NEOGENE ARC MAGMATISM IN A CONTINENTAL SETTING FROM PATAGONIA TO ALASKA

Neogene arc magmatic rocks along the backbone of the Americas are found in the Aleutian and Cascade arcs, the Mexican to Central American arc, and the Northern, Central, Southern, and Austral Andean Volcanic Zones in South America. The distribution and chemistry of these magmatic rocks have recorded episodes of ridge collision, shallowing and steepening of subduction zones, and plateau uplift along the margin. Source components in the magmas include melts and fluids from subducting slabs, the asthenospheric mantle wedge, subducted sediment, upper plate mantle and crusts entering the mantle through forearc subduction erosion, and upper pate contamination as the magmas ascend. Their relative proportions depend on convergence parameters, the condition of the subducting slab, the pre-existing configuration of the margin, and climate. A widely debated factor in discussing these components has been the role of slab melting in producing eclogitic trace-element signatures in adakitic magmas. The most convincing slab melts are associated with young hot subducting plates linked to slab windows. Elsewhere, major slab melt contributions are controversial. Where the upper plate crust is > 40 km thick, in situ crust is a likely factor in producing an adakitic signature. Where adakitic magmas erupt in unstable and migrating arc configurations, upper plate components introduced into the mantle by forearc subduction erosion must be considered. The composition and amount of subducted sediment is an additional factor. A continuing challenge is to separate the effects of subducted sediment, forearc eroded crust, and in situ crust. In the backarc, strong evidence for shallowing and steepening subduction zones can come from broadening and narrowing magmatic belts where the influence of arc-like components increases and then decreases. Shallowing subduction zones are associated with a cooling mantle wedge, hydration and diminishing magmatism, and steepening ones with a thickening mantle wedge that can produce mafic magma flare-ups leading to extensive melting in the upper plate. Delamination of thickened continental crust above subduction zones can increase mantle production of mafic magma and extensive crustal melting. Understanding the presence or lack of synchronicity in the style of magmatic events along the entire margin is an ongoing challenge.