NEW 40AR/39AR AGES FROM THE CEROCAHUI BASIN REGION OF THE NORTHERN SIERRA MADRE OCCIDENTAL, CHIHUAHUA, MEXICO: IMPLICATIONS FOR IGNIMBRITE CORRELATION AND THE TIMING OF SYNEXTENSIONAL DEPOSITION

The Cerocahui basin is a half-graben located adjacent of the Copper Canyon region of the northern Sierra Madre Occidental silicic large igneous province. Previous studies in the area have identified late Oligocene alluvial and mafic-intermediate volcanic synextensional basin deposits underlain by several Oligocene silicic ignimbrite outflow sheets (Parajes formation). Age control of the study area is limited; this study presents new ⁴⁰Ar/³⁹Ar ages to further constrain depositional ages and the timing of extension.

The Chepe ignimbrite, the lowest stratigraphic unit of the Parajes formation, is exposed in a horst block on the eastern edge of the Cerocahui basin and yields an age of 34.68 ± 0.14 Ma (2σ). Previous work tentatively correlated this ignimbrite to the regionally extensive ~29.9 Ma Divisadero tuff based its stratigraphic position below younger dated rocks in the study area and its distinct crystal-rich phenocryst assemblage (~30% embayed quartz, plagioclase, biotite, hornblende, and sanidine; to 2 mm) that is similar to samples collected from the Divisadero tuff type-locality. However, the correlation of these two ignimbrites is highly unlikely based on our new age data. Instead, the Chepe ignimbrite is more likely correlated to the previously identified 34.1 ± 0.9 Ma Vista tuff, which was erupted from the Las Varas caldera ~125 km NNE of this study area. An outflow unit of the Vista tuff is identified ~70 km S of the caldera and is nearly identical in appearance to the Divisadero tuff, suggesting that this tentative correlation with the Chepe ignimbrite is plausible. The KM ignimbrite is near the stratigraphic top of the Parajes formation, with our new data yielding an age of 27.46 ± 0.30 Ma. The age of the KM ignimbrite corresponds well with previous zircon U-Pb LA ICP-MS dating from the underlying Puerto Blanco ignimbrite (27.58 ± 0.26 Ma) and from an ignimbrite near the base of the overlying Cerocahui basin deposits (28.1 ± 0.8 Ma).

A basalt lava unit that conformably caps the Cerocahui basin alluvial deposits yields an age of 23.59 ± 0.13 Ma. Based on previous work, this new age suggests a depositional rate of ~300 m/Myr in the basin near the fault margin. The basalt lava unit is relatively flat-lying and has only minor offset across the basin bounding fault, suggesting that extension in the study area was waning by ~23.6 Ma.