An Integrated Petrochemical and Geophysical View into the Processes Producing the Cerro Galan Caldera system in the Central Andes

Abstract:

The 6 to 2 Ma Cerro Galan caldera system in the southern Puna of the Central Andes erupted some of the world’s classic giant monotonous intermediate silicic ignimbrites. Chronological, geochemical and seismic data provide an integrated view into the processes that produced these ignimbrites whose ultimate origin is linked with steepening of the subducting Nazca slab following aseismic ridge subduction and lithospheric delamination. The Galan caldera system reflects ~6 Ma of magma buildup interrupted by generally increasingly larger eruptions of pumice-rich, crystal-poor ignimbrites (e.g., 4 of 8-35 km³ from 6.4 to 5.5 Ma; 190 km³ at 4.8 Ma; 390 km³ at 4.7 Ma; see Folkes et al. 2011) and culminating with the 2.126-2.056 Ma crystal-rich, pumice-poor Galan ignimbrite (630 km³). A relative incompatible trace element enrichment in successive ignimbrites along with seismic images support pre-eruptive removal of eutectic crystals in magma mush chambers at ~15-20 km that are in turn fed by ~50:50 mixtures of mantle and crustally-derived magmas derived from below. Biotite Ar/Ar ages that are >0.6 Ma older than sanidine ages for the Galan event support prolonged pre-eruption crystallization as do U/Pb zircon ages in other Puna ignimbrites. In accord with mantle input, the ages of the Cerro Galan ignimbrites are bracketed by the ages of bordering 6.6 to 0.02 Ma mafic lavas whose pattern of calc-alkaline to intraplate-like chemistry reflects a changing mantle input that can be reconciled with the delamination of basal crust and lithosphere imaged by seismic S-wave and attenuation tomography. Temporal changes in mafic magma compositions also correlate with evolving ignimbrite compositions. Other support for lithospheric removal comes from shear-wave splitting, receiver function images and earthquake distributions that reveal regional crustal thicknesses, compositional variations and thermal constraints. Recent ambient noise tomography suggesting a mid-crustal mush volume of 22,000 km³ with up to 22% melt currently under the Cerro Galan Caldera (Delph et al. 2017) is supported by an earthquake swarm beneath the resurgent dome and is in accord with a 1 to 5-10 volcanic to plutonic ratio.