UNDERSTANDING THE DEEP MAGMATIC PROCESSES CONTROLLING THE EARLY EVOLUTION OF ARC MAGMAS: A COMPARISON OF THE CENTRAL VOLCANIC ZONE IN NORTHERN CHILE WITH THE ALEUTIAN ISLAND ARC

The Central Volcanic Zone (CVZ) of the central Andes is the archetypal example of a continental magmatic arc. Extreme crustal thicknesses beneath the arc (>70 km in some areas) coupled with extensive upper crustal magmatism since the Miocene, associated with a regional magmatic flare-up, have resulted in near absence of primitive, mantle-derived magmas reaching the surface, and a major gap in our understanding of the source(s) and characteristics of the primitive magmas feeding the CVZ. Using detailed mineral/melt inclusion chemistry, thermobarometry, and phase equilibria from mafic cumulates preserved in intermediate lavas erupted in the CVZ, we provide critical constraints on the nature and composition of these primitive magmas. We see clear evidence for the production of high-Mg basalt in the mantle wedge, followed by the ascent, stalling and subsequent modification of the basalt at the crust-mantle boundary, prior to ascent into the lower crust. Interestingly, the magmatic pathways and processes recorded in minerals and glasses from the CVZ share similarities with those recorded in the Aleutian Island arc off the coast of Alaska (U.S.). In this study, we compare and contrast the magmatic processes controlling the nature and compositions of primitive mantle-derived magmas from both arcs, with an emphasis on the role of crustal thickness and composition on the early evolution of magmas in both systems. In addition, we explore the role of the crust-mantle boundary as a universal feature controlling the composition of arc-derived magmas globally.