The Mt. Tule dike swarm is located approximatey 65 miles east of San Diego near Jacumba, California. It is a layered pegmatite-aplite system forming a bowl-shaped structure in map view, opening to the north, and occurs within a metasedimentary roof pendant that lies near the center of the La Pasta pluton. Bordering the dike system on the south is a garnet-two-mica monzogranite. E. R. Mapper was used to plot this dike system on a SPOT panchromatic image with 10-meter resolution. Although showing a variety of strikes and dips, the general trend for the dikes is a north-northwest strike along the western side with a moderate dip to the southwest, an east-west strike along the southern side with a moderate dip to the south, and a north-northeast strike along the eastern side with a moderate dip to the southeast. A central zone in the dike structure contains sparse dikes with subhorizontal attitudes. The La Pasta pluton intruded and engulfed the Triassic (?) metasedimentary roof pendant at 94 ± 1 Ma, partially melting the roof pendant to create the garnettwo-mica monzogranite which has been dated at 93 ± 1 Ma and the dike system around the borders. It is not clear why the steeply dipping dikes are limited to the margins of the roof pendant, but their distribution may be related to the degree of partial melting in the metasedimentary package.
