New Mexico Geological Society Annual Spring Meeting — Abstracts

The Questa caldera of the Latir volcanic field offers an opportunity to study the magmatic cycle of a single caldera from inception to cessation. The unique nature of only one caldera within the field implies that both the volcanic record and cooling history of the plutons were not complicated by later generations of volcanism and plutonism. Preliminary ⁴⁰Ar/³⁹Ar dating of volcanic and plutonic rocks from the Latir volcanic field suggests a prolonged magmatic and cooling history. New ages indicate a ~2 m.y period of precaldera volcanism followed by a magmatic climax with the eruption of the caldera-forming Amalia tuff at 25.26±0.01 Ma. Ages determined from the subvolcanic plutons indicate a variety of cooling histories reflecting variable depths of emplacement and uplift.

Single-crystal laser-fusion of sanidine from the tuff of Tetilla Peak indicates that the earliest precaldera volcanism within the Latir field began at 28.15±0.05 Ma. The rhyolite of Cordova Creek, which was previously thought to be a similar age as the tuff of Tetilla Peak based on stratigraphy and K-Ar dating, erupted much later at 25.48±0.03 Ma. The similar stratigraphic position between the two units, along with the new, younger age of the rhyolite of Cordova Creek suggests that the rate of precaldera volcanism was not constant prior to the eruption of the Questa caldera, but rather culminated in the several hundred thousand years prior to the eruption of the Amalia tuff. We hope to test this hypothesis with continued dating of intermediate composition precaldera volcanic rocks which have stratigraphic positions between the rhyolite of Cordova Creek and the Amalia tuff,.

The exposed plutons within the field, thought to represent the subvolcanic batholith, provide the opportunity to examine the ⁴⁰Ar/³⁹Ar thermochronology of batholiths that underlie calderas. Dates from hornblende, biotite, and K-feldspar (500°C, 350°C, and ~250°C respective closer temperatures) and age spectra from plutonic K-feldspars show that the thermal history of subvolcanic batholiths varies depending on location of emplacement. Ages from the four northern resurgent plutons are between 26.70 ± 0.10 Ma and 24.89±0.03 Ma. Ages older than the Amalia tuff reflect excess argon within the K-feldspars. Plutons that mark the southern caldera margin were emplaced between 24.77±0.06 Ma and 23.63±0.19 Ma. Age spectra indicate that the southern caldera margin plutons partially reset K-feldspars of the resurgent plutons and they themselves were partially reset by two plutons, Rio Hondo and Lucero Peak, located outside the southern caldera margin. Age spectra from both Rio Hondo and Lucero Peak plutons, suggests a prolonged cooling history in the southern region of the Latir field. We propose to use K-feldspar thermal modeling to better understand the cooling and uplift history of these plutons.