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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 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 Volcan Ollagiie 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 '43Nd/'44Nd ratios with decreasing Yb contents for basaltic andesite samples indicate that the deep continental crust beneath Ollagiie 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.

The arid climate of the Altiplano has preserved a volcanic history of ∼11 million years at the Aucanquilcha Volcanic Cluster (AVC), northern Chile, which is built on thick continental crust. The AVC has a systematic temporal, spatial, compositional and mineralogical development shared by other long-lived volcanic complexes, indicating a common pattern in continental magmatism with implications for the development of underlying plutonic complexes, that in turn create batholiths.The AVC is a ∼700-km2, Tertiary to Recent cluster of at least 19 volcanoes that have erupted andesite and dacite lavas (∼55 to 68 wt.% SiO2) and a small ash-flow tuff, totalling 327 ± 20 km3. Forty 40Ar/39Ar ages for the AVC range from 10·97 ± 0·35 to 0·24 ± 0·05 Ma and define three major 1·5 to 3 million-year pulses of volcanism followed by the present pulse expressed as Volcán Aucanquilcha. The first stage of activity (∼11–8 Ma, Alconcha Group) produced seven volcanoes and the 2-km3 Ujina ignimbrite and is a crudely bimodal suite of pyroxene andesite and dacite. After a possible two million year hiatus, the second stage of volcanism (∼6–4 Ma, Gordo Group) produced at least five volcanoes ranging from pyroxene andesite to dacite. The third stage (∼4–2 Ma, Polan Group) represents a voluminous pulse of activity, with eruption of at least another five volcanoes, broadly distributed in the centre of the AVC, and composed dominantly of biotite amphibole dacite; andesites at this stage occur as magmatic inclusions. The most recent activity (1 Ma to recent) is in the centre of the AVC at Volcán Aucanquilcha, a potentially active composite volcano made of biotite-amphibole dacite with andesite and dacite magmatic inclusions.These successive eruptive groups describe (1) a spatial pattern of volcanism from peripheral to central, (2) a corresponding change from compositionally diverse andesite-dacite volcanism to compositionally increasingly restricted and increasingly silicic dacite, (3) a change from early anhydrous mafic silicate assemblages (pyroxene dominant) to later biotite amphibole dacite, (4) an abrupt increase in eruption rate; and (5) the onset of pervasive hydrothermal alteration.The evolutionary succession of the 327-km3 AVC is similar to other long-lived intermediate volcanic complexes of very different volumes, e.g., eastern Nevada (thousands of km3, Gans et al. 1989; Grunder 1995), Yanacocha, Perú (tens of km3, Longo 2005), and the San Juan Volcanic System (tens of thousands of km3, Lipman 2007) and finds an analogue in the 10-m. y. history and incremental growth of the Cretaceous Tuolumne Intrusive Suite (Coleman et al. 2004; Glazner et al. 2004). The present authors interpret the AVC to reflect episodic sampling of the protracted and fitful development of an integrated and silicic middle to upper crustal magma reservoir over a period of at least 11 million years.