High precision U/Pb zircon dating of the Chaltén Plutonic Complex (Cerro Fitz Roy, Patagonia) and its relationship to arc migration in the southernmost Andes

Abstract

[1] We report new high-precision U/Pb ages and geochemical data from the Chaltén Plutonic Complex to better understand the link between magmatism and tectonics in Southern Patagonia. This small intrusion located in the back-arc region east of the Patagonian Batholith provides important insights on the role of arc migration and subduction erosion. The Chaltén Plutonic Complex consists of a suite of calc-alkaline gabbroic to granitic rocks, which were emplaced over 530 kyr between 16.90 AE 0.05 Ma and 16.37 AE 0.02 Ma. A synthesis of age and geochemical data from other intrusions in Patagonia reveals (a) striking similarities between the Chaltén Plutonic Complex and the Neogene intrusions of the batholith and differences to other back-arc intrusions such as Torres del Paine (b) a distinct E-W trend of calc-alkaline magmatic activity between 20 and 17 Ma. We propose that this trend reflects the eastward migration of the magmatic arc, and the consistent age pattern between the subduction segments north and south of the Chile triple junction suggests a causal relation with a period of fast subduction of the Farallón-Nazca plate during the Early Miocene. Previously proposed flat slab models are not consistent with the present location and morphology of the Southern Patagonian Batholith. We advocate, alternatively, that migration of the magmatic arc is caused by subduction erosion due to the increasing subduction velocities during the Early Miocene.

Exhumation of the Neuquén Basin in the southern Central Andes (Malargüe fold and thrust belt) from field data and low-temperature thermochronology

“…It is proposed that i) rapid and anomalously wide migration of the orogenic front towards the continental interior follows arc expansion that modifies the thermal and therefore the mechanical structure of the lithosphere, favoring shortening; followed by ii) shut down of arc magmatism producing a volcanic arc gap, and consequent "refrigeration" of the upper plate through removal of the mantle wedge and lowering of basal heat flow. Fossil shallow subduction settings along western South America have been proposed on similar grounds: anomalous foreland deformation and arc expansions/migrations (James and Sacks, 1999;Kay et al, 2006;Ramos and Folguera, 2005;Kay and Coira, 2009;Espinoza et al, 2010;Ramírez de Arellano et al, 2011;González Guillot et al, 2011;Spagnuolo et al, 2012;Gianni et al, 2015, Fennell et al, 2015.…”

Cretaceous Intraplate Contraction in Southern Patagonia: A Far‐Field Response to Changing Subduction Dynamics?

“…Inset diagram is a time‐space plot (Coney & Reynolds, ) of Cretaceous arc magmatism. Eocene to early Miocene arc ages are from Suárez and De La Cruz () and Ramírez de Arellano et al (). (d) Proposed geodynamic model linking intraplate contraction in the Deseado fold and thrust belt to changes in slab angle.…”

“…It is noteworthy that the documented arc waning‐gap stage preceded the eruption of Eocene volcanic plateaux by 25 Ma, which hampers linking the volcanic gap entirely to the formation of the Aluk‐Farrallon slab window (V. A. Ramos & Kay, ; Figure c). Finally, normal subduction would have resumed sometime during the latest Eocene to Early Miocene when the arc reestablished to the west (Hervé et al, ; Ramírez de Arellano et al, ; Figure c).…”

“…Subduction erosion is expected to produce arc migration as it removes crustal material from the forearc wedge (e.g., Kay et al, 2005). As mentioned above, this process has been postulated to explain Neogene magmatism based on geochemistry of arc rocks and the virtual absence of forearc crust between 48°S and 51°S (Ramírez de Arellano et al, 2012). Chemical evidences for subduction erosion usually involve a decrease in εNd values and an increase in 87 Sr/ 86 Sr ratios.…”

“…We elaborated a time-space plot diagram which is helpful to determine spatial changes in arc position through time (Coney & Reynolds, 1977; Figure 9c). To construct this diagram, the paleotrench was restored taking into account a 50km forearc loss by Neogene subduction erosion (Ramírez de Arellano et al, 2012). Since the contribution of Cretaceous deformation to bulk shortening in the Austral Patagonian Andes is unknown, plotted arc-totrench distances represent a minimum value.…”