DEFORMATION AND HIGH T-LOW P METAMORPHISM OF AN ACCRETIONARY PRISM SEQUENCE, CENTRAL ARGENTINA

The Tuclamè Formation in the northwest of the Sierras de Cordoba, central Argentina, is an impure meta-sandstone, presumed to be part of an accretionary prism derived from the margin of Gondwana in the Late Proterozoic/Early Cambrian. The Tuclamè Formation has been subjected to deformation and to high T-low P metamorphism. The first recognizable deformation and metamorphic event was diffusional mass transfer that resulted in a pressure solution cleavage. This was followed by upright folding with a shallow northerly plunge. Early-formed quartz veins were buckled along with the pressure solution seams. These structures are consistent with shortening above an orthogonal convergent plate boundary. Late in the deformation history, high T- low P metamorphism produced large porphyroblasts (~3 cm x 1.5 cm) of cordierite with some staurolite. Biotite grains inside the porphyroblasts show relict crenulations. The porphyroblasts grew after the folding event, coincident with a regional static metamorphic event in the middle Cambrian (ca. 520 Ma). The coexistence of cordierite and staurolite and the presence of Regime II recrystallized quartz grains constrains the temperature and pressure for the Tuclamè Formation metamorphism to 500-550° C and £ 3 kbars.

The matrix to the porphyroblasts has a least one strong biotite foliation and one recognizable muscovite foliation; these foliations wrap around the porphyroblasts, which indent each other in most samples, indicating continued compression and pressure solution after porphyroblast growth. Retrogression of the cordierite produced new muscovite with cooling ages that are ca. 450-477 Ma (⁴⁰Ar/³⁹Ar analyses by M. Krol), coincident with a major Ordovician (Famatinian, equivalent to Taconic) terrane emplacement event to the west of the Sierras de Cordoba.