GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 51-4
Presentation Time: 9:00 AM-5:30 PM


HUMPHREYS, Mary, MICHELFELDER, Gary S., HOFFMAN, Max L. and RENTZ, Shannon Porter, Department of Geography, Geology, and Planning, Missouri State University, 901 S. National Ave., Springfield, MO 65897

The Rio Grande Rift in southern New Mexico began between 36-28 Ma marking a change in magma composition from mafic to andesite compositions to bimodal volcanism. During the early Oligocene southwestern New Mexico compressional arc magmatism due to the subduction of the Farallon plate transitioned to extensional stress magmatism of the Rio Grande rift. The Rubio Peak Formation marks the end of continental arc magmatism and the beginning of extension. The Rubio Peak Formation petrologically ranges from basalt lava flows and dikes to dacite ash flow tuffs. The overlying Bell Top Formation is the stratigraphically oldest bimodal ash flow tuff formation. These tuffs range in age from 36 to 29 Ma and represents a transition in magma composition and source.

Here, we use petrography combined with whole rock and mineral major and trace element geochemistry to gain insight on the formation from lava flows of the Rubio Peak Formation and compare these data to data from similar age rhyolite tuffs from the region. We also compare 40Ar/39Ar ages from the Rubio Peak to the 40Ar/39Ar and U/Pb ages of Bell Tops 2 and 3. This will help determine if the source of both formations came are linked or if the Rubio Peak was a product of early crustal contamination before fractional crystallization and is not the parental melt of the Bell Top tuffs. Trace element compositions of the Rubio Peak are distinct and do not reflect a parental source from crustal assimilation or by fractional crystallization. Plagioclase compositions of the two formations are unique and core compositions of the Bell Top tuffs do not match any analyzed composition from the Rubio Peak Formation. We suggest that the Bell Top tuffs are the result of partial melting of the upper crust as a result of thermal priming and MASH development during the emplacement of the Rubio Peak and Rubio Peak equivalent andesites and basalts in southern New Mexico.