NEW ZIRCON U-PB AGES DEFINE A STRONGLY EPISODIC HISTORY OF MAGMATISM FOR THE PERMO-TRIASSIC CHOIYOI SILICIC LARGE IGNEOUS PROVINCE OF CHILE AND ARGENTINA

The Choiyoi rhyolite large igneous province of Chile and Argentina (23°S-42°S) was emplaced along the southwest margin of Gondwana in a subduction-related magmatic arc setting. The Mendoza and Rio Blanco river sections (32°S-33°S) provide important insight into the petrology and timing of Choiyoi magmatism. Here, Choiyoi volcanic strata occur in a ~40 km wide north trending structural belt with Choiyoi rocks deposited unconformably across Carboniferous basement along the western side of the belt and depositionally overlain by Middle Triassic Cuyo Basin strata on the eastern side. Eighteen new laser ablation ICPMS zircon U-Pb crystallization ages from samples collected across this belt define a strongly bimodal age distribution with peaks at ~248 Ma and 266 Ma. The older peak dominates the distribution with thirteen of the eighteen ages yielding a weighted mean of 266.3±1.4 Ma (95% conf.). The age peaks from in situ samples are also strongly expressed in the detrital zircon U-Pb age record of Neogene foreland basins along this segment of the margin, and together are consistent with a strongly episodic flareups in Choiyoi magmatism. The older peak in magmatism has been ascribed to flat-slab subduction and slab breakoff during the waning stages of subduction regime. Whole rock geochemistry suggest a transition from calc-alkaline magmatism at ~266 Ma to more alkaline/shoshonitic compositions at ~248 Ma as previously suggested for Choiyoi rocks to the north (La Pampa province) and the southeast (San Rafael Massif).