New Mexico Geological Society Annual Spring Meeting — Abstracts

Abundant NNE- to NNW-trending mafic dikes near Riley, New Mexico are coeval with and appear to radiate from the Oligocene Socorro-Magdalena caldera cluster (SMCC, 32 -24 Ma). The westward migrating SMCC was emplaced in an ENE-trending zone (reactivated Laramide Morenci zone) during early opening of the Rio Grande rift. The mafic dikes intrude Cretaceous and Tertiary sandstones, shales, and conglomerates on the SE margin of the Colorado Plateau.

The mafic dikes are subdivided into three petrographic categories: kersantite (lamprophyric), feldspathoidal, and basaltic. Kersantite dikes contain large (~4 cm) clots of biotite in a groundmass of plagioclase and clinopyroxene. These groundmass phases commonly occur as pseudomorphs that are "replaced" by magmatic (?) carbonate. Feldspathoidal dikes typically lack plagioclase but contain nepheline, titaniferous clinopyroxene, minor phlogopite, and rare microphenocrysts of leucite. Carbonate is usually absent in the feldspathoidal dikes. Basaltic dikes contain clinopyroxene and rare olivine phenocrysts in a groundmass of plagioclase + clinopyroxene ± biotite. Despite differences in petrography, the dikes are chemically similar. When plotted on a total alkali-silica diagram, the dikes fall in the shoshonite, potassic trachybasalt, and basalt fields. Trace element plots, along with age data and spatial relationships, suggest the dikes have a common or similar magma source. Enrichment of HFS elements indicates that this source must be from a small-degree (ie. small volume) mantle melt or must include a significant crustal component. The large volume of the Oligocene Socorro-Magdalena magmatic system (7000 km3 ) implies that crustal assimilation was a significant factor in its evolution.