Interrelated pressure-temperature-time-paths of medium to high pressure metamorphic rocks in the Sierra Pie de Palo (W-Argentina): Evolution of a “hard” collisional wedge during an Ordovician microcontinent-arc collision

Abstract

The Sierra Pie de Palo (W-Argentina) is part of an Ordovician collisional wedge between the Cuyania microcontinent and the Ordovician Famatinian arc along the west Gondwana protomargin. The flatly E-dipping collisional wedge formed by polyphase shearing. The lower plate consists of Neoproterozoic-Cambrian metasedimentary rocks and Mesoproterozoic mafic-ultramafic basement of eastern Cuyania. The Famatinian forearc represents the upper plate composed of Mesoproterozoic metasedimentary rocks intruded by Mesoproterozoic and Ordovician plutons and overlying Neoproterozoic metasedimentary rocks. Pseudosection modeling on 20 samples distributed over the entire wedge yields a range of recorded maximum PT conditions of 7–14 kbar/480–590 °C along metamorphic gradients of 10–20 °C/km. Clockwise PT-paths are common starting from a midcrustal level with gradients of 20–30 °C to maximum depth followed by thermal relaxation. Local anticlockwise PT-paths are observed (1) in high-grade basement klippen of the overriding Famatinian arc (8–10 kbar/700–765 °C) caused by late emplacement onto the nappe stack and (2) within late upper crustal ductile zones with normal shear sense.Isotopic ages of (re)crystallisation of metamorphic minerals and assemblages comprise: 429 ± 2 to 434 ± 7 Ma (Lu-Hf; garnet), 460 ± 6 Ma (UPb monazite in garnet), 404 ± 7 to 422 ± 8 Ma (Rb-Sr; white mica) and 403 ± 4 to 440 ± 8 Ma (40Ar/39Ar white mica). Including previous data, ages cluster around major peaks at 464 ± 1 Ma, 436 ± 3 Ma, 427 ± 3 Ma, 415 ± 2 Ma and 405 ± 1 Ma related to deformation during three orogenic stages: (1) subduction of the lower plate (eastern Cuyania) to the basis of the wedge at 470–455 Ma and later burial of the upper plate (Famatinian forearc) to similar depths at 440–415 Ma, a characteristic of a “hard” collision; (2) exhumation mainly by erosion and (3) late dissection of the wedge by upper crustal thrusts and some normal faulting at the top of the wedge during arrival of the colliding microcontinent Chilenia (405–360 Ma) at the new Devonian leading margin.