U-PB AND 40AR/39AR GEOCHRONOLOGY OF RHYOLITIC TUFFS AND LAVAS OF THE SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE, CHIHUAHUA AND DURANGO BORDER REGION, NORTHERN MEXICO

We present new U-Pb (zircon-SIMS) and ⁴⁰Ar/³⁹Ar (biotite, hornblende, sanidine) ages from rhyolitic ignimbrites and lavas collected on a recently established geological transect along Highway 24 across the north-central portion of the Sierra Madre Occidental silicic large igneous province (SMO), approximately along the border between Chihuahua and Durango states, between the towns of Hidalgo de Parral and Guadalupe y Calvo. This is the first new transect to be established across the SMO in over 20 years, and was chosen specifically because it straddles the boundary between extended and non-extended parts of the SMO. Samples were chosen specifically to constrain the tops and bottoms of stratigraphic sections exposed in river canyons and basin-bounding fault scarps; therefore, they bracket the ages of numerous non-dated tuffs in between. These data provide absolute and relative age constraints on stratigraphic sections described in the field and insight into the temporal distribution of Eocene-Oligocene silicic volcanism. Data indicate progressive younging to the southwest along the transect, with a succession of exclusively Eocene rhyolites in the northeast (c. 41 – 37 Ma), Eocene rhyolite overlain by Oligocene rhyolites in the center (c. 39 – 33 Ma), and exclusively Oligocene rhyolites in the southwest (c. 33 – 31 Ma). A 40.4 Ma ignimbrite overlies Mesozoic metasedimentary basement at the northwestern end of the transect, approximately 20 km west of Parral. A 31.3 Ma ignimbrite overlies a Cretaceous ignimbrite (66.4 Ma) at the western end of the transect at Guadalupe y Calvo. Using these data we also present some tentative chronostratigraphic correlations along the transect. 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 during the waning stages of SMO volcanism. However, the temporal pattern of volcanism along the Chihuahua-Durango border is unlike that in semi-adjacent (>250 km) areas. There is a near-absence of volcanic units dated to between c. 37 – 33 Ma (n = 1), suggesting a waning of volcanism at this time. Furthermore, evidence of Early and Middle Eocene (>42 Ma) rhyolites and Miocene (c. 24 – 22 Ma) rhyolites is absent along our transect, but characteristic of semi-adjacent areas.