Geochemical, Isotopic and Single Crystal 40Ar/39Ar Age Constraints on the Evolution of the Cerro Galan Ignimbrites

Abstract

The giant ignimbrites that erupted from the Cerro Galan 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. Major and trace element analyses of Cerro Galan ignimbrites (68-71% SiO2) that include 65 new analyses can be interpreted by evolution at three crustal levels. AFC modeling and new fractionation corrected d18O values from quartz (+7.63-8.85%o) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (87Sr/86Sr ~ 0.7055) and crustal melts (87Sr/86Sr near 0.715-0.735). 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 produces hybrid melt 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 800-850ºC The mush zone was 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 temperature as low as 680ºC just preceding eruption. 40Ar/39Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galan 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 Galan ignimbrites.