Eleven million years of arc volcanism at the Aucanquilcha Volcanic Cluster, Northern Chilean Andes: implications for the life span and emplacement of plutons

The arid climate of the Altiplano has preserved a volcanic history of similar to 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 similar to 700-km(2),Tertiary to Recent cluster of at least 19 volcanoes that have erupted andesite and dacite lavas (similar to 55 to 68 wt.% SiO2) and a small ash-flow tuff, totalling 327 +/- 20 km(3). Forty(40)Ar/Ar-39 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 Volcan Aucanquilcha. The first stage of activity (similar to 11-8 Ma, Alconcha Group) produced seven volcanoes and the 2-km(3) 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 (similar to 6-4 Ma, Gordo Group) produced at least five volcanoes ranging from pyroxene andesite to dacite. The third stage (similar to 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 Volcan 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-km(3) AVC is similar to other long- lived intermediate volcanic complexes of very different Volumes, e.g., eastern Nevada (thousands of km(3), Gans et al. 1989; Grunder 1995), Yanacocha, Peru (tens of km(3) Longo 2005), and the San Juan Volcanic System (tens of thousands of km(3), 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.