The Los Frailes Ignimbrite Complex sits in the backarc of the Andean Central Volcanic Zone (CVZ) and is the most easterly of the large Altiplano volcanic centers. Despite its large size (2000 km3) and substantial mineralization in its satellite units, the majority of the Los Frailes Complex remains poorly described with conflicting age assessments of the main Los Frailes ignimbrite. Processes related to its emplacement include: variable crustal thickening and uplift over a steepening subducted slab, episodes of delamination of the mantle-lithosphere and lower crust and deep crustal flow. Based on 25 new analyses and the works of previous sub-regional studies, a three tier crustal magma evolution is proposed for the Los Frailes Complex, similar to models suggested for Puna ignimbrites. The crust-to-mantle mixing ratio of the erupted mass is put near 50:50 based on new fractionation corrected [delta]18OQuartz analyses (+9.43-10.79‰). AFC models incorporating new 87Sr/86Sr (0.710-0.713) and 143 Nd/144Nd (0.5121-0.5123) ƐNd (-9 to -6) ratios and the strongly peraluminous character of the complex support a metapelitic crustal end-member and silicic crustal base. Melting and mixing near the Moho is established based on steep HREE patterns (Sm/Yb=4-12) and very high Sr content (400-650 ppm Sr) while middle crustal plagioclase removal creates negative Eu anomalies (Eu/Eu*=0.6-0.9). High crystal content and reported cordierite are the results of low pressure crystallization in upper crustal magma chambers prior to eruption. Coherent temporal trends of REE and HFSE ratios display the effects of crustal thickening and delamination on Moho-depth AFC processes. Two distinct ratio changes at 10-12 Ma and 2-4 Ma are proposed to correlate with two discreet delamination events below the Los Frailes Complex. The 10-12 Ma event is supported by corresponding shifts in regional brittle deformation and isotopic character as well as proposed rapid uplift at that time. Major eruptions of the Livicucho and Condor Nasa ignimbrites and emplacement of the large main Los Frailes ignimbrite mark the volcanic expression. The age of the main Los Frailes ignimbrite is established at 1.5-3.5 Ma, in agreement with Barke et al. (2007), based on two new 40Ar/39Ar sanidine ages ~1.521 Ma. In addition, seismic tomographic studies show missing lithosphere directly below the complex which corresponds with a 3He/4He emissions study suggesting recent mantle melting. These findings demonstrate that delamination in the southern Altiplano is episodic with the results of the 2-4 Ma event still observable today. ii