THE ROLE OF CRUSTAL MELTING IN DACITE PRODUCTION AT THE AUCANQUILCHA VOLCANIC CLUSTER, CHILE: ISOTOPIC AND TRACE ELEMENT CONSTRAINTS

The correlation of large silicic systems with crustal thickness is first order evidence that the continental crust is important in the production of evolved magmas. Differentiation is enhanced by the presence of a cool crustal path that promotes crystallization, a low-density lid to stall magmas, or a compressional environment that inhibits eruption. Partial melt of the crust or bulk wall rock may also be material contributors to magma through assimilation and mixing. The Aucanquilcha Volcanic Cluster (AVC) of the central Andes is an example of significant material involvement of crust in the production of dacites. At the AVC, systematic centralization of volcanism corresponds with an abrupt increase in eruptive volume and a change to biotite-amphibole dacite. This magmatic pulse occurs about half-way through the eleven million year lifetime of the AVC system. Such pulsing may be a common feature of large silicic systems, reflecting thermal maturation and partial melting of fertile crustal lithologies to produce voluminous dacites. Lavas of the AVC show an increase in Dy/Yb (<3.5) and 87Sr/86Sr (<0.7068) through time which we attribute to the increasing influence of partially melted crust with system incubation and maturation. The elevated Dy/Yb ratios with time suggest that the incorporated crust had a garnet-bearing restite. Early lavas show little variation in Dy/Yb with SiO2, consistent with fractionation and modal occurrence of pyroxene; late lavas show a broad decrease in Dy/Yb with increasing SiO2, consistent with fractionation and modal occurrence of amphibole. Whole rock 87Sr/86Sr values of AVC lavas increase through time, with a pronounced inflection accompanying the construction of most recent edifice, Volcán Aucanquilcha. While the trend for dacites is relatively smooth, 87Sr/86Sr for andesites are much more variable, not only providing the baseline signature for a given time but often the highest values as well. For example, recently erupted basaltic andesite cinder cones adjacent to the AVC (Poruñita and Luna de Tierra) do not have primitive 87Sr/86Sr values with respect to dacites, and in fact Poruñita has a higher 87Sr/86Sr ratio (0.706826; Mattioli et al., 2006) than any analyzed lava of the AVC. The increase in 87Sr/86Sr and Dy/Yb through time both suggest that the extent of crustal contamination has increased and/or the material being assimilated has changed. We envision a scenario where the progressive development of the AVC batholith was accompanied by the maturation and partial melting of the surrounding crust. Through time, the hot crustal aureole likely ascended through the lower and middle crust, attendant to the batholith. The process of dacite generation homogenizes crustal and mantle components so that the erupted products display attenuated, albeit detectable, variation. Continued recharge of mantle-derived magma maintains a relatively buffered isotopic signature within the central plexus. Conversely, the “outlying” andesite 87Sr/86Sr values illustrate the heterogeneous components that dacites comprise. Presumably bypassing the main batholithic center, their isotopic signatures do not reflect the homogenization that occurs with continued recharge and mid-upper crustal AFC processes.