THE CERRO COLORADO CINDER CONE, CERROS DEL RIO VOLCANIC FIELD: A STUDY OF MELT EVOLUTION AND PLUMBING SYSTEM DYNAMICS

Cerro Colorado is a 2.5-2.6 Ma dissected cinder cone in the Cerros del Rio volcanic field, north-central New Mexico. The volcano stands 2225 m in elevation and includes a well-defined 325 m diameter central vent characterized by inward dipping beds of moderately to strongly welded and highly oxidized scoriae deposits. These transition sharply to periclinally dipping wall facies composed of vesiculated fragments, oxidized and unoxidized cinders, spatter agglutinate, and lava flows. While Cerro Colorado units follow general cinder cone facies patterns (central crater facies, proximal and distal wall facies) and granularity trends (decreasing cinder size away from summit), exposed sections deviate from the idealized cinder cone models. Petrographic and powder xray diffraction studies concluded that the various volcanic facies showed similar mineral makeup (olivine + augite + plagioclase + magnetite ± quartz xenocrysts). Textures were predominantly porphyritic, with olivine and augite phenocrysts in a framework of plagioclase microlites ± glass. Several wall facies outcrops show multiple and regular alternations between explosive (pyroclastic falls) and effusive (lava flows) cycles. Pyroclastic deposits are highly vesiculated while lava flows are noticeably non-vesicular. These observations indicate that Cerro Colorado eruptions fluctuated from undegassed to degassed events, implying the existence of a shallow magma storage reservoir. Poorly vesiculated late-stage lava lobes suggest that as gas pressure dropped, the eruption evolved from pyroclastic fountains to lava effusions, following the expected cinder cone eruptive pattern. The presence of several subvertical lava conduits (dikes) in the wall and crater facies scoria layers suggests that the system contained lava channels. Our data indicate that the growth of the monogenetic Cerro Colorado cinder cone occurred via a multi-feeder geometry system, rather than a simple central axis feeder, challenging the simple feeder-dike plumbing system cinder cone model.