V003-0013
Temporal, Spatial, and Geochemical Variations in Eocene-Oligocene Ignimbrite Flare-Up Volcanism in Southern New Mexico

Monday, 7 December 2020
Poster
Karissa Brianne Vermillion, New Mexico State University Main Campus, Department of Geological Sciences, Las Cruces, NM, United States, Emily R Johnson, USGS Cascades Volcano Observatory, Vancouver, WA, United States and Matt Heizler, New Mexico Institute of Mining and Technology, Socorro, United States
Abstract:
The Paleogene ignimbrite flare-up across much of the present-day southwestern U.S. produced widespread silicic, caldera-forming eruptions. In southern New Mexico, the ignimbrite flare-up generated the Mogollon-Datil (MDVF) and Boot Heel (BHVF) volcanic fields between ~36 and 21 Ma. Previous studies have proposed an approximately NNW migration of volcanism over time due to rollback of the Farallon slab. Here we present high-precision, single-crystal, laser fusion 40Ar/39Ar geochronology and whole-rock and feldspar geochemistry to investigate temporal, spatial and geochemical variations in magmatism across the MDVF and BHVF.

Our samples come from tuffs spanning ~250 km from the eastern MDVF to the southwestern BHVF; our new eruption ages range from 36.5 to 33.2 Ma. Our results show that the initiation of caldera volcanism was widespread across southern New Mexico. The oldest silicic volcanism began around 36.6 Ma in the MDVF, but caldera volcanism was continuous across ~200 km of southern New Mexico at ~36.5 – 35.5 Ma. We find a slight westward age progression of the youngest tuffs, but overall, our results do not indicate a strong westward migration of the initiation of caldera volcanism.

We also find temporal variations in whole rock and sanidine geochemistry. For example, sanidine Ba concentrations increase over time in both volcanic fields. Tuffs older than 35.5 Ma tend to be crystal poor and contain sanidine with lower Ba (<0.16 wt%), whereas younger tuffs (<35.5 Ma) are crystal rich, with sanidine that have variable but higher Ba concentrations (<1.4 wt%). There are also temporal changes in whole rock geochemistry; for example, Eu anomalies in the rhyolites decrease over time in the MDVF, consistent with an increase in tuff crystallinity. Together, these results could indicate a greater role of magma recharge over time, where new magma remelted crystal cumulates and resulted in remobilization and eruption of crystal-rich tuffs with high Ba sanidine.

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