New Mexico Geological Society Annual Spring Meeting — Abstracts

An unnamed felsic volcanic unit located in the southern Sangre de Cristo Mountains is thought to have erupted as a lava flow during the Latest Eocene. This newly recognized outcrop occurs in the Rosina Peak 7.5 minute quadrangle about 15 km north of Pecos, NM and is potentially the first rift-related volcanic rock of this age found within the southern Sangre de Cristo Mountains. The age of the unit overlaps ages of 28 to 34 Ma for calc-alkaline to alkaline intrusives in the Ortiz Mountains 50 km to the southwest. The age is also nearly identical to ages obtained for the lower Picuris Tuff in the Taos area 75 km to the north (Bauer, personal communication).

The regional geology consists of Proterozoic tonalite, amphibolites, and granite overlain by Carboniferous limestones and shales of the La Posada and the Alamitos formations. The felsic lava crops out on a ledge above the Pecos River and is fault bound by coarse sandstones and limestones of the La Posada formation. The unit occupies an area of 0.01 km² and has an approximate true vertical thickness of 25 m. In outcrop it is white and exhibits platy fracturing. Hand samples are porphyritic, with abundant phenocrysts of predominantly biotite and plagioclase. Minor phenocryst phases include quartz, sanidine, and xenocrystic muscovite. In thin-section the rock has a groundmass of plagioclase; quartz and rare biotite microlites and a trace of glass. The microlites display a notable alignment in the direction of flow. Plagioclase phenocrysts show extensive oscillatory zoning and are occasionally glomerocrystic. Quartz is rounded and resorbed. Muscovite xenocrysts have reaction rims of biotite. ⁴⁰Ar/³⁹Ar step heating of biotite phenocrysts at the NM Geochronology Research Laboratory yielded a plateau age of 35.05±0.53 Ma (2σ error). The plateau age is defined by a weighted mean of nine steps comprising nearly 93% of the total ³⁹Ar released. The plateau age is virtually indistinguishable from the total gas age of 34.5±1.1 Ma and is thought to represent the age of emplacement.

The unit described does not contain any pumice or volcanic ash. Therefore it is not an ash fall or ash flow deposit. Subhorizontal platy jointing is commonly associated with slowly cooled lava flows. Thus we conclude that the deposit is likely a lava flow. However it is impossible at this time to exclude a shallow intrusive genesis. Questions that remain to be answered include the nature of the faulting which preserved this outcrop, and the absence of a nearby vent for this unit. An eroded eruptive center in the southern Sangre de Cristo Mountains is a possibility.