New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Toledo Rhyolite (CTR) comprises domes and tephra erupted during the interval between two caldera forming ignimbrites, the Tshirege Member (TM or upper) and Otowi Member (OM or lower) of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. As such, they record important petrogenetic information regarding the evolution of the Bandelier magma system during this intervaL Data were collected in two ⁴⁰Ar/³⁹Ar laboratories and intercalibration was secured by utilizing the same standard (Fish Canyon Tuff sanidine) and running repeated analyses of a sample from the OM Bandelier Tuff, for which identical ages were obtained. Crystal populations from 20 samples range from essentially homogenous juvenile material to populations with significant components of both juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define ⁴⁰Ar/³⁹Ar ages which agree with stratigraphic constraints. Reliable isochron ages were obtained for the majority of these sanidine analyses. In contrast, plagioclase analyses are distinctly more scattered and do not define reasonable ages. The ⁴⁰Ar/³⁹Ar ages for the OM Bandelier Tuff (1.608 ± 0.010 Ma) and the TM Bandelier Tuff (1.225 ± 0.008 Ma) yield a repose interval of 380 ± 20 ka between caldera collapse events. ⁴⁰Ar/³⁹Ar dates on pumice fall units within the CTR tephra indicate that eruptive activity occurred at 1.59, 1.54, 1.48, 1.37 and 1.22 Ma. ⁴⁰Ar/³⁹Ar dating of CTR domes indicates these were also being erupted within the caldera at 1.54, 1.45, 1.38-1.34 and 1.27 Ma. The dates obtained indicate that CTR eruptive activity producing both tephra and domes occurred during discrete intervals at approximately 1.54 Ma, 1.48-1.45 Ma, 1.35-1.37 Ma, and 1.27 Ma. The interval from 1.379±0.012 Ma to 1.336 ±0.018 Ma was particularly active as 7 of 16 units dated were erupted during this interval. These ⁴⁰Ar/³⁹Ar data in some cases confirm suspected differentiation sequences suggested by geochemical relationships among domes and tephra whereas in others they show that apparently related samples are of distinctly differing ages and cannot be comagmatic.