Chapter 21 - Cenozoic arc-related magmatism in the southern Central and North Patagonian Andes

Abstract

Evolution of arc magmatism along the southern Central and North Patagonian Andes (~29° to 46°S) has been controlled by variable geodynamic parameters. Geochemical variations in arc-related magmas allow the correlation of magmatic evolution with the evolving tectonic setting of the Andes. This review highlights changes in arc magmatism throughout the Cenozoic by comparing the geochemical evolution of arc-related products from multiple Andean segments. During Paleocene to middle Eocene time, extensional conditions prevailed and arc magmatism was restricted to the volcanic front; significantly, in North Patagonia an enriched asthenospheric influx is registered in the retroarc due to development of a slab window. By late Eocene to early Oligocene time, oblique subduction of the Farallon plate beneath the South American plate influenced the development of low-volume arc magmatism, with mostly tholeiitic composition and strongly controlled by extensional structures. Breakup of the Farallon plate by the late Oligocene led to widespread extension along the margin and a change to more typical tholeiitic to calc-alkaline Andean-type volcanism in forearc, intra-arc, and retroarc basins along the southern Central Andes. In North Patagonia, the arc front retreated toward the Chilean trench by late Oligocene-early Miocene time, while arc-derived magmas with a mostly tholeiitic composition were emplaced along intra-arc and retroarc basins. Miocene to middle Pliocene arc magmatism shows strong differences between the southern Central and North Patagonian Andes. To the north, a shallow subduction regime led to an eastward expansion of calc-alkaline arc magmatism in the present-day Pampean-Chilean flat-slab segment and late Miocene Payenia shallow subduction segment. In contrast, coeval arc magmatism in North Patagonia was mainly restricted to the main Andean axis. Overall, major differences and similarities in the geochemical evolution of Cenozoic arc magmatism along the southern Central and North Patagonian Andes are controlled by changes in the subduction zone geometry, variable convergence rates, collision of ridges, and associated processes that affected different segments of the Andes.