LOS FRAILES IGNIMBRITES OF THE CENTRAL ANDEAN ALTIPLANO PLATEAU: CHEMISTRY, EVOLUTION AND TECTONIC SETTING

A large number of very voluminous backarc dacitic ignimbrites of the large monotonous type have erupted on the high Central Andean Puna-Altiplano Plateau since 10 Ma. The best studied of are the southern Puna Galan complex and the central plateau Puna Altiplano Volcanic (APVC) complexes. In contrast, the giant < 25 Ma backarc los Frailes complex in the Altiplano to the north has been little studied with most work focused on the > 12-14 Ma units associated with world class Ag, Sn and Zn deposits. Here, we show that the voluminous < 14 Ma K-rich biotite-bearing andesitic to dacitic (58-69% SiO₂) los Frailes complex ignimbrites differ from the Galan and APVC ignimbrites in having a more peraluminous chemistry, extreme HREE depletion (Sm/Yb=5-12) and LREE enrichment (up to 90 ppm La) and more arc like HFSE/LREE ratios (La/Ta=35-50). At the same time, their ⁸⁷Sr/⁸⁶Sr ratios (~0.711-0.712) are in the range of the Galan and APVC ignimbrites. Their extreme HREE depletion requires garnet as a deep restitic phase and the presence of cordierite crystals requires equilibration at < ~14 km before eruption. The los Frailes ignimbrites are best interpreted as hybrid mixtures of enriched decompression mantle magmas and deep crustal melts that experienced a multistage crustal evolution before eruption. Temporal variations in their Sm/Yb ratios can be correlated with the regional shortening and uplift history of the Altiplano suggesting that:: a) very high Sm/Yb ratios (7- 12) in 14-12 Ma ignimbrites reflect a very thick underlying crust (> 70 km?) related to 26-17 Ma crustal shortening beneath the plateau, b) lower ratios (5-7 Ma) in 8-7 Ma flows reflect lithospheric foundering in an important pulse of plateau uplift (Garzione et al.2008), and c) a return to higher ratios in voluminous < 5Ma flows reflects renewed thickening in response to foreland shortening and crustal flow beneath the plateau. Ages of many units remain uncertain, but chemical similarities of widespread, volumetrically important flows with an ignimbrite flow dated at 2.2 Ma by Barke et al. (2007) support important < 3 Ma (800 km3) activity above low velocity mantle and crustal low velocity anomalies seen in seismic profiles (e.g., Beck and Zandt 2002).