Links between crustal melting, plate boundary forces, and syn-convergent exhumation in the Sierras Pampeanas, Argentina

Transitions from convergence to extension during an orogenic cycle result from the dynamic interaction between plate bounding forces, the thermal and rheologic evolution of the lithosphere, and contrasts in gravitational potential energy within an orogen. The presence of melt in the middle and lower crust, in particular, exerts a profound effect on the rheology of orogenic belts and in facilitating a change from convergence to extension and orogenic collapse. Determining whether or not melting was as an effective driving mechanism of extension within a given orogen requires accurately constraining the timing and duration of melting in the crust with respect to plate convergence, crustal thickening, and lithospheric extension. The Sierras Pampeanas of northwest Argentina record the transition from a Cambrian convergent margin to an Ordovician collisional orogen with the accretion of the allochthonous Precordillera terrane. Regional convergence associated with Famatina arc magmatism initiated as a result of east dipping subduction by ∼515-495 Ma and the majority of arc magmatism occurred from ∼485 to 465 Ma. Initial collision of the Precordillera terrane with the Famatina arc margin began by ~470 Ma and the terrane had fully collided by the Late Ordovician (∼458-449 Ma). Syn-convergent extension within the Sierra de Pie de Palo initiated at middle to lower crustal depths at ~436 Ma and continued through ~417 Ma. We present new U-Pb zircon and sphene, Lu-Hf garnet, and 40Ar/39Ar amphibole and mica ages and thermobarometry from lower crustal granulite facies migmatites of the Loma de Las Charcas. These data, coupled with existing regional isotopic ages and one-dimensional thermal modeling, suggest that: 1) regional peak granulite facies metamorphism occurred at ~465 Ma with near isothermal temperatures of ~850° C from ~5-12 kb within the Famatina arc; 2) Ordovician melts remained at temperatures above their solidus for 20-30 million years following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence; and 3) the onset of syn-convergent extension occurred after regional migmatites cooled beneath their solidus, and was synchronous with the establishment of a new plate margin to the west of the Precordillera terrane at ~436 Ma. The presence of large volumes of melt throughout the middle and lower crust was insufficient to drive extension during regional convergence and terrane accretion. Exhumation was likely the result of a change in the plate bounding forces.