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 + andr0.29, spes0.13, pyr0.02, alm0.55) to rim (gros + andr0.22, spes0.05, pyr0.04, alm0.69) 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 Kwhite mica of the dominant S2m-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 S2m-mylonitic foliation are slightly wrapped around sigmoidal syn-kinematic plagioclase (Ab76-Ab83) containing a Si-foliation. The outermost garnet rim (gros + andr0.08, spes0.07, pyr0.05, alm0.80) also grew syn-kinematically with the S2m, 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 byexhumation with intense shearing and ending with a contact metamorphism in the Jurassic, prior to Andean orogeny.