Geochemical, isotopic and single crystal 40Ar/39Ar age constraints on the evolution of the Cerro Galán ignimbrites

Abstract

The giant ignimbrites that erupted from the Cerro Galán caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4-7), high Sr contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid crustal levels create large negative Eu anomalies (Eu/Eu*=0.45-0.70) and variable trace element enrichment in a crystallizing mush zone with a temperature near 800-850°C. The mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic crystallization occurs at temperatures as low as 680°C just preceding eruption. 40 Ar/ 39 Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galán ignimbrite erupted in more than one batch including a~2.13 Ma intracaldera flow and outflows to the west and north at near 2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas, whose changing chemistry reflects their evolution over a steepening subduction zone, could trigger the eruptions of the Cerro Galán ignimbrites.

The origin and crust/mantle mass balance of Central Andean ignimbrite magmatism constrained by oxygen and strontium isotopes and erupted volumes

“…The APVC appears to be the most long-lived (and best-dated) ignimbrite complex in the CVZ. Both the APVC and Cerro Galán are potentially still active, even though the age and volume relations suggest that their main period of activity has waned (Kay et al 2010(Kay et al , 2011Salisbury et al 2011;Mulcahy et al 2014). The large ignimbrite centres further north apparently were active for shorter time periods and are now extinct.…”

“…6a, starting with a mantle value of δ 18 O ≈ 5.8 ‰ (Harmon and Hoefs 1995), clearly reveal that the majority of Central Andean magmatic values are ~0.5-1.5 ‰ above the differentiation line and thus a crustal component with a heavy oxygen isotopic composition must have been involved in their origin. The two samples from the Oxaya plateau ignimbrites have the lowest δ 18 O values in our data set, whereas the plateau ignimbrites from the southern CVZ (APVC and Cerro Galán; Kay et al 2010Kay et al , 2011 are considerably heavier in δ 18 O than the rest of all volcanic rocks from the CVZ.…”

“…Therefore, the clustering of age data needs to be considered with care, and field relations become important to identify the ages of large plateau-forming ignimbrites, in particular in southern Peru, where large clusters of ignimbrites may only have a few age determinations. Irrespectively, this compilation of ignimbrite ages across the CVZ shows well the northsouth progression in the eruption of the voluminous dacitic ignimbrites starting at ~24 Ma with the Nazca ignimbrites in the northernmost part of the CVZ and ending with the Cerro Galán ignimbrites (Kay and Coira 2009;Kay et al 2011). The APVC appears to be the most long-lived (and best-dated) ignimbrite complex in the CVZ.…”

Time-dependent deformation of Uturuncu volcano, Bolivia, constrained by GPS and InSAR measurements and implications for source models

“…Note that all depths in this paper will be with respect to average surface elevation, which is ~4.6 km within 100 km of Uturuncu. In the Central Andes, petrological determination of pre-eruptive conditions implies that magma accumulates at different depths for varying periods of time (e.g., Kay et al, 2010), and the uplift at Uturuncu is believed to be related to partial melt in the APMB where andesitic magmas evolve to dacitic composition prior to ascent to shallower pre-eruptive reservoirs (Muir et al, 2014a(Muir et al, , 2014b.…”

The Evaluation of Brine Prospects and the Requirement for Modifications to Filing Standards

“…Mixing with pre-existing subsurface brines has also been suggested (Risacher et al, 2003), but neither of these hypotheses point to an ultimate source for the anomalous values of Li and other minor elements. Other explanations for the high values of Li and B include weathering of Neogene volcanic rocks (Rettig et al, 1980;Orti and Alonso, 2000;Kay et al, 2010), or Palaeozoic basement (Ortis and Alonso, 2000;Kasemann et al, 2004), and transport from hydrothermal volcanic activity (Orti and Alonso, 2000;Gibert et al, 2009;personal communication, Godfrey 2010).…”