O magmatismo do Oligo-Mioceno na Província Magmática de Somuncurá, Argentina, e a relação com a subdução andina

Abstract

The alkaline complex of the Sierra de Apas, situated in the Somuncurá Magmatic Province of Northern Patagonia, Argentina, developed throughout the Oligocene-Miocene epoch. This dissertation comprehensively examines various methodologies employed to investigate the magmatic evolution of the Sierra de Apas. The research integrates petrographic, lithogeochemical, isotopic (Sr-Nd), and geochronological (U-Pb in apatite) analyses. The region is characterized by bimodal magmatism, comprising basaltic and trachytic lavas, succeeded by more evolved lithologies such as rhyolites. The U-Pb ages obtained suggest that volcanic events occurred, on average, approximately 25 million years ago, correlating with the development of calderas and magmatic collapses within the area. The volcanic succession is categorized into eleven volcanic elements, encompassing three principal evolutionary events: pre-collapse, syn-collapse, and post-collapse. Lithogeochemical data reveal SiO₂ concentrations ranging from 42 to 78 wt.%, alongside enrichments in incompatible elements such as Zr (600-2000 ppm) and Nb (50-80 ppm). Analyses of rare earth elements indicate elevated concentrations of Y and La, particularly in trachytic and commenditic rocks. The predominant influence of fractional crystallization is evidenced by a distinct separation between mafic and felsic magmas in Harker diagrams. Isotopic values exhibit variability, with ⁸⁷Sr/⁸⁶Sr(m) ranging from 0.703723 to 0.900385 and ¹⁴³Nd/¹⁴⁴Nd(m) spanning 0.512606 to 0.512894, indicating mixed contributions from a depleted mantle and crustal processes. Isotopic modeling reveals that trachytes and comendites display 87Sr/86Sr(i) values between 0.703690 and 0.767785, reflecting a pronounced degree of crustal contamination. The εNd(i) values ranged from -0.3 to +5.3, underscoring the influence of more juvenile mantle sources in certain samples. Geochemical modeling diagrams, such as Nb vs. La, indicate that fractional crystallization was the prevailing process, whereas the behavior of select trachytes and commendites suggests crustal assimilation. The magmatic evolution was also characterized by partial melting and mantle recharge processes, indicating a complex magmatic system with interactions between mantle and crust in a tectonically modified environment shaped by Andean subduction. In conclusion, this dissertation enhances the understanding of intraplate magmatism in Patagonia, elucidating the interplay between subduction and mantle plumes while providing novel insights into the evolution of alkaline magmas and their association with regional tectonic processes.