THE CHOIYOI SILICIC LARGE IGNEOUS PROVINCE OF ARGENTINA AND CHILE AND A THREE PHASE VOLCANISM-MASS EXTINCTION MODEL FOR THE END-PALEOZOIC BIOTIC CRISIS

The Permo-Triassic Choiyoi magmatic province of central and southern Argentina and Chile (23°-42°S) with an aerial extent of ~664,000 km2 and dominant rhyolite-ignimbrite composition ranks amongst the largest silicic large igneous provinces on earth. Laser ablation ICP-MS zircon U-Pb ages from key Argentina transects define a strongly episodic history of magmatism with a major peak in volcanic activity at 266±4 Ma. This includes ~266 Ma emplacement ages throughout >1 km thick sections that attest to rapid generation and emplacement of silicic magmas. Zircon U-Pb ages from a regional N-S oriented dike swarm suggest that a substantial component of Choiyoi pyroclastic rocks were fed from fissure eruptions. Polar volcanic eruptions are more likely to inject gas and ash into the stratosphere and the Choyoi province was emplaced at extremely high paleolatitudes of ~60°S-80°S. Substantial stratosphere injection from Plinian eruptions under these circumstances would be expected at 100-year to decadal or even annual frequency with concomitant climatic impact. The Choiyoi flareup of silicic volcanic activity overlaps broadly with several major events occurring around this time including the Kamura cooling event, a decline in diversity of fusulinoids, prolonged stepwise tetrapod extinction, and carbon isotope excursions, external causes for which have gone unrecognized. These events precede the two closely spaced major mass extinction events that close out the Paleozoic Era, the ‘end-Guadalupian’ extinction at ~260 Ma and end-Permian mass extinctions at ~251 Ma each of which are each associated with major flood basalt eruption, the Emeishan Traps (260‐257 Ma) and the Siberian Traps (253‐250 Ma), respectively. The Choiyoi flareup at 270-262 Ma was therefore the first in a sequence of three closely spaced LIP magmatic events that may have collectively contributed to sustained environmental instability driving the end-Paleozoic biotic crisis.