New Mexico Geological Society Annual Spring Meeting — Abstracts

A conceptual model for relating pluton assembly to caldera eruptions and associated volcanism was developed using ⁴⁰Ar/³⁹Ar dating of volcanic and plutonic rocks associated with the Questa Caldera, Latir volcanic field. The volcanic geochronology provides point-in-time information about magmatism whereas the thermochronology of exposed plutonic rocks establishes their emplacement and cooling histories. ⁴⁰Ar/³⁹Ar dating indicates that volcanism spanned 6 Ma and the plutons experienced various emplacement and thermal histories.

Precaldera volcanism began at 28.3 Ma and ended at 25.3 Ma. The combination of the published geochemistry with ages of precaldera volcanism from this study suggests that the earliest magmatism was dominated by multiple, small magma chambers, rather than a single, large magma chamber. The Questa caldera formed during the eruption of the Amalia Tuff. Sanidine from thirteen samples yielded a mean age of 25.23 ±0.05 Ma for the Amalia Tuff.

Four resurgent plutons were emplaced, crystallized, and rapidly cooled to 150ºC within 500 ka of caldera collapse. A biotite from the previously undated Canada Pinabete pluton yielded an age 25.28 ±0.08 Ma. Because the Canada Pinabete pluton and Amalia Tuff are geochemically similar and the ages are analytically indistinguishable, the Canada Pinabete pluton is interpreted as non-erupted Amalia Tuff. This supports the idea that not all ignimbrite magma chambers completely empty during eruption and some plutons can be directly correlated to large-scale ignimbrite sheets. Three post-caldera rhyolites yielded sanidine ages between 24.9 and 25.0 Ma indicating volcanism was coeval with emplacement of the resurgent plutons.

Following resurgent plutonism, plutons were emplaced along the southern caldera margin and south of caldera margin. In contrast to resurgence, these plutons exhibit protracted and complex cooling histories. U-Pb zircon and ⁴⁰Ar/³⁹Ar biotite ages combined with K-feldspar multiple diffusion domain thermal modeling indicate the various thermal histories of the postcaldera plutons is attributed to incremental emplacement and subsequent reheating events. An age of 22.5 Ma from a postcaldera andesite suggests that volcanism was coeval with the youngest pluton emplacement.