Tectono-magmatic evolution of the Loncon Complex: evidence from Lu-Hf isotopes and whole-rock geochemistry

Thematic Session: Crustal Evolution of the Pre-Andes and Patagônia

Tectono-magmatic evolution of the Loncon Complex: evidence from Lu-Hf isotopes and whole-rock geochemistry

Stremel^1,3, A. J., Benedini^1,3, L., Gregori^1,3, D. A., Geraldes², M., Strazzere^1,3, L., Pavon Pivetta^1,3, M. C., and Costa do Santos^2,4 A.

¹ Departamento de Geología, Universidad Nacional del Sur, Av. Alem 1253, Bahía Blanca, Argentina, [email protected], [email protected], [email protected], [email protected], [email protected],

² Departamento de Mineralogia e Petrologia Ígnea, Universidad del Estado de Río de Janeiro, Brazil. [email protected]

³ Instituto Geológico del Sur, CONICET-Universidad Nacional del Sur

⁴Tektos and GeoAtlântico groups (UERJ)/Geobiotec, Departamento de Geociências, Universidade de Aveiro, 3810-193. ORCID: 0000-0003-2526-8620.

        Located in northwestern Patagonia, the Loncón Complex represents a pull-apart depocenter formed during the Upper Sinemurian syn-rift phase (Stremel et al., 2024). New whole rock geochemistry and zircon Lu–Hf isotopic data allow to partially constrain the geodynamic evolution of the Lower Jurassic back-arc volcanism at these latitudes (~ 41° S - 70° 05’ W). Characteristically, the volcanic rocks of the Complex, consist of subalkaline andesites and rhyolites, belonging to the calc-alkaline series. This magmatism is characterized by significant enrichment in light rare-earth elements and large-ion lithophile elements, as well as depletion in high field strength elements, and relatively high Nb/U, Ta/U, moderate La/Yb, and low Th/Ta and Sr/Y ratios.

       In addition, the εHf_(t) values complement the geodynamic scenario in terms of advancing and restraining orogens during the upper Sinemurian. In this sense, the related magmatism consists of a heterogeneous isotopic reservoir, showing negative and positive εHf (t) values (varying from - 2.76 to +22.1 and considering "t" age of 193.08 ± 1.13 Ma). The new data suggest limited crustal contamination during magma ascent in similar way with Barros et al., 2023. Our compiled data indicates that the rocks were derived from the partial melting of depleted and metasomatized mantles, which underwent the continental lithosphere together. The petrogenetic characteristics of the Loncon Complex are consistent with the slab break-off model, previously proposed by Navarrete et al., 2019, in which lithospheric stretching progressively limits the crustal participation in the magma reservoirs. This tectonic process and the related intraplate extension are considered slightly previous to the Chon Aike mantle plume episode.            

Barros, M., Pavón Pivetta, C., Gregori, D.A., Benedini, L., Strazzere, L., Geraldes, M.C., and Marcos, P., 2023, Early jurassic tectonomagmatic stages in the western North Patagonian Region, Argentina: Insights from the comallo volcanic sedimentary complex: International Geology Review, p. 1–25. doi:10.1080/00206814.2023.2225579 

Navarrete, C., Gianni, G., Encinas, A., Márquez, M., Kamerbeek, Y., Valle, N., Folguera, A., 2019a. Upper Triassic to Middle Jurassic geodynamic evolution of southwestern Gondwana: from a large flat-slab to a mantle plume suction in a rollback subduction setting. Earth Sci. Rev. 194, 125–159. https://doi.org/10.1016/ j.earscirev.2019.05.002.

Stremel, A. J., Benedini, L., Strazzere, L., Gregori, D.A., Bahía, M. E., Geraldes, M.C., Pavón Pivetta, M. C., Barros, M., Costa dos Santos, A. (2024): Geochronological and stratigraphical characterization of lower Jurassic tectonically-controlled felsic tuff ring volcanism in Northwestern Patagonia, Argentina, International Geology Review, DOI: 10.1080/00206814.2023.2301365