Intracrustal Derivation of Na-Rich Andesitic and Dacitic Magmas: An Example from Volcán Ollagüe, Andean Central Volcanic Zone

Andesitic and dacitic lavas erupted from Quaternary stratovolcanoes in the Central Volcanic Zone (CVZ) of the Andes have major and trace element compositions similar to proposed melts of subducted mid-ocean ridge basalt and Na-rich trondjhemites, tonalites, and granodiorites (TTG) in Early Archean high-grade gneiss terrains. Suites of lavas from individual volcanoes have average La/Yb of 25-45, Sr/Y of 30-65, Yb contents of 1.5-1.1 ppm, and$Na_{2}O$contents > 3.8 wt %, with lavas erupted on the volcanic front displaying the most affinity to the Early Archean suites. Because slab-melting is not involved in the derivation of the CVZ magmas, these data indicate that rocks with Early Archean TTG compositions can also be produced through intracrustal differentiation processes. Isotopic, major element, and REE data from Volcán Ollagüe are consistent with a model incorporating contamination of mantle-derived basaltic magmas in garnet-bearing deep continental crust to produce basaltic andesite magma, followed by fractionation of a plagioclase-dominated mineral assemblage in shallow crustal magma chambers to produce andesitic and dacitic magma with compositional features similar to Na-rich TTG. Slight offset between parental basaltic andesite and andesite samples on diagrams illustrating variation of REE suggest that the shallow crustal magma chambers are density stratified with basaltic andesite at the base overlain by zones of andesitic and dacitic magma. Furthermore, strongly increasing La/Yb and decreasing$^{143}Nd/^{144}Nd$ratios with decreasing Yb contents for basaltic andesite samples indicate that the deep continental crust beneath Ollagüe is probably more silicic and enriched in incompatible trace elements relative to crust beneath volcanoes located farther west along the present arc front of the central Andes. Based on these results, it is suggested that contamination of mantle-derived basaltic magmas in garnet-bearing mafic continental crust is an alternative mechanism to slab-melting to produce the Early Archean TTG suites and younger rocks with similar compositions. This model and the data summarized here may provide support for models advocating relatively early growth of the earth's continental crust.