Early-mid Cenozoic extensional arc volcanism of the North Patagonian and the Southern Central Andes (33-43°S)

Abstract

The volcanic arc of the Southern Central and Northern Patagonian Andes (33-43°S; Chile and Argentina) in the early to middle Cenozoic exhibits distinctive features that shed light on the dynamics of subduction-related volcanic activity in extensional tectonic settings. The late Eocene to early Miocene interval is particularly significant in the Andes, as it records a widespread extensional regime leading to the opening of forearc, intra-arc, and backarc basins, accompanied by prolific volcanism. This period precedes the final major uplift phase of the Andes. It is hypothesized that this extensional event occurred in two phases, associated with changes in the geometry of the subducting slab: i) oblique convergence at low rates during the late Eocene to early Oligocene, and ii) orthogonal convergence at high rates during the late Oligocene to early Miocene. This study presents new field, petrographic, geochemical, and isotopic data from volcanic sequences in the Northern Patagonian Andes, along with an extensive compilation of geochemical data from the Southern Central Andes, aiming to analyze petrogenetic processes at different stages of arc evolution. During the late Eocene to early Oligocene, subduction-related volcanism manifests in two parallel belts located 100 and 300 km from the trench. In this period, both belts exhibit intermediate composition and a relatively homogeneous geochemical signature, transitioning between tholeiitic and calc-alkaline series. Multi-element patterns show enrichment in mobile elements (Rb, Sr, Ba, Th, U, Pb) compared to high field-strength elements and rare earth elements. Moreover, Sr and Nd isotopes indicate a derivation from depleted mantle sources with limited crustal contributions. Trace element ratios used as tectonic discriminants (Ba/Ta, La/Ta, Th/Yb, among others) suggest development in a subduction environment with contributions from fluids derived from the subducting slab. In contrast, the record of volcanic activity during the late Oligocene to early Miocene, at the peak of the extensional regime, displays significant geochemical differences along the Andes. In the Southern Central Andes, volcanism displays more basic compositions and less influence from the subducting slab. Conversely, in the Northern Patagonian Andes, subaerial and subaqueous lava flows intercalate with marine deposits, indicating a regime of rapid subsidence. These rocks exhibit geochemical signatures both transitional to arc, E-MORB, and even OIB. These geochemical singularities suggest a genesis primarily dominated by decompression melting of a heterogeneous mantle wedge. In summary, this study emphasizes how tectonic conditions play a fundamental role in the magmatic response of subduction-associated volcanism in the Southern Central Andes and Northern Patagonian Andes. The geological and geochemical evidence presented reveals that the stress conditions of the margin significantly influence the composition, distribution, and evolution of magmatic manifestations in these volcanic arcs, highlighting the complexity and sensitivity of these systems to tectonic conditions.