REVISED TIMING OF OLIGOCENE MAGMATISM AND EXTENSIONAL BASIN DEVELOPMENT IN THE CEROCAHUI-GUAZAPARES REGION OF THE NORTHERN SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE, CHIHUAHUA, MEXICO

Previous studies by Murray & Busby in the Cerocahui-Guazapares region of the northern Sierra Madre Occidental identified three late Oligocene synextensional rock units deposited during the North American mid-Cenozoic ignimbrite flare-up: 1) ca. 27.5 Ma Parajes group, a ~1 km-thick sequence of welded silicic outflow ignimbrite sheets; 2) ca. 27–24.5 Ma Témoris formation‒alluvial deposits, mafic-intermediate lavas/intrusions, and distal nonwelded silicic ignimbrites, and 3) ca. 24.5–23 Ma Sierra Guazapares formation‒silicic vent/proximal ignimbrites, lavas, and plugs. Age control in this region is limited; this study presents new ⁴⁰Ar/³⁹Ar ages that further refine our previous ages of deposition and extension determined by U-Pb ICP-MS zircon dating.

The Chepe ignimbrite, the lowest stratigraphic unit of the Parajes group, yields an age of ca. 34.5 Ma. Based its relative stratigraphic position and its distinct crystal-rich phenocryst assemblage (~30% quartz, plagioclase, biotite, hornblende, and sanidine; to 2 mm), we previously tentatively correlated this ignimbrite to the regionally extensive ~29.9 Ma Divisadero tuff. However, our new age data makes this correlation highly unlikely; instead, the Chepe ignimbrite is more likely correlated to the previously identified ca. 34.1 Vista tuff erupted ~125 km NNE of this study area. An outflow unit of the Vista tuff is found ~70 km south of the source caldera and is nearly identical in appearance to the Divisadero tuff, suggesting the correlation with the Chepe ignimbrite is plausible. A new date from the KM ignimbrite, near the stratigraphic top of the Parajes group, yields an age of ca. 27.5 Ma, which corresponds well (within error) of our previous zircon U-Pb LA ICP-MS dates from the underlying & overlaying ignimbrites.

In the Cerocahui basin, Témoris formation alluvial deposits are deposited over Parajes group ignimbrites and exhibit fanning bedding dips that decrease upsection. These deposits are capped by a conformable flat-lying basalt lava that yields an age of ca. 23.5 Ma. With our previous dating, this new age suggests a depositional rate of up to 300 m/Myr in the basin. The basalt lava has only minor offset across the basin bounding fault, suggesting that extension in the study region was active since at least ca. 27.5 Ma and was negligible after ca. 23.5 Ma.