Effusive volcanic microcosm of a regional ignimbrite flare-up: Prolonged life cycle of the Chaxas Complex, northern Chile, and its influence on modern volcanic arc character

Volcanological and temporal development of the Chaxas Complex on the western edge of the Altiplano-Puna Volcanic Complex of northern Chile is reported here in full for the first time. The Chaxas Complex is a collection of spatially and chronologically related lava domes and associated volcaniclastic deposits that was thought to be constructed between 8.35 Ma and 1.09 Ma, spanning almost the entire duration of the Altiplano-Puna Volcanic Complex flare-up. Prior studies also suggested that the 4.18 ± 0.03 Ma Puripicar ignimbrite was discharged from a vent near the Chaxas Complex. However, development of the Chaxas Complex has not been detailed until now. Redefined stratigraphy and volcanic history indicate a younger and shorter history for the Chaxas Complex than previously thought. The oldest unit described is the 5.44 ± 0.01 Ma Agua Perdida Rhyolite, which immediately underlies the 4.18 ± 0.03 Ma Puripicar ignimbrite that was sourced from buried vents to the north of the Chaxas Complex. Puripicar is in turn overlain by the 3.73 ± 0.02 Ma Embaucador Rhyolite. Following eruption of the Embaucador, the Chaxas Complex produced effusive domes, a sequence of local ignimbrites, block-and-ash flows, debris avalanches, and lahars between 3.54 ± 0.33 Ma and 1.242 ± 0.049 Ma. The 0.98 ± 0.03 Ma Purico ignimbrite caps the Chaxas sequence in the southeast. Stacked on top of the Chaxas Complex and the buried Puripicar source vent to the west and north is a thick (up to 1 km) pile of dacitic (62%−66% SiO2) coulées and lava flows (the arc platform lavas) that form a basal platform to the 23-km-long Sairecabur-Escalante-Colorados-Putana arc volcano cluster, which define an unusually spatially dense zone of composite edifices. Based on mineralogy and geochemistry of the arc platform lavas, they are identified as a transitional sequence between the end of the Chaxas Complex and the development of the modern arc. U-Pb zircon ages of the arc platform lavas are as young as 0.07 ± 0.01 Ma but also carry zircon crystal cargo that chronologically overlaps with those in eruptions of the Chaxas Complex, connoting that intrusions of the Puripicar-Chaxas magmatic system are being sampled by younger arc magmas. The anomalously dense spatial density of this volcano cluster may reflect the filtering effect of a now solidified intrusive complex that birthed the Puripicar supereruption. Crustal priming by large silicic systems followed by focused “steady-state” arc activity is posited as a primary mechanism to overprint characteristic volcano spacing in the Central Volcanic Zone.