THE PETROLOGY AND GEOCHEMISTRY OF A MIOCENE UNDIFFERENTIATED DACITE IN THE MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO

The Mogollon-Datil volcanic field in southwestern New Mexico contains intermediate composition volcanic rocks that represent a transition time period between arc magmatism and extensional magmatism. In the Alma quadrangle these rocks are mapped as undifferentiated dacite flows at Spurgeon Mesa. These rocks have been dated at 25.2 Ma, and at type location are 280 meters thick. Previous investigations have described these rocks as fine-grained porphyritic pyroxene bearing dacite with equal amounts of orthopyroxene and plagioclase. The source of the dacite remains unknown. This study combines thin section petrography to identify mineral textures and whole rock geochemistry to characterize the rocks as a whole. Results from two different samples from the undifferentiated lava flows have shown differences in mineral size and assemblage. Five samples from the undifferentiated dacite were analyzed by XRF and ICPMS for major and trace element geochemistry. These data are combined with 31 previously analyzed andesite and dacite samples that are geographically adjacent to the undifferentiated dacite.

Dacite phenocrysts include plagioclase, clinopyroxenes and rare orthopyroxene and biotite. All samples are crystal-poor to crystal-rich ranging from 6-45% phenocrysts. Plagioclase phenocrysts are anhedral to subhedral with sieving and resorption textures. Pyroxenes are typically subhedral phenocrysts occurring in glomerocrysts. Samples are andesite to dacite ranging from 58.6-62.5 wt% SiO₂. FeO and MgO range from 5.7-6.2 wt% and 2.1-3.2 wt%, respectively. Sr ranges from 475-600 ppm with Sr/Y ratios ranging from 11-20. Adjacent intermediate composition rocks of the Mineral Creek and Last Chance Andesite are similar in composition ranging from 51.2-61.7 wt% SiO₂ and 4.7-10.93 wt% FeO. Preliminary data suggests that the undifferentiated dacites of Spurgeon Mesa are likely related to the Mineral Creek Andesite based on major and trace element data and petrography. The Last Chance Andesite is more mafic and contains higher trace element contents and ratios suggesting it is from a different magmatic source.