New Mexico Geological Society Annual Spring Meeting — Abstracts

The Burro Mountains of New Mexico lie at the southernmost edge of the Mogollon-Datil volcanic field. New U-Pb zircon ages from intermediate and felsic lava flows, hypabyssal intrusions, and ash-flow tuffs provide new age constraints on timing of magmatism in the Burro Mts. The youngest andesite lava flow was analyzed for Sr-isotope compositions to determine degree of and source of contamination by country rock.

U-Pb LA-MC-ICPMS and SHRIMP zircon ages of volcanic rocks in the Saddlerock area of the Burro Mts. indicate a compositional transition at ~34 Ma from intermediate stratocone magmatism to felsic magmatism of the Schoolhouse Mountain caldera. Weighted mean ages for Saddle Rock Canyon Fm. andesite lava flows ranged from 33.5 to 35.3 Ma. Andesite breccias interfingered with the first overlying rhyolite ashflow tuff and tuff-breccia of the Kerr Canyon Fm., which yielded an age of 34.3 ± 0.4 Ma. The Kerr Canyon latite yielded an age of 34 ± 0.98 Ma, and flow-banded rhyolite plug which intruded the latite, an age of 34.7 ± 1.6 Ma. A rhyolite dike, which crosscut an andesite dike (33.5 ± 0.3 Ma), yielded an age of 33.7 ± 0.4 Ma. Another dike, mapped previously as an Oligocene rhyolite, yielded a weighted mean age of 75 ± 1.3 Ma. Current mapping and geochronology suggest that ~75 Ma magmatism in the Burro Mts. was more extensive than previously determined, and provided moderate xenocrystic zircon contamination in Paleogene volcanic rocks.

Single-crystal TIMS analysis of 7 plagioclase crystals from the youngest Mangas Creek andesite lava flow have (⁸⁷Sr/86Sr)_o values from 0.7074 to 0.7079 when age-corrected to a U-Pb LA-MC-ICPMS zircon age of 33.8 ± 0.7 Ma. Eruption of the andesite lava through thick intracaldera deposits and lack of assimilation of Proterozoic basement rock can explain the (⁸⁷Sr/86Sr)_o signatures observed.