CHARACTERIZATION OF SUBVOLCANIC CONDUITS OF THE BASALTIC INTRUSIVE COMPLEX OF THE SAN RAFAEL SWELL, CENTRAL UTAH

Exposures of subvolcanic plumbing systems the fed monogenetic basaltic volcanic fields from the upper few kilometers are relatively rare despite their relative abundance in extensional systems such as the U.S. Basin and Range Province. In this study we present one of few studies of such a system, the basaltic intrusive complex of the San Rafael Swell of central Utah. This complex is located near the northwestern margin of the Colorado Plateau and is likely associated with decompression melting related to lithospheric thinning at the transition into the adjacent Basin and Range. The complex is comprised of numerous dikes, sills, and, perhaps, three to four dozen volcanic conduits. Erosion has removed nearly all traces of the volcanoes but provided an ideal natural laboratory for studying the plumbing system of a monogenetic volcanic field.

The volcanic conduits are everywhere associated with dikes and can, in most places, be shown to have developed through one or more of the sills. In a few rare examples it can be demonstrated that the conduit interacted (i.e. exchanged melt) with adjacent conduits. Many of the conduits appear to form in the step-over between en echelon segments of dikes. Other conduits appear to nucleate on dikes, beginning as narrow zones of brecciation of the bounding host rocks. In all examples these conduits evolve into complex systems with multiple mappable horizons of distinct combinations of melt and host rocks. These include host rock breccia that may or may not develop a preferred orientation (clast-defined foliation), mixtures of host rock breccia and melt, and melt ranging from densely choked with host rock xenoliths to xenolith-free melt (typically within the core of the conduit). It appears that as the conduit develops it is characterized by an evolution where early breccias become partially mixed and fluidized by the ascending melt.