New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent ⁴⁰Ar/³⁹Ar dating provides a start to advancing our understanding of how plutons in southwestern New Mexico relate to caldera volcanism and mineralization. Unlike the well-established geochronology of southwest NM caldera volcanism, only a sparse dataset exists for the regional plutons. Eleven ⁴⁰Ar/³⁹Ar dates from the plutons indicate a variety of relationships to the regional caldera volcanism and mineralization. Some caldera-related plutons were emplaced either syn-caldera collapse or as late as ~3 m.y. after caldera formation. Calderaindependent plutons are less understood, largely because they lack detailed geochronology and thermochronology.

Some plutons are exposed within or adjacent to calderas. Three plutons in southwestern New Mexico are spatially associated with calderas. One calderapluton pair, the Juniper caldera and Animas quartz monzonite stock, have indistinguishable ages of 33.6 Ma, which suggests that the pluton is associated with the magmatic system that caused caldera collapse. The temporal relationships of the other calderaplutons pairs, Organ caldera (36.2 Ma) /Organ batholith (33.3 Ma) and Steins Caldera (34.4 Ma)/Granite Gap stock (33.1 Ma), indicate that plutons are as much as 3 m.y. younger than caldera formation. Mapping suggests the Organ pluton is made up of multiple intrusions; the granite of Granite Peak (youngest), Sugarloaf Peak quartz monzonite and Organ Needle quartz syenite (oldest). This might represent slow cooling of syn-caldera emplaced plutons or prolonged growth of a subcaldera batholith. A study of caldera-related plutons in the Latir volcanic field, northern NM, suggests that plutons record the timing of peak and waning stage of caldera magmatism.

Some of the regional plutons are located as much as 80 km away from known calderas. Ages of eight plutons range from 32.6 to 39.1 Ma, but in general are older than the caldera-related plutons and several are related to tungsten, molybdenum, and beryllium mineralization. The origin of the intrusions is not well understood, but some may represent unerupted magma chambers or roots of calderas, which were eroded or buried beneath rift sediments. Additional geochronology is necessary to understand the relationships of caldera-related plutonism, calderaindependent plutonism, mineralization, and volcanism.