Counterclockwise P-T-t path, cyclic crustal anatexis and magmatic tempo in accretionary orogens: A case study in the Devonian arc of Central Patagonia, Argentina

Abstract

An intriguing aspect of accretionary orogen dynamics is the crustal growth of the West Gondwana margin throughout the Terra Australis Orogeny, particularly regarding the development of the Patagonian crust and whether it involved periods of collisional or non-collisional orogenesis. This study examines the Devonian tectono-thermal evolution of the western North Patagonian Massif (southern South America), through the investigation of high-grade metamorphic rocks associated with the Devonian arc of Central Patagonia. Phase equilibrium modelling and geothermobarometry indicate an initial MP-HT metamorphic event (M1) under contractional conditions at the amphibolite- to granulite-facies transition, with metamorphic conditions of 6.7 ± 1 Kbar and ∼ 760 °C, previously dated at 379 ± 5 Ma. Successive crustal thickening and associated anatexis are recorded by metamorphic conditions at 8.9 ± 1 Kbar and ∼ 750 °C. Zircon U-Pb geochronology indicates leucosome crystallization at 360 ± 2 Ma, while an associated megacrystic granite records a 368 ± 2 Ma crystallization age, suggesting it represents an earlier magmatic pulse within the prolonged anatectic scenario. Decompression and cooling led to zircon crystallization near the solidus, marking an MP-MT (M₂) metamorphic event at amphibolite facies conditions  (∼ 6.5 Kbar and 620 °C), recorded by zircon ages of 353 ± 2 Ma. A final retrograde metamorphic event (M₃) is recorded at ∼ 660 °C and ∼ 4.6 Kbar with monazite ages of 323 ± 5 Ma. This event is linked to decompression and intrusion of gondwanide magmatism. These metamorphic conditions reveals an overall counterclockwise P-T-t path for the Taquetren range from the Devonian to Carboniferous. Combined with the study of magmatic addition rates in the arc and forearc during the Devonian, this pattern supports an accretionary orogen model characterized by alternating periods of advancing and retreating subduction, potentially driven by trenchward and landward slab folding, which may account for short-cycle variation (∼15 Myr). These dynamic slab movements may have influenced variations in magmatic activity in the forearc and arc, coupled with shifts in metamorphic conditions and tectonic regimes.