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
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.