ISOTOPIC CONSTRAINT ON ANDESITE AND DACITE PETROGENESIS AT THE AUCANQUILCHA VOLCANIC CLUSTER, NORTHERN CHILE: IMPLICATIONS FOR CRUSTAL GROWTH VS. CRUSTAL RECYCLING

Whole rock Sr isotope data from the Aucanquilcha Volcanic Cluster (AVC) in northern Chile help constrain the petrogenetic processes leading to the production of andesites and dacites in this long-lived (11 my), on-arc volcanic center. The AVC is situated in the central Andes volcanic zone, peripheral to the Altiplano-Puna Volcanic Complex (APVC) ignimbrite province. The AVC experienced an eruptive flare-up about half way through its lifetime, coincident with the major pulse observed in the APVC at ~5 Ma. This, paired with the centralization and homogenization of erupted lavas through time, suggests that the AVC is the surface manifestation of a batholith, and as such, documents the time-transgressive evolution of a magmatic system.

The Central Andean crust is thick (60-70 km). The absence of true basalts, coupled with elevated ⁸⁷Sr/⁸⁶Sr, attest to significant crustal involvement in the production of lavas. At the AVC, ⁸⁷Sr/⁸⁶Sr ratios range from 0.70509 to 0.70680, with a gradual increase throughout the system’s life. ⁸⁷Sr/⁸⁶Sr of andesites and dacites increase in tandem through time, though dacite values are typically slightly elevated, suggesting more crustal involvement for the more evolved lavas. For the first ~9 my, the observed increase in ⁸⁷Sr/⁸⁶Sr is gradual until the eruption of the modern arc volcano, Volcán Aucanquilcha, which displays a relatively large range of ⁸⁷Sr/⁸⁶Sr from ~0.7058 to ~0.7068.

The observed increase in ⁸⁷Sr/⁸⁶Sr suggests that the arc crust contribution to magmas is increasing through time. Alternatively, the nature of the assimilated crust changes through time. The most abrupt and dramatic change is observed in the final stage of AVC activity at Volcán Aucanquilcha. The shift in ⁸⁷Sr/⁸⁶Sr, along with evidence for shallower magma ponding (from amphibole barometry) and cooler magma temperatures and xenocrysts (from zircon geochemistry and age data) suggest that the final stage of the AVC batholith was likely an upper crustal cupola which ascended to within a few kilometers of the surface and assimilated appreciable upper crust.