Remnants of the Latir volcanic field and cogenetic plutonic rocks are exceptionally exposed along the east margin of the present-day Rio Grande rift by topographic and structural relief in the Sangre de Cristo Mountains of northern New Mexico. Evolution of the magmatic system associated with the Latir field, which culminated in eruption of a regional ash flow sheet (the Amalia Tuff) and collapse of the Questa caldera 26 m.y. ago, has been documented by 74 new potassium-argon (K-Ar) and fission track (F-T) ages. The bulk of the precaldera volcanism, ash flow eruptions and caldera formation, and initial crystallization of the associated shallow granitic batholith took place between 28 and 25 Ma; enconomically important molybdenum mineralization is released to smaller granitic intrusions along the south margin of the Questa caldera at about 23 Ma. Interpretation of the radiogenic ages within this relatively restricted time span is complicated by widespread thermal resetting of earlier parts of the igneous sequence by later intrusions. Many samples yielded discordant ages for different mineral phases. Thermal blocking temperatures decrease in the order: K-Ar sanidine > K-Ar biotite > F-T zircon ≫ F-T apatite. The F-T results are especially useful indicators of cooling and uplift rates.

Upper portions of the subvolcanic batholith, that underlay the Questa caldera, cooled to about 100¿C within about a million years of emplacement; uplift of the batholith increases to the south along this segment of the Sangre de Cristo Mountains. Activity in the Latar volcanic field was concurrent with southwest directed extension along the early Rio Grande rift zone in northern New Mexico and southern Colorado. The geometry of this early rifting is compatible with interpretation as back arc extension related to a subduction system dipping gently beneath the cordilleran region of the American plate. The Latir field lies at the southern end of a southward migrating Tertiary magmatic locus, along which much of the southern Rocky Mountains was blanketed by volcanic rocks in Eocene and Oligocene time. This southward migration of igneous activity through the Rocky Mountains may have been a critical precursor that, by heating and weakening the lithosphere, helped localize initial extensional deformation in the eastern Cordilleran region. Ages of rift-related basaltic volcanism in the Questa-Latir area span an apparent magma gap 20-10 m.y. ago, inferred by others for more southern sectors of the Rio Grande rift. Rift-related volcanism was nearly continuous through Miocene and Pliocene time in the northern New Mexico sector of the rift, perhaps because of intersection with the diffuse northeast trending Jemez magmatic zone.