Metamorphic P-T-d evolution path of ductile-sheared rocks of Cerro Catedral, North Patagonian Andes of Argentina: From high-P/T Late Paleozoic progression to low-P/T Jurassic overprint

Abstract

A petrological study, combined with field and microstructural evaluation of metapelites, metabasites and a tonalitic to quartz-dioritic rock of Cerro Catedral, North Patagonian Andes of Argentina, reveals the protracted polyphase deformational and metamorphic evolution of the area during the Late Paleozoic to Mesozoic. The 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H was used to model the bulk-rock composition of a mylonitic garnet- and andalusite-bearing micaschist. For this purpose, a P-T pseudosection was calculated using the GeoPs software and contoured with isopleths and mineral isomodes of interest. Refractory zoned garnet typically exhibits a prograde bell-shaped compositional zoning. Its core (gros + andr_0.29, spes_0.13, pyr_0.02, alm_0.55) to rim (gros + andr_0.22, spes_0.05, pyr_0.04, alm_0.69) compositions yield a prograde path from initial conditions of ∼9 kbar and 490–500 °C to ∼12 kbar and 510 °C. The remnant compositions (Si: 3.10–3.18 apfu) of K-white mica of the dominant S_2m-foliation support these physical conditions. The, thus, calculated low geothermal gradient of 11–14°Ckm⁻¹ suggests that the studied rocks were transported to a maximum depth of ∼45 km in a shallow paleosubduction zone. Microstructures of coalescent quartz ribbons might indicate a peak temperature around 650 °C after peak pressure conditions. These quartz ribbons of S_2m-mylonitic foliation are slightly wrapped around sigmoidal syn-kinematic plagioclase (Ab₇₆-Ab₈₃) containing a S_i-foliation. The outermost garnet rim (gros + andr_0.08, spes_0.07, pyr_0.05, alm_0.80) also grew syn-kinematically with the S_2m, and indicates a dominant tectonic exhumation into the middle crust (5.5 kbar ≈ 20 km) with a subsequent cooling to 550 °C. A randomly oriented mineral assemblage of andalusite and biotite indicates a subsequent low-pressure (<3 kbar) thermal overprint between 627 ± 24 °C and 664 ± 50 °C, calculated by Ti-in-biotite geothermometers, near the ductile-deformed tonalite pluton. High Y₂O₃ (>1.5 wt%) homogeneous monazite yielded a Middle Jurassic age (170 ± 4 Ma). Subsequent cooling produced large muscovite laths (Si: ∼3.0 apfu) in metapelites prior to the generation of upward open folds and a S₃-crenulation cleavage below 500 °C and late low-grade S-C mylonites. Microstructures in minerals indicate high-to low-temperatures solid-state deformation in the igneous and country rocks during the cooling phase. A final retrogression by chlorite and sericite overprinted the metapelites and the intrusive at < 400 °C. The deciphered ẞ-shaped P-T-d path provides constraints on the geotectonic evolution of this particular sector of the North Patagonian Andes, from a Gondwanan subduction setting, followed by exhumation with intense shearing and ending with a contact metamorphism in the Jurassic, prior to Andean orogeny.