New Mexico Geological Society Annual Spring Meeting — Abstracts

The northern Burro Mountains exposes Paleogene intrusive and extrusive rocks that are part of the extensive Mogollon-Datil volcanic field (MDVF). Zircons from a granitic pluton, felsic and intermediate dikes, and an intermediate flow from the Burro Mountains, southwest New Mexico, were analyzed for U-Pb ages by Laser Ablation MC-ICP-MS. One of the dated dikes, an andesite porphyry, had plagioclase phenocrysts that were analyzed for Sr isotope compositions using TIMS.

Analyses of 9 plagioclase crystals from the plagioclase-phyric andesitic dike have (⁸⁷Sr/⁸⁶Sr)_o values from 0.712 to 0.715 when age-corrected to 34.5 Ma. Previously published whole-rock Sr data from other MDVF andesites have lower (⁸⁷Sr/⁸⁶Sr)_o: 0.706- 0.709 (Davis et al., 1993). Assimilation of Proterozoic basement can explain high initial ⁸⁷Sr/⁸⁶Sr signatures observed in the northern Burro Mountains.

The Hummingbird granite pluton southwest of the Red Rock area was mapped by Hedlund (1978) as Precambrian, but zircon dating yielded a much younger age at 32 ± 2 Ma. A granophyric dike also from the western area was ~74 Ma. A plagioclasephyric andesite dike has a weighted mean ²⁰⁶Pb/²³⁸U age of 34.5 ± 2.6 Ma and an andesite flow has an age of 32.8 ± 4.1 Ma. The dike cuts the Cretaceous Beartooth Quartzite, and the flow is stratigraphically below rhyolitic ash-flow tuffs. More precise dating of Paleogene dikes and flows will help constrain the timing of the transition from andesite to rhyolite magmatism in the MDVF. The presence of coeval intrusive rocks and tuffs indicates that the deeper levels of the magmatic system were exposed during NW tilting of crustal blocks along Neogene normal faults.