GEOLOGY OF THE CENTRAL SAN MATEO MOUNTAINS, NEW MEXICO

The San Mateo Mountains are a north-south trending uplifted fault block within the Mogollon-Datil volcanic field of southwestern New Mexico. They consist mostly of Oligocene rhyolitic ash-flow tuffs, lavas, and intrusions. 1:24,000 scale geologic mapping and 40Ar/39Ar geochronology were employed to 1) determine rock distribution and stratigraphy 2) make correlations with rocks mapped elsewhere in this volcanic field and 3) establish the timing of eruptive events.

Geologic mapping reveals a rhyolite ash-flow tuff overlain by rhyolite lava. Granitic porphyries intrude these units. On top of the lava lies a volcaniclastic sandstone that is overlain by another ash-flow tuff. At the top of this sequence is a volcaniclastic sandstone.

Samples from the volcanic units were dated using the single-crystal laser fusion 40Ar/39Ar method on sanidine. The lowest ash-flow tuff correlates in age and petrology with the Vicks Peak Tuff and the other ash-flow tuff correlates with the Turkey Springs Tuff. The Vicks Peak Tuff was erupted between 28.34 and 28.28 Ma. Within the next 40,000 years viscous pulses of rhyolite lava erupted over the Vicks Peak Tuff. Within the next 100,000 years silicic magma bodies intruded the Vicks Peak Tuff and rhyolite lava. There was a break in volcanic activity between 28.3 and 24.3 Ma, during which time a volcaniclastic sandstone was deposited. The Turkey Springs Tuff then erupted at 24.3 Ma and was subsequently covered by another volcaniclastic sandstone.

Preliminary interpretations of this data suggest that 1) 40Ar/39Ar dates record two distinct pulses of volcanism; one between 28.32 and 28.23 Ma and the other at 24.3 Ma, 2) the eruption of the Vicks Peak Tuff followed by rhyolite lava during the first pulse may represent progressive degassing of a common magma chamber, 3) the unexposed base of the Vicks Peak Tuff and the occurrence of rhyolite lava and granitic porphyries may suggest that these rocks are within or near the source cauldron of the Vicks Peak Tuff.