INCREASED ERUPTION TEMPO FOLLOWING A LONG ERUPTION HIATUS PRIOR TO THE TSHIREGE CALDERA-FORMING ERUPTION IN THE JEMEZ MAGMATIC SYSTEM, NEW MEXICO, USA

The Otowi and Tshirege members of the Bandelier Tuff erupted at 1605 and 1232 ka, respectively, during catastrophic caldera-forming eruptions. In between these events several dozen eruptions of rhyolite lava and tephra occurred, collectively known as the Valle Toledo Member of the Cerro Toledo Formation. New high-precision ⁴⁰Ar/³⁹Ar sanidine ages for these eruptions, together with detailed petrography, document previously unrecognized temporal and mineralogical changes leading up to the Tshirege eruption. For ~10 kyr following the Otowi eruption, the maximum average recurrence interval between eruptions was 1 kyr. Afterwards, from 1595-1350 ka, eruptions occurred less frequently, approximately every 9 kyr. This slower-tempo period was followed by a ~100 kyr hiatus in volcanic activity. Beginning at 1240.5 ±2.1 ka, the system transitioned back into a high-tempo state, with at least six explosive and two effusive eruptions within 8.6 ±2.5 kyr of the Tshirege event. These post-hiatus rhyolites, as well as the Tshirege eruption, tapped rhyolite that contain either biotite, fayalite, or both phases. In contrast, all rhyolites erupted before the hiatus lack fayalite, and instead contain biotite, pyroxene, and/or amphibole. This shift in mineralogy is evidence that a major change in magma storage conditions occurred while the system matured and primed itself for an eruption that ultimately erupted ~400 km³ of magma. Collectively, the shifts in eruption tempo and phase assemblage offer insight into what a potential future pre-caldera Jemez system might look like. Post-Tshirege volcanism in the Jemez system has mostly been effusive eruptions of biotite-bearing rhyolite. Eruption tempo is commonly slower (overall average recurrence interval of ~35 kyr) than it was before the Tshirege eruption. The most recent activity at ~70 ka contrasts with older post-Tshirege volcanism in that following an eruption hiatus of ~450 kyr, significant explosive eruptions occurred. This shift away from predominantly effusive eruptions is reminiscent of the sequence of explosive eruptions that ended the eruption hiatus prior to the Tshirege caldera-forming eruption.