THE PAIRIQUE CORDIERITE-BIOTITE DACITES: A CLUE TO THE PETROGENESIS OF PERALUMINOUS MAGMAS IN THE APVC REGION

Upper Miocene back-arc volcanism erupted in the 21º-24ºS segment of the central Andean plateau is mostly represented by calc-alkaline andesites to rhyolites of metaluminous to weak peraluminous affinities, i.e. their mafic mineral assemblages include biotite and/or other, less aluminous phases. In the Pairique area (22º52'S - 66º49'W) however, a group of 11.5 to 10.5 Ma dacite and rhyolite units shows magmatic cordierite as an ubiquitous accesory phase. These rocks are comparable to the Lachlan S-type granites, suggesting more crustal affinities than the typical Andean volcanic rocks. In Pairique, cordierite-bearing volcanic rocks are pre- and post-dated by high-K, calc-alkaline andesites and dacites, and they show evidence of mingling with andesitic magmas in pre-eruptive magma chambers. Textural and chemical features of key phenocryst phases (plagioclase with resorbed poikilitic cores An80-60, and orthopyroxene with dusty cores En80), the common presence of noritic cumulates, as well as whole-rock chemical compositions (FeOt+MgO to 7 %, CaO >2.0 %), and high crystallization temperatures of cordierite (>800ºC), all suggest that the peraluminous volcanic rocks did not originate by crustal melting alone. The best way to explain the Pairique magmas is through assimilation/reaction of crustal protoliths or melts with mantle-derived magmas at relatively low pressures (~4 kbar). A similar origin has been suggested for peraluminous magmas from the Inner Arc of Perú and Bolivia. Pairique magmas overlap compositionally with metaluminous to weak peraluminous dacites from the APVC. The latter are hybrids magmas dominated by crustal components (50-70%), probably formed at mid- to lower crustal levels (>20-30 km). Peraluminous magmatism instead would have involved the participation of crustal components that better match mature (pelitic) upper crustal sources. Thus, formation of peraluminous magmas in this part of the Andes may be a combined effect of: a) a compositional stratification of the crust, with a pelitic upper level, and a lower level of metagreywacke composition, or constituted by rocks modified during older melting events (Paleozoic); and b) a favourable (transtensive) tectonic setting that allowed mantle-derived magmas to invade the upper crust and to react with it to generate peraluminous melts.