Jurassic to Paleogene magmatic evolution of the Southern Central Andes (36–42º S) triggered by changes in tectonic style

Abstract

Arc magmatic evolution is influenced by various factors, including changes in tectonic style related to subduction. In the Southern Central Andes, tectonic conditions shifted from extensional/transtensional during the early Andean stage (Early Jurassic-Early Cretaceous) to compressive/transpressive during the Late Cretaceous followed by a short-lived extensional event in the Late Cretaceous–Paleogene. To better understand magmatic evolution in this region, we present new U-Pb zircon ages, REE and Hf isotope in zircon data, geochemical analyses, and Sr-Nd isotopic data from three intrusive and subvolcanic units: the Pre-Cuyo Group (Varvarco), the Naunauco Late Cretaceous-Paleogene Belt (Varvarco-Butalón-Andacollo and Naunauco-Colipilli), and the North Patagonian Batholith (Villa Pehuenia). In Varvarco (∼36.8°S), a Jurassic age of 184 ± 3 Ma was obtained for the Jurassic Host Granite, confirming Jurassic magmatism in the area. These rocks show (ƐNd)i of −0.80 and (ƐHf)i between + 0.31 and + 5.26. Late Cretaceous-Paleogene tonalites and associated dikes (ca. 67–65 Ma), including the Butalón Tonalite, show (ƐNd)i from −0.13 to + 1.09, and (ƐHf)i from + 1.00 to + 7.81. All rocks in the area yielded Mesoproterozoic–Neoproterozoic model ages. At Naunauco and Andacollo (∼37.7°S), Late Cretaceous-Paleogene volcanic rocks from the Colipilli and Cayanta formations yield (ƐNd)i values between + 1.89 and + 4.05 and Neoproterozoic model ages, indicating a more juvenile source. At Villa Pehuenia (∼38.9°S), the Jurassic Moquehue Granite (184 ± 2 Ma) is characterized by (ƐNd)i of + 0.88 and (ƐHf)i values between + 1.71 and + 4.98. The Paso de Icalma Granodiorite (71 ± 1 Ma) has (ƐNd)i of + 2.93 and (ƐHf)i between + 6.99 and + 9.13, with Meso- to Neoproterozoic model ages. Isotopic and geochemical data suggest that Early Jurassic-Early Cretaceous magmatism had a more crustal source signature, whereas Late Cretaceous–Paleogene magmatism involved more juvenile sources. U-Pb ages and literature data support a Late Cretaceous–Paleogene steady-state event at ∼67 Ma. These data reflect changes in the tectonic style of the subducted slab. During the Jurassic-Early Cretaceous, the dominant slab roll-back and slab tearing produced extension in the back-arc region and migration of magmatism from the coast of Chile eastwards, developing back-arc deposits in an extensional environment. In the Late Cretaceous-Paleogene, the decrease in the absolute velocity of the South American plate led to an extensional regime, while the development of a slab window associated with subduction of the Farallon–Aluk ridge, together with crustal thinning, contributed to the establishment and maintenance of steady-state magmatic conditions. Preliminary zircon REE data suggest porphyry-type base metal mineralization potential for the Moquehue Granite and Butalón Tonalite.