DOUBLE DATING AND OXYGEN ISOTOPE ANALYSIS OF SILICIC LAVAS AND IGNIMBRITES FROM THE SIERRA MADRE OCCIDENTAL LARGE IGNEOUS PROVINCE (MEXICO): INSIGHTS INTO THE SOURCES AND TIMESCALES OF MAGMA GENERATION IN AN ARC FLARE-UP

During the Late Eocene to Early Miocene, northern Mexico was affected by widespread silicic magmatism and extension coeval with the westward migration of the Cordilleran volcanic arc, forming the 300,000 km² Sierra Madre Occidental (SMO) plateau. The SMO is the largest continuous rhyolitic ignimbrite province in the world. The rhyolitic magmas are interpreted to be the products of assimilation and fractional crystallization (AFC) processes with up to 60% mantle addition to the lower crust, as constrained by hafnium and oxygen isotopes in zircon. Here, we compare new zircon δ¹⁸O (SIMS-UCLA) and geochronology data from a subset of ten lavas and ten ignimbrites collected in a west (youngest) to east transect across the central SMO in the Chihuahua-Durango border region to ascertain if there is a difference in source components and the timescales of magma genesis. This is the first study of its kind in the SMO.

Preliminary data analysis suggests that the oxygen isotopes of the silicic lavas and ignimbrites are indistinguishable, at between 6 – 7.5 per mil; this agrees with previous studies that indicate no assimilation of upper crustal lithologies. Double-dating by U-Pb in zircon (SIMS-UCLA) and Ar/Ar (Auburn) analyses of sanidine (both step-heating and total fusion) constrain the duration of magma genesis through to eruption. Previous double-dating studies have indicated a ~1.5 - 2 Myr period between the zircon and Ar/Ar ages, interpreted to be the maximum duration of silicic magma genesis: analysis of our data continue, but will be the first to test if there is a systematic difference in the duration of magma genesis of lavas and ignimbrites. Our hypothesis is that ignimbrites, much more voluminous than the lavas, will have longer durations reflecting the extra time required to accumulate large, shallow magma bodies eventually erupted in caldera-forming events.