The geochemistry of a dying continental arc: the Incapillo Caldera and Dome Complex of the southernmost Central Andean Volcanic Zone (∼28°S)

The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (similar to 28 degrees S), where the steeply dipping (similar to 30 degrees) segment of the subducting Nazca plate transitions into the Chilean flatslab to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3-1 Ma effusive phase characterized by similar to 40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive caldera-forming event at 0.51 Ma that produced the 14 km(3) Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO2 = 67-72 wt%; La/Yb = 37-56; Ba/La = 16-28; La/Ta = 30-50; Sr-87/Sr-86 = 0.70638-0.70669; epsilon (Nd) = -4.2 to -4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6-8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano-Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (> 40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated adakite-like dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.