NEW INSIGHT INTO THE FORMATION OF DIATREMES IN THE NORTHERN NAVAJO VOLCANIC FIELD, WEBER MOUNTAIN, SOUTHWESTERN COLORADO

A series of northeast-trending, phlogopite-rich alkaline basalt dikes and related maar diatremes form the northern extent of the Oligocene Navajo volcanic field in southwestern Colorado. A detailed field investigation of the dike-diatreme system at Weber Mountain reveals a history of pre- and post-eruptive dike emplacement and lateral subsurface volcanic eruption. We propose that the emplacement and evolution of this system was driven by basaltic magmas that emplaced along pre-existing northeast structural fabrics in the crust. All of the maar diatremes exposed in the Mesa Verde area provide evidence of lateral eruptions near the upper contact of the Mancos Shale, rather than vertical eruptions with roof collapse. This lends evidence to the hypothesis that the formation of subsurface diatremes in parts of this system were determined by gas overpressures at a given stratigraphic horizon, rather than interaction of the magmas with regional aquifer systems.

At Weber Mountain, an apron of bedded basaltic tuff and tuff breccia developed along the eastern margin of the northeast-trending system of dikes, and fragments of older dikes were entrained in this eruptive phase. Subsequent dike emplacement followed northwest and east-west trends, and crosscut the tuff breccia apron after the eruptive phase. Preliminary petrologic and petrochemical studies indicate that the basaltic dikes and tuffs at this site are similar in many respects to those in other diatreme systems in the Navajo volcanic field, but they have some anomalous characteristics that could reflect variable compositions caused by differentiation of similar magmas or perhaps different mantle magma sources.