Abstract

The youngest eruptions in the Jemez Volcanic field occurred in the southwestern moat of the Valles Caldera, producing two series: a younger El Cajete series (ECS) with lower SiO₂ (72-76 wt%), consisting of three members; in ascending order: the El Cajete Pumice Fall (ECP), Battleship Rock Tuff (BAT), and Banco Bonito Lava Flow (BB); and an older South Mountain Rhyolite with higher SiO₂ (76-79 wt%). Petrological, mineralogical, and geochemical data show that SMR and ECS belong to two different magmatic systems. ECS came from a hotter, less silicic, deeper magma chamber. Mineral equilibria calculations indicate that the ECS formed over a pressure interval of ~2.52 -3.68 kbar and a temperature range of ~834 to 868oC; SMR yielded a lower, narrower pressure range of about 1.24 kbar and a lower equilibration temperature of ~728 - 736^oC.

The dynamic difference of the eruptions caused the ECS to have a reverse compositional zonation. With increasing silica in ECP--BAT-BB, whole-rock trace elements show the following changes: Rb increases, Nb slightly increases, whereas commonly incompatible elements, such as Sr, Ba, and Zr, decrease. A moderately-deep subjacent magma chamber developed under the caldera and the magma built the compositional gradients during a low energy episode in the chamber. Magma recharge induced the eruption of ECS. Chemical variations in the different members of ECS correlated with the changing eruption energy. The strongest early eruption tapped into the deeper part of magma chamber and caused mixing of different chemical zones in the magma. Later eruptions were less energetic and only released the more felsic melt from the upper part of the magma chamber.