New Mexico Geological Society Annual Spring Meeting — Abstracts

Timing and petrogenesis of a Mesoproterozoic mafic dike swarm in southwestern North America is poorly understood. Existing diabase ages range from 1200 Ma-1080 Ma, but most have poor precision. Diabase samples collected from the Organ and Burro Mountains of southern New Mexico and from the Salt River Canyon, Sierra Ancha, Pinaleno, Santa Catalina, Hualapai, and Garnet Mountains of eastern and northwestern Arizona intrude Proterozoic sediments and granites in the form of dikes and sills. Diabases are typically medium grained with ophitic and subophitic texture and have plagioclase laths, near ~1 mm in length. Subhedral olivine crystals are present in most New Mexico and SE Arizona diabase, but they are uncommon in NW Arizona diabase. Feldspar compositions for the Burro and Sierra Ancha Mtns. range from An .321 to An 70.43. Pyroxene compositions of the Burro and Sierra Ancha Mtns. reflect the presence of both clinopyroxene (augite) and orthopyroxene (hypersthene), and pigeonite. Strontium isotope analyses of Burro Mtns. diabase show ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.700624- 0.705408 for single plagioclase crystals, which spans higher than whole rock values ranging from 0.702076-0.703484 indicating likely assimilation of the host granite by the dikes. We obtained a U-Pb zircon age of 1115±13 Ma (2s) using LA-MC-ICP-MS on a dike from Salt River Canyon in Arizona. This age is older than U-Pb baddeleyite ages of 1087±3 Ma and 1069±3 Ma obtained from two diabase sills in California (Heaman and Grotzinger, 1992). This discrepancy suggests there are two different aged dike swarms or there was a long lived magmatic interval. Dikes with ages closer to 1100 Ma may be related to the Midcontinent Rift (Hammond, 1990)