New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent studies of the Sierra Cuchillo laccolith and surrounding volcanic sequences have refined the geochronology and petrologic understanding of the area. New U-Pb zircon data on the laccolith, previously dated at 49 Ma by Jahns (1978), place intrusion and cooling at 38.6 ± 0.7 -1.2 Ma. The laccolith is a fine-grained granitic intrusion containing at least three pegmatite-like concentric zones of alteration formed by late-stage volatile fluids. The source of the laccolith, at least in part, is assimilated Precambrian crust. This is supported by zircon cores dated at 1.455 Ga + 15 Ma. Six volcanic packages surround Sierra Cuchillo; two have been hypothesized as being related to the laccolith (McMillan, 1978). The original interpretation of these sequences places the undifferentiated Latite-Andesite sequences directly above the Mesozoic equivalent to the Dakota Sandstone. This is overlain by the Dacite-Rhyolite sequence (Jahns, in press). U-Pb zircon dating places the Latite-Andesite sequence at 36.2 + 0.7 Ma, and the Dacite-Rhyolite sequence at 36.6 + 0.7 Ma. Geochemically the Latite-Andesite sequence is very similar to the Sierra Cuchillo laccolith containing a Sr concentration of 837 ppm, and a Nb concentration of 9 ppm. The Dacite-Rhyolite sequence contains slightly higher concentrations of Rb, Th, Nb, Zr, and Y in the basal member, but the top member is similar to Sierra Cuchillo.

Many smaller intrusions surround the Sierra Cuchillo laccolith including the Willow Springs dome and the Vindicator sill. The samples from the Vindicator sill, dated at 37.7 + 0.7 Ma, have trace element concentrations very similar to Sierra Cuchillo. These samples indicate that the sill should actually be considered part of the laccolith. Small rhyolitic intrusions, formally considered satellite intrusions, contain zircons of 28.0 Ma. These are also geochemically distinct from Sierra Cuchillo with extremely high Nb (79-200 ppm), and Y (36-93 ppm) concentrations, and extremely low Sr concentrations (9-16 ppm).

The Sierra Cuchillo laccolith is unique among igneous rocks of similar age. Most mid-Tertiary rhyolitic rocks were erupted from large calderas with regional ash-flow sheets. Sierra Cuchillo was emplaced at a shallow depth, and may correlate only to local, rather than regional, volcanic sequences.