Masters Thesis

U-Pb and 40Ar/39Ar geochronology of rhyolitic tuffs and lavas of the Sierra Madre Occidental silicic large igneous province: Insights into Eocene-Oligocene silicic volcanism from a new sample transect, Chihuahua and Durango border region, northern Mexico

I present seventeen new U-Pb and eight 40Ar/39Ar ages from a previously unstudied geological traverse across the central Sierra Madre Occidental silicic large igneous province along the Chihuahua-Durango state border in Mexico. These data indicate progressive younging of volcanism to the southwest, with a succession of exclusively Eocene rhyolites in the northeast (c. 40 – 37 Ma), Eocene rhyolites overlain by Oligocene rhyolites in the center (c. 39 – 33 Ma), and primarily Oligocene rhyolites in the southwest (c. 34 – 30 Ma). A 39.6 Ma ignimbrite overlies Mesozoic metasedimentary basement at the northeastern end of the transect, approximately 20 km west of Hidalgo del Parral. A 29.5 Ma ignimbrite overlies a Cretaceous ignimbrite (67.0 Ma) at the western end of the transect at Guadalupe y Calvo. Southwestward younging of the volcanic stratigraphy agrees with data from elsewhere in the SMO, and all are inferred to record southwestward slab-rollback of the Farallon slab and arc migration during the waning stages of Sierra Madre Occidental volcanism. The non-uniform rate of Farallon slab retreat was approximated using available age and spatial data. These rates were 15.1 km/Ma through the northern sector, 11.7 km/Ma through the central sector, and 13.7 km/Ma through the southern sector. These values are in close agreement with reported rates of arc-trench retreat in modern arcs. In addition, I used these geochronological data to constrain the longevity of these silicic magmas to ≤2.2 Myrs and to attempt to correlate between ignimbrites. One correlation between three deposits is observed: the informally-identified 38.5 Ma, ≥26 km3 Ojito tuff.

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