METAMORPHIC EVOLUTION OF THE SIERRA DE MAZ: IMPLICATIONS FOR THE TIMING OF TERRANE ACCRETION ON THE WESTERN MARGIN OF GONDWANA

The Mesoproterozoic MARA terrane of South America is separated from the Paleozoic margin of Argentina by the present-day Valle Fertil fault zone. The accretion history of the MARA terrane to Gondwana and the evolution of the Valle Fertil fault zone remain uncertain. We present multi-equilibrium thermobarometry and garnet geochronology to constrain the metamorphic and tectonic evolution of the MARA terrane. The Maz shear zone in Sierra de Maz separates units with distinct metamorphic histories. Grt-Pl-Bt-Ms±Chl±Amp metapelites to the west preserve a single phase of prograde garnet growth at 564 ± 91 ˚C and 8.2 ± 1.4 kbar. A post-tectonic garnet Lu-Hf age dates amphibolite facies metamorphism at 423 ± 10 Ma (2σ). Within the shear zone, Grt-Pl-St-Bt-Ms metapelites were metamorphosed at 653 ± 27 ˚C and 7.7 ± 1.1 kbar. A pre-tectonic garnet age of 430 ± 5 Ma overlaps with a shear zone titanite age of 433 ± 9 Ma. Syn- to post-tectonic garnet in amphibolite east of the Maz shear zone record an age of 425 ± 10 Ma. Staurolite zone rocks to the east have multiple garnet populations and produce mixed Lu-Hf isochrons that range in age from 760 to 600 Ma. Along the eastern margin of the Sierra de Maz, Grt-Kfs-Sil-Bt metapelites preserve a single phase of garnet growth and were metamorphosed at 876 ± 56 ˚C and 10.3 ± 1.6 kbar. A garnet Lu-Hf age from this assemblage dates granulite facies conditions at 1050 ± 46 Ma. In contrast, a Sm-Nd garnet age from the same sample of 629 ± 22 Ma may record an unrecognized metamorphic event. The Sierras de Ramaditas preserve Grt-Sil-Bt±Crd metapelites with a single phase of garnet growth and were metamorphosed at 856 ± 160 ˚C and 5.5 ± 1.5 kbar. A garnet Lu-Hf age from this assemblage dates granulite facies conditions at 452 ± 14 Ma. The Sierra de Maz preserves two dominant metamorphic events at 1.2-1.0 Ga and 430-420 Ma and Ordovician garnet ages associated with the Famatina arc (470-460 Ma) are currently absent. Pre-, syn-, and post-tectonic garnet ages date the onset of deformation in the Maz shear at 430-425 Ma associated with sinistral transpression in the Valle Fertil fault zone. Granulite facies assemblages in the Sierras de Maz and Ramaditas have different pressures and ages suggesting the two regions preserve different metamorphic, and possibly, tectonic histories.