The link between collisional tectonics and arc magmatism in the Paleozoic Famatina arc, Argentina

Abstract:

Studies of the Famatina margin of northwest Argentina highlight the challenges in understanding the links between Cordilleran arc processes, terrane accretion, and lower crustal deformation, metamorphism, and magmatism. The evolution of the Famatina arc suggests that regional convergence and lower crustal shortening drove voluminous arc magmatism during terrane accretion, pervasive melting of the middle and lower crust did not result in regional extension, and the cessation in arc magmatism and late syn-convergent extension coincided with the end of terrane accretion and a change in the plate margin. The Famatina arc extends more than 1800 km along strike and records the transition from a Cambrian convergent margin to an Ordovician collisional orogen with the accretion of the allochthonous Precordillera terrane. The Famatina arc was established on mid-Proterozoic basement following the onset of regional convergence and east-dipping subduction by ~515-495 Ma. The peak in arc magmatism in the central portion of the Famatina arc occurred from ~485-465 Ma and was coeval with the initial collision of the Precordillera terrane, major crustal shortening accommodated along a series of oblique lower-crustal shear zones, and back arc shortening. Steeply dipping reverse-sense ductile fabrics within the arc have hornblende cooling ages of >445 Ma and suggest that voluminous arc magmatism was concurrent with intra-arc shortening. Intrusion of mafic and intermediate magmas during the peak in arc magmatism lead to widespread granulite facies metamorphism of the middle and lower arc crust from ~470-465 Ma. Pelitic rocks melted in this event remained above the solidus for ~20-30 Ma, throughout the episode of peak convergence. Pressure-temperature paths from middle and lower crustal migmatites suggest that cooling from peak conditions was near isobaric. Decreased convergence associated with the end of Precordillera terrane accretion at ~436 Ma facilitated the onset of syn-convergent extension, cooling and exhumation from granulite facies conditions, and the termination of Famatina arc magmatism.