The 40ar/39ar Geochronology And Thermochronology Of The Latir , New Mexico: Implications For Silicic Volcanism

Matthew J. Zimmerer

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 associated 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 at 25.24 ± 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 28.13 ± 0.06 Ma. The 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 25.42 ± 0.05 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 . 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 . Dates from biotite and K-feldspar (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 indicate rapid emplacement as early as 24.84 ± 0.11 Ma. Plutons that mark the southern caldera margin were emplaced between 24.78 ± 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.

Biographical Sketch

Matt Zimmerer received a BS in geology at the University of Kentucky in May 2005. While working toward his degree, Matt worked at the Center for Applied Energy Research. There he researched the occurrence of mercury in coal and coal combustion by-products. Matt began his MS degree in the fall of 2005, working under Bill McIntosh studying the 40Ar/39Ar geochronology of the Latir volcanic field. While not working toward his research, you can probably find Matt riding his bike, working on his bike, talking about bikes, or just looking at bikes. Following the completion of his MS degree (which seems to talk a long time for Argon students) Matt will either continue his education in geochemistry or go for a really long bike ride.