TIMING OF INTENSE MAGMATIC EPISODES IN THE NORTHERN SIERRA MADRE OCCIDENTAL, MEXICO

The approx. 400,000-km ² calc-alkalic Sierra Madre Occidental (SMO) is the largest ignimbrite-dominated volcanic province in North America. Within the northern SMO brief periods of intense magmatism have been identified using bulk-sample and single-crystal laser-fusion ⁴⁰Ar- ³⁹Ar dating of sanidine and anorthoclase associated with two areas of concentrated mapping. Area 1 extends for 300 km at 28-28.5° N latitude, from Yecora (Sonora) eastward to Chihuahua City. Area 2 extends northward from Chihuahua City for 160 km along 106-106.5° W longitude. In area 1, forty-five ages range from 28.1 to 40.9 Ma. There are concentrations of fourteen ages between 33.1 and 33.9 Ma and of ten ages between 34.8 and 36.0 Ma. Rocks having these ages are present across the entire area. Four ages from distinctive peralkaline ignimbrites (ferroaugite rhyolites) are clustered at 29.82 ± 0.05 Ma. In area 2, thirty ages range from 29.3 to 45.4 Ma. Thirteen ages for the ferroaugite rhyolites are from 29.3 to 30.2 Ma. Except for one age at 30.0 Ma, the other ages range between 33.6 and 45.4 Ma. Of these eleven ages are between 44.1 and 45.4 Ma.

These three major episodes (33.1-33.9 Ma, 34.8-36.0 Ma, and 44.1-45.4 Ma) are clearly regional in extent. The two youngest are not well represented in area 2, but they match activity in the nearby Datil-Mogollon (New Mexico) and Trans-Pecos (Texas) volcanic fields. The oldest episode is older than those in any adjacent field. However, it may be unrecognized in area 1 due to extensive younger cover. A minor peak of 28-30 Ma activity in the western part of area 1 is more prominent to the south in the Copper Canyon area, and near Durango City at latitude 24°. The brief pulse of peralkaline volcanism at 29-30 Ma in areas to the east of the SMO may be a response to intensification of tensional stress there. A definitive explanation for the other brief outbursts remains elusive.