AN 8 MA MAGMATIC LOCUS WITH A 3.5 M.Y. ERUPTIVE HISTORY: CERRO CHAXAS IN THE ALTIPLANO-PUNA VOLCANIC COMPLEX OF THE CENTRAL ANDES

Cerro Chaxas records the rapid onset and lifecycle of a magmatic locus during upper crustal flare-up magmatism. Prior geochronologic and field data indicated that volcanic activity at Chaxas spanned nearly the entire duration of APVC flare-up magmatism from 8.35-2.64 Ma; possibly including the ≥1000 km3 dacitic Puripicar ignimbrite eruption (4.18 Ma). This study presents new stratigraphic and geochronologic data (U-Pb in zircon) that shows volcanic activity at Chaxas initiated with a pyroclastic flow of crystal poor rhyolite that is temporally indistinguishable (4.04±0.5 Ma) from the Puripicar. Following emplacement of the Puripicar, from 3.25-1.55 Ma, block and ash (B&A) flows generated from effusing crystal rich domes (≥50% crystallinity) were deposited with sporadic small volume pyroclastic flows (PFs) and fallouts. These units have overlapping zircon age distributions indicating prolonged formation and destruction of post-Puripicar domes. Dome clasts displaying differing lithologies separated by serrated margins in hand sample and small PFs coeval with B&A flows corroborate the extended presence of an endogenously growing dome complex. The eruptive record of Cerro Chaxas thus records the peak to waning stages of the regional flare-up.

Magmatic history at Chaxas is inferred via in-situ zircon trace elements and ages, and pressure estimates derived from the MELTS geobarometer. Data indicates that while the initiation of volcanism from Chaxas at 4-4.5Ma was abrupt, consolidation of zircon saturated magma in the upper crust (≤300 MPa) began between 7-8 Ma. Zircon age distributions in young eruptions overlap with those from older eruptions and have jumps in their cumulative distribution functions, implying late Chaxas magmas repeatedly sampled zircon antecrysts during ascent. Trace elements and Ti-in-zircon temperatures for younger zircons have more variability (i.e., diverging trends) than older zircons. Magma storage is interpreted to have transitioned from a and large unitary homogenous system to a more dispersed system with discrete melt regions as the regional flare-up dissipated. Finally, we will present population size dependent birth-death models to simulate evolution of zircon populations as the Chaxas system evolved, and contrast these with models appropriate for small volume systems.