Geochemistry of the Active Azufre--Planchon--Peteroa Volcanic Complex, Chile (35 15'S): Evidence for Multiple Sources and Processes in a Cordilleran Arc Magmatic System

Abstract

Along strike of the Quaternary magmatic arc in the Southern Volcanic Zone of the Andes, there is a south to north increase in crustal thickness, and the lavas define systematic geochemical trends which have been attributed to variations in the proportions and compositions of mantle-and crustal-derived components. Realistic interpretations of these regional geochemical trends requires an understanding of the sources and processes that control lava compositions at individual volcanoes. Because it is in an important geophysical and geochemical transition zone, we studied the Azufre—Planchon—Peteroa volcanic complex, a nested group of three volcanoes <0·55 m.y. in age located at 35°15′S in the Southern Volcanic Zone of the Andes. North of this complex at 33–35°S the continental crust is thick, basalts are absent, and there is abundant evidence for crustal components in the evolved lavas, but south of 37°S, where the crust is relatively thin, basaltic lavas are abundant and the contribution of continental crust to the lavas is less obvious. In addition to its location, this volcanic complex is important because there is a diversity of lava compositions, and it is the northernmost exposure of recent basaltic volcanism on the volcanic front. Therefore, the lavas of this complex can be used to identify the relative roles of mantle, lower-crustal and upper-crustal sources and processes at a single location. Volcan Azufre is the oldest and largest volcano of the complex; it is a multi-cycle, bimodal, basaltic andesite–dacite stratovolcano. Volcan Planchon is the northernmost basalt-bearing volcano along the volcanic front of the Southern Andes, and Volcan Peteroa, the youngest volcano of the complex, has erupted mixed magmas of andesitic and dacitic composition. Most basaltic andesite lavas at Azufre and Planchon are related by a plagioclase-poor, anhydrous mineral fractionating assemblage. High-alumina basalt is produced from a tholeiitic parent by an ∼4–8 kbar fractionating assemblage. During this moderatepressure crystallization, the magmas also incorporated a crustal component with high La/Yb and high abundances of Rb, Cs and Th. Based on the chemical characteristics of the added component and the inferred depth of crystallization, the crustal source may have been garnet granulite derived from solidified arc magmas in the lower to middle continental crust. At Planchon, the role of crustal assimilation has increased with decreasing eruption age probably because crustal temperatures have increased during continued volcanism. Azufre dacite lavas formed at low pressures by fractionation of a plagioclase-rich assemblage. These dacite lavas contain an upper-crustal component, probably derived in part from limestone, with high values of 87Sr/86Sr and 18O/16O. Thus two depths (upper and lower crust) of crystallization and associated crustal assimilation are evident in Planchon–Azufre lavas. Peteroa, the focus of recent volcanism, consists of calc-alkaline andesite and dacite eruptive products whose textures and compositions indicate an important role for magma mixing. Therefore, the volcanism evolved from a tholeiitic system of basalt and subordinate dacite (Planchon–Azufre) to a calc-alkaline system with abundant mixed lavas of intermediate composition (Peteroa). In addition to crustal thickness, two important parameters which controlled the diversity of lava composition in this complex are magma supply rate from the mantle and crustal temperature. Both parameters varied with time, and they must be considered in broader interpretations of along-strike geochemical trends.