Laccoliths of the Ortiz porphyry belt, Santa Fe County, New Mexico

Twelve distinct laccoliths can be identified in the Ortiz porphyry belt. The laccoliths are composed of plagioclase-orthoclase-hornblende-quartz porphyry, which can be classified as quartz andesite by its modal assemblage. Laccoliths from 33.2 to 36.2 Ma and related sills in the Ortiz porphyry belt, which intrude the entire range of the approximately 3-km-thick (1.9-mi-thick) Phanerozoic stratigraphic section, predate mid-Tertiary movement on the Tijeras‐Canoncito fault system and 27.9‐31.4 Ma stocks, dikes, and base- and precious-metal mineralization. The range of depth of formation of concordant intrusions in the Ortiz porphyry belt is about three times that commonly observed in other laccolith groups. Two hypotheses are proposed to explain this feature: (1) Pre 34-Ma movement on the Tijeras‐Canoncito fault system placed laccolith-hosting Pennsylvanian and Permian strata on the southeast side of the fault system to the same elevation as Mesozoic strata on the northwest side, allowing the laccoliths and sills to intrude in a narrower depth range; and (2) Growing volcanic edifices, contemporaneous with the intrusion of laccoliths and sills, caused the range of neutral buoyancy, in which concordant intrusive bodies would form, to rise over time. Both of these models may have worked together. The transition from Laramide east-westdirected compressional regional stress field (with vertical least compressive stress) to the Rio Grande rift-related extensional stress field (horizontal least compressive stress) is constrained by the ages of the older laccoliths and the younger stocks, and by their field relations. Igneous rocks of the Ortiz porphyry belt straddle the transition. Laccoliths intruded near the waning of the Laramide stress regime during the interval from 33.2 to 36.2 Ma and may have occupied Laramide structures such as folds and arches. Post-laccolith movement on the Tijeras‐ Canoncito fault system and subsequent intrusion of 27.9‐31.4 Ma discordant stocks and dikes indicate a change in regional stress fields, constraining the timing of the transition to the period from 31.4 to 33.2 Ma.