EXHUMATION OF MODERATE PRESSURE (12 KBAR) KYANITE-STAUROLITE-BEARING SCHISTS DRIVEN BY CONTINENTAL SUBDUCTION DURING THE COMPRESSIONAL CLOSURE OF A BACK ARC BASIN IN THE PATAGONIAN ANDES

The Patagonian Andes record a history Cretaceous-Neogene orogenesis that began with the compressional inversion of a Jurassic back arc basin called the Rocas Verdes. This quasi-oceanic rift was the only one of a series of Jurassic back arc basins south of Ecuador that was floored by basaltic crust with mid-ocean ridge affinities. The southernmost part of the region includes rare kyanite-staurolite-bearing schists located north of the Beagle Channel (55° S latitude). Structural and metamorphic studies combined with U-Pb and U-Th-Pb dating of zircon and monazite, respectively, and ⁴⁰Ar-³⁹Ar thermochronology reveal a history of continental subduction in a back arc setting followed by the exhumation of rocks that were tectonically buried to depths of >35 km. ⁴⁰Ar-³⁹Ar ages on metamorphic muscovite indicate that back arc basin inversion began by ~98 Ma, forming a ~60 km wide thrust wedge between an active volcanic arc and the South American continent. U-Pb ages on zircon and cross cutting relationships with granitic plutons indicate that subduction of the basaltic floor of the back arc basin followed by continental subduction southward beneath the arc occurred prior to ~86 Ma. The transition from subducting basaltic crust to continental subduction appears to have caused a partial obduction of the basin floor. After ~86 Ma, continued shortening resulted in the uplift and exhumation of the kyanite-staurolite schists, which formed in underthrust continental crust. Pseudosection modelling (in NCKFMASHTO) of the schists suggest a P-T path dominated by decompression from 12 to 9 kbar at T≈620°C, coupled with garnet mode decreasing from 5% to less than 1%. U-Th-Pb dating of monazite indicates that exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed next to the youngest granite plutons include sillimanite-bearing migmatites formed at P≈6 kbar after ~72 Ma. Metamorphism induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back arc basin, was terminated by thrusting-controlled exhumation with the rocks at P≈9 kbar by ~73 Ma and 6 kbar by ~70 Ma. Final exhumation after the Neogene occurred by transtensional faulting related to the present-day South America-Scotia plate boundary.