COMPOSITION AND TIMING OF VOLCANISM ALONG THE WESTERN MARGIN OF THE TAOS PLATEAU VOLCANIC FIELD, COLORADO AND NEW MEXICO

New geochemistry and geochronology complement field observations in characterizing the composition and timing of volcanism of the westernmost Taos Plateau volcanic field (TPVF), which coincides with the western margin of the San Luis Basin segment of the Rio Grande Rift. Small to medium sized cinder cones with 0.5 to 2 km basal diameter capping earlier lava flows are the most common vent type and are associated with mafic to intermediate compositions that commonly contain abundant quartz and plagioclase xenocrysts. Low-relief shield volcanoes are probably more abundant than presently known and are sources of olivine tholeiites of the Servilleta Basalt. San Antonio Mountain is unique along the west side with a base diameter of about 8 km and about 750 m vertical relief, which is comparable to Ute Mountain or Guadalupe Mountain in the central to eastern part of the TPVF.

New ⁴⁰Ar/³⁹Ar geochronology on lava flows and volcanic centers in the area between Petaca Peak (south) and Los Mogotes (north) indicates three episodes of volcanism. From 4.8 to 4.3 Ma cinder cones and lava flows are mostly mildly alkaline (MgO = 5.6 to 6.7 wt %, Na₂0+K₂O (TA) = 4.5 to 5.7 wt %) and include Cinder pits (4.8 Ma), Los Mogotes (4.6 Ma), Chino Peak (4.4 Ma), and Petaca Peak (4.3 Ma). The peak of volcanism and most compositionally diverse along the west side of the TPVF occurred between 3.4 to 2.8 Ma. During this period tholeiitic olivine basalt and basaltic andesite lavas from unknown sources are the most primitive compositions (MgO = 5.6 to 7 wt %, TA = 3.4 to 5 wt %) but are less abundant than mildly alkaline basaltic trachyandesites and trachyandesites (MgO = 3 to 5 wt %, TA = 6 to 7 wt %) that originate from numerous cinder cones including the paired vents at Los Cerritos de la Cruz (3.4 Ma), a vent north of the Rio de Los Pinos (3.2 Ma), Red Hill (2.8 Ma), and a vent on the northeast flank of San Antonio Mountain (2.8 Ma). San Antonio Mountain was active around 3 Ma and is composed of a precursor andesite (MgO = 2.8 wt %, TA = 6.8) followed by dacitic (MgO = 2 wt %, TA = 7 wt %) lavas that compose most of the edifice. Tholeiitic olivine basalts (MgO = 6 to 6.8 wt %, TA = 4 to 5 wt %) of a 2.4-Ma lava flow and a 2.3-Ma paired vent complex immediately east of San Antonio Mountain, Pinabetoso Peaks, are the youngest activity recognized on the west side of the TPVF.