The geochronology of volcanic and plutonic rocks at the Questa caldera: Constraints on the origin of caldera-related silicic magmas

Volcanic and plutonic rocks exposed in the Latir volcanic field, northern New Mexico, provide an opportunity to examine the timing of caldera-related magmatic processes. Fifty-five mineral separates were dated using the 40Ar/39Ar method. Volcanism began at 28.5 Ma and ended at 22.7 Ma, climaxing with the eruption of the high-SiO2 peralkaline Amalia Tuff from the Questa caldera at 25.39 +/- 0.04 Ma. Following caldera collapse, four resurgent plutons were emplaced between 25.4 and 25.0 Ma. Previous geochemical studies suggested that the peralkaline phases of two resurgent plutons and a peralkaline ring dike represent an intrusive equivalent to the Amalia Tuff. A new analysis of K-feldspar from the ring dike yielded a plateau age of 25.38 +/- 0.04 Ma, supporting this previous conclusion. Only these small-volume peralkaline intrusions are temporally indistinguishable from syncaldera magmatism. The structurally lower resurgent plutons were emplaced similar to 100-400 k.y. after the Amalia Tuff eruption and are therefore too young to be the non-erupted, more-mafic parts of Amalia Tuff magma chamber, implying that the Amalia Tuff magma was not generated via in situ differentiation in the upper crust. Following resurgent plutonism, five additional plutons were emplaced between 24.9 and 19.3 Ma along the southern caldera margin and south of the caldera. Thermochronology indicates that most plutons rapidly cooled following emplacement, but that some were subsequently reheated during protracted postcaldera pluton emplacement. The timing of pluton emplacement at the Questa caldera indicates that only small volumes of the intrusive rocks are the same age as the caldera eruption and that most of the intrusive rocks record postcaldera magmatic processes.