantly at different volcanoes. To isolate the major processes and avoid the complex overprinting of multiple processes in the main Jemez volcanic field (JVF), we obtained whole-rock major- and trace-element compositions and mineral compositions from a small basalt-to-trachyandesite/dacite volcanic field (Cerros del Rio, CDR) adjacent to the main JVF. Lavas of the CDR field were erupted during development of the middle to upper crustal magma chamber (4-1 Ma) that resulted in formation of Toledo and Valles calderas at 1.6-1.2 Ma, but were not affected by mixing with the main chamber. The mafic end members of this suite typically range from basanite/alkali olivine basalt to hawaiite (SiO2 44-52%; Na2O+K2O 4.2-6.3%), likely derived from an ocean-island-basalt (OIB)-modified lithospheric source (McMillan, N. Mex. Geol. Soc. Guidebook 49, 107-116, 1998). Compositions range to trachyandesite and dacite (SiO2 65-67%; Na2O+K2O 6.9-7.2%) (classification of Le Bas et al., J. Petrol., 27, 745-750, 1986). Trace elements (e.g., Th, Zr) are not well correlated with major elements. Phenocrysts range from olivine and plagioclase in the least evolved lavas to high- and low-Ca pyroxene and sodic plagioclase in the intermediate to silicic members. Primitive members of this series contain gabbroic xenoliths and evolved members are typified by quartzofeldspathic xenoliths and by xenocrysts of quartz and feldspar, which we infer were derived from middle to upper crust. Using only whole-rock compositions observed in the CDR, we can model the major-element compositional range assuming removal of observed phenocryst phases coupled with addition of observed xenolithic/xenocrystic material. Modeling of trace elements, assuming world-wide average values (Rudnick and Gao, Treatise on Geochem., Chap. 3, 2004), is not compatible with a significant lower crustal component in these magmas. We conclude that the simplest model for the origin of this basalt-to-trachyandesite/dacite suite is fractional crystallization of OIB-modified basalt combined with significant assimilation of middle to upper crustal rocks. Although we cannot rule out more complicated models involving partial fusion of lower crust and mixing of magmas, we do not think they are necessary or likely.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting