Geochronology and geochemistry of the Cerro Toledo Rhyolite
— Terry L. Spell, Philip R. Kyle, and Joel Baker

The Cerro Toledo Rhyolite (CTR) comprises domes and tephra erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member (TMBT) and Otowi Member (OMBT) of the Bandelier Tuff, in the Jemez volcanic field, New Mexico. The ⁴⁰Ar/³⁹Ar ages for the OMBT (1.608 ± 0.010 Ma) and the TMBT (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 erupted within the caldera at 1.54, 1.45, 1.38-1.34 and 1.27 Ma. Analyzed crystal populations range from being fairly homogenous juvenile material to very heterogeneous mixed magmatic and xenocrystic assemblages. 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, 1.37-1.35 Ma, and 1.27 Ma. The interval from 1.34-1.38 Ma was particularly active as 7 of 16 units dated are constrained to these ages. All CTR samples are high-silica rhyolites that are extremely depleted in minor elements such as Fe and Mg. In contrast to relatively constant major and minor element compositions, trace elements such as Nb vary widely, reflecting distinct differences in differentiation. CTR tephra samples generally exhibit lower (⁸⁷Sr/ ⁸⁶Sr)_i sanidine than for whole pumice and may reflect Rb loss and Sr exchange with meteoric water during hydration of glass. Sanidine ( ⁸⁷Sr/⁸⁶Sr)_i ranges between 0.70482 to 0.70593, except for one sample with an ( ⁸⁷Sr/ ⁸⁶Sr)_i of 0.74821. This sample may be contaminated by Precambrian xenocrysts, although ⁴⁰Ar/ ³⁹Ar data indicates complete equilibration of the argon isotopic system. e_Nd values for CTR tephra range from -0.2 to -2.6 and show an irregular decrease with decreasing age. The isotopic data indicate a significant mantle-derived component in CTR magmas. Geochemical data suggests the oldest CTR tephra is closely related to the OMBT and probably represents residual material left in the OMBT magma chamber. The youngest tephra has an ⁴⁰Ar/³⁹Ar date indistinguishable from the TMBT and is geochemically similar and probably represents the earliest phase of the TMBT eruption. Although geochemical trends suggest CTR tephra and domes from 1.54 to 1.22 Ma record differentiation producing TMBT magmas the variable e_Nd values for CTR tephra samples precludes this simple interpretation. These data suggest that the CTR represents a sequence of eruptions that either tapped several separate magma bodies during the 380 ka interval between the OMBT and TMBT, or tapped an open system magma chamber which was replenished prior to 1.55, 1.46, 1.38 and 1.22 Ma, rather than progressive evolution of a single closed system magma chamber.