EXAMINING ASH FALL SEQUENCES IN CALK-ALKALINE SUBDUCTION RELATED VOLCANISM, SOUTHERN NEW MEXICO

The Mogollon-Datil volcanic field (MDVF) is a 40-20 ma cluster of caldera activity in southern New Mexico tied to the subduction of oceanic lithosphere beneath the North American continental plate. The calk-alkaline magmatism of the three calderas in this field (Mogollon, Bursum, and Gila Cliff Dwellings) produced several voluminous regionally dispersed ash flow tuffs. This study will specifically examine the volcanic rocks: Cooney Formation (a compositionally zoned rhyolitic to quartz latite ash flow tuff containing quartz>k-feldspar>plagioclase>biotite, pumice and lithic fragments. Rb ranges from 213-317ppm, Sr from 104-108ppm, and ⁸⁷Sr/⁸⁶Sr_m from 0.71326-0.71534), Davis Canyon Tuff (a phenocryst poor, high-silica rhyolite ash flow tuff containing sanidine>quartz>sodic plagioclase. Rb ranges from 214-230ppm, Sr from 42.2-63.2ppm, and ⁸⁷Sr/⁸⁶Sr_m from 0.71383-0.71464), and the Shelley Peak Tuff (a compositionally zoned, crystal-rich rhyolite tuff containing sodic plagioclase>minor sanidine>biotite>cpx. Rb ranges from 154-213ppm, Sr from 105-245ppm, and ⁸⁷Sr/⁸⁶Sr_m from 0.70944-0.7112) to further elucidate their petrogenic origins, attempt to determine if they may be of the same magmatic source, and yield data that could help model processes that would generate magma of these particular compositions.

This study will examine compositional variation between the Davis Canyon, Shelley Peak and Cooney Tuffs and help with understanding the magma plumbing system during each eruption. More specifically, we will evaluate possible crustal components of these units, along with looking for geochemical signatures of arc or rift related magmatism. We will compare previous geochronology results (K-Ar and U-Pb fission track) and whole rock major and trace element geochemistry to data obtained via ⁴⁰Ar/³⁹Ar and U/Pb dates and new whole rock Sr, Nd and Pb isotopic ratios.