GEOLOGY AND GEOCHRONOLOGY OF THE CERROS DEL RIO VOLCANIC FIELD, NEW MEXICO

The Cerros del Rio volcanic field (CDRVF) is one of several middle Pliocene to Pleistocene basaltic volcanic fields of the axial Rio Grande Rift in central and northern New Mexico. The CDRVF is the 2nd largest, occupying 700 km² and representing up to 120 km³ of mafic magma erupted during the 2.7 to 1.1 m.y. interval. Most of the lavas erupted during the 2.7 to 2.2 m.y. interval and were deeply incised forming paleovalleys subsequently filled by the Otowi Member (1.61 Ma) of the Bandelier Tuff. Late stage, less voluminous Cerros del Rio lavas (1.1 Ma) overlie reworked deposits of the Tshirege Member (1.22 Ma) of the Bandelier Tuff in the western part of the field and flowed over steep escarpments of the La Bajada fault in White Rock Canyon.

Eruptive centers of the CDRVF are typically central vent volcanoes, ranging from low-relief shield centers to steep-sided, breached cinder and spatter cone remnants. Lavas range from 45 to 64 wt% SiO₂ and exhibit strong correlation of geomorphic form with whole-rock chemistry. Low-silica, subalkaline basaltic lavas (less than 52 wt% SiO₂) are thin (less than 3-4 m), far-traveled, and erupted from broad shield volcanoes. Transitional to mildly alkaline basalts and basaltic andesites (50-63 wt% SiO₂ and more than 5 wt% Na₂O+K₂O) occur as thick (to 30 m), discontinuous lavas erupted from high-relief, steep-sided vents. Dacite lavas (more than 63 wt% SiO₂) occur high in the section and are related to mid-stage dome growth and eruption of thick (to 50 m) blocky lava flows from poorly defined vent areas. Mapped volcanic centers represent short eruptive events that likely were derived from rapidly evolving reservoir-conduit systems. Compositional variation of magma chemistry in space and time is complex and likely related to episodic interactions between newly arrived magma inputs with pockets of evolved magma and/or crystal mush, leading to a diversity of eruptive products as opposed to differentiation in large long-lived reservoirs.