Experimental and petrological constraints on long-term magma dynamics and post-climactic eruptions at the Cerro Galán caldera system, NW Argentina

Abstract Cerro Galan in NW Argentina records > 3.5 Myr of magmatic evolution of a major resurgent caldera complex. Beginning at 5.72 Ma, nine rhyodacitic ignimbrites (68–71 wt% SiO 2 ) with a combined minimum volume of > 1200 km 3 (Dense Rock Equivalent; DRE) have been erupted. The youngest of those ignimbrites is the eponymous, geochemically homogenous, caldera-forming 2.08 ± 0.02 Ma Cerro Galan Ignimbrite (CGI; > 630 km 3 DRE). Following this climactic supereruption, structural and magmatic resurgence led to the formation of a resurgent dome and post-climactic lava domes and their associated pyroclastic deposits. A clear transition from amphibole to sanidine-bearing magmas occurred during the evolution of Cerro Galan and is inferred to represent a shallowing of the magma system. We test this hypothesis here using experimental phase equilibria. We conducted a series of phase equilibria experiments on the post-climactic dome lithologies under H 2 O-saturated conditions using cold seal Waspaloy pressure vessels with an intrinsic log f O 2 of NNO + 1 ± 0.5 across a temperature-pressure range of 750–900 ° C and 50–200 MPa (P H2O = P total ), respectively. Petrologic and geochemical analysis of the post-climactic lithologies shows that the natural phase assemblage (plagioclase + quartz + biotite + sanidine + Fe-Ti oxides ± apatite ± zircon) is stable at H2O ) and 805–815 °C. Applying experimental results to the CGI pumice, which has the same phenocryst phase assemblage and modal abundance, whole rock and phenocryst chemistry, and overlapping temperature and f O 2 as the post-climactic deposits, suggests that these pre-eruptive conditions (P H2O Amphibole in the early Cerro Galan ignimbrites (Toconquis Group; 5.72–4.51 Ma, and the Cueva Negra Ignimbrite, 3.77 ± 0.08 Ma) records crystallization across a range of pressures (500 to 200 ± 60 MPa). In the interval between the eruption of the Cueva Negra ignimbrite and the CGI (2.08 ± 0.02 Ma) the complex magma system shallowed and stalled at low pressures ( The post-climactic history of Cerro Galan is informed through a detailed investigation of the textural differences among the post-climactic dome lithologies, and a comparison of those textures with previously published decompression experiments. These suggest that the highly vesiculated, pumiceous clasts with rare microlites represent magma stored within the core of the lava dome that decompressed relatively rapidly (0.003–0.0003 MPa s −1 ) and evolved via closed system degassing. Resulting over-pressure of the dome may have triggered superficial explosion. In contrast, dense clasts with abundant crystalline silica precipitates represent more typical dome-forming magmas that decompressed more slowly ( −1 ), evolved via open system degassing, and form the outer carapace of a lava dome. Integrating decompression histories with results from new phase equilibria experiments suggests that during post-climactic volcanic activity at Cerro Galan, remnant CGI dome-forming magmas ascended from the shallow magma reservoir (