INSIGHT INTO MIDDLE TERTIARY CRUSTAL EXTENSION AND MANTLE MAGMATISM ALONG THE NORTHERN MARGIN OF THE SAN JUAN BASIN AT THE ONSET OF VOLCANISM IN THE SAN JUAN MOUNTAINS, SOUTHWESTERN COLORADO

Swarms of middle Tertiary mafic dikes are exposed along the northern edge of the San Juan Basin in southwestern Colorado. The emplacement of mantle magmas that gave rise to these dikes was close in time and space to large-scale volcanic events in the adjacent San Juan Mountains. The dike swarms reflect widespread incipient extension and invasion of mantle magmas that could have played an important role in regional magmatic events, but the history of these dikes is poorly documented and uncertain.

Preliminary studies establish that middle Tertiary mafic intrusive rocks in the region vary from phlogophite-rich minette to pyroxene-rich gabbro. Some of these rocks were involved in the formation of diatreme centers at approximately 27 Ma on the northeastern edge of the Navajo volcanic field. The dominant chemical signatures of mafic dike rocks in the northern San Juan Basin indicate that they have alkaline-potassic and LREE-enriched affinities. These chemical trends are consistent with the production of parent magmas from melting of metasomatized lithospheric mantle, as proposed for magmas in the Oligocene Navajo volcanic field.

The trends of middle Tertiary mafic dike swarms in southwestern Colorado fall mostly between 350° and 45°. There are also subordinate sets of dikes with roughly east-west trends. In most locations the dikes either cut or were emplaced into pre-existing faults and regional joint sets, but in some areas the dikes are displaced by conjugate sets of normal faults. Dikes trends and fracture patterns are consistent with north to northeast compression accompanied by the onset of regional crustal extension. This provided avenues for the invasion of mantle magmas that could have triggered crustal melting and extensive volcanism in the adjacent San Juan Mountains.