New Mexico Geological Society Annual Spring Meeting — Abstracts

Many NNE- and NNW-striking mafic dikes (~100) are exposed across an eight km-wide swath of gently SW-dipping preOligocene strata on the SE Colorado Plateau near Riley, New Mexico. Basaltic dikes are dominant and typically contain sparse phenocrysts of augite and olivine in a groundmass of plagioclase microlites and intergranular pyroxene. Minette and nephelinite dikes are rare; the minettes contain 3-4 cm biotite clots and the nephelinite dikes contain phenocrysts of titanaugite and leucite trapezohedrons. Most of the dikes are highly altered; pyroxene is commonly replaced by calcite and siderite and olivine is commonly replaced by serpentine and minor chalcedony.

The subparallel dikes commonly show en echelon steps and several dikes show zigzag or branching patterns, which demonstrate that the NNE and NNW trends are penecontemporaneous. A few dikes have baked and reduced wallrock aureoles as much as 20 m wide indicating that they probably erupted and fed lava flows that were once laterally continuous with the Oligocene La Jara Peak Basaltic Andesite, which is preserved in the adjacent Bear Mountains. The width of one apparent feeder dike decreases north of the contact metamorphic zone and terminates within ~200 m. One northward branching dike forms two successive forks and thins abruptly northward at each fork, which indicates that magmatic pressure drops to the north. Eocene sandstone beds are locally upturned or folded at dike stepovers, with the steepest beds dipping north. Near dike intersection points, younger dikes often abruptly change strike, which suggests that older dikes formed a stress guide during subsequent dike propagation. These observations support the hypothesis that the dikes propagated northward from the mafic roots of the Oligocene Socorro-Magdalena caldera cluster (24-32 Ma). Preliminary 40Ar/39Ar age determinations indicate the dikes are 26-30 million years old.