Petrogenesis of peraluminous magmas in the Central Andean backarc: the Huayra Huasi Volcanic Complex, NW Argentina

The Huayra Huasi Volcanic Complex of Miocene age (11.88 ± 0.15 Ma U–Pb zircon) was emplaced in the Northern Puna plateau of Argentina, spatially associated with ignimbrites erupted from Altiplano–Puna Volcanic Complex calderas. The complex comprises biotite-bearing dacites and low-SiO2 rhyolites in the northern area and high-SiO2 rhyolites in the south, all with peraluminous compositions (A/CNK > 1.0–1.22). The units have broadly similar initial Sr and Nd isotopic ratios (87Sr/86Sr ∼ 0.71013–0.71225 and eNd ∼ −5.4 to −7.0) and are composed of plagioclase, quartz, sanidine and biotite as the main phenocryst phases. All units host macroscopic microgranular enclaves and xenoliths of sillimanite–biotite schists, sillimanite- and sillimanite–garnet gneisses, as well as fibrous alumina-rich microxenoliths, the latter being especially abundant in the southern rhyolites. Petrographic, mineral and whole-rock geochemistry, geothermometric and isotopic data indicate that all units of the complex originated by contamination of andesite magmas through assimilation of upper crustal lithologies in early stages of magma evolution. The fibrous alumina-rich microxenoliths are composed almost entirely of refractory minerals (sill + Kfsp ± Pl ± Bt) and interpreted as peritectic or restitic products of partial melting of assimilated metasedimentary rocks similar to the unmodified metamorphic xenoliths in the complex. Geochemical modeling indicates that, after early-stage contamination, each magmatic unit evolved separately. Whereas the northern dacites and low-SiO2 rhyolites underwent assimilation and fractional crystallization throughout their history, the southern rhyolites mainly evolved via fractional crystallization of felsic phases alone. This study shows that the peraluminous nature of felsic magmas do not necessarily originate by partial melting of crustal material but can be acquired by metaluminous magmas during later evolution. The processes shown here of assimilation and fractional crystallization and pure fractional crystallization has relevance for other igneous bodies of similar compositions in the Puna backarc and worldwide.