MIDDLE ORDOVICIAN AND EARLY DEVONIAN METAMORPHIC AND DEFORMATION EVENTS IN THE LOMA DE LAS CHACRAS, SIERRAS PAMPEANAS, SAN JUAN PROVINCE, WESTERN ARGENTINA

The Loma de las Chacras in the Sierras Pampeanas, western Argentina, sits at the boundary between two major terranes: an Ordovician igneous complex, the Famatina arc, and the upper Cambrian-lower Ordovician carbonate platform of the Precordillera terrane. The timing and mechanism of the collision between the arc and continental block are greatly debated, with dates ranging from early Ordovician to middle Devonian, and emplacement by either dominantly convergent or translational processes. Our field and geochronologic study of the eastern margin of Loma de las Chacras finds evidence for multiple metamorphic and deformation events spanning at least 60 Ma. The dominant deformation is a 200 meter wide, NE trending, SE dipping mylonite-ultramylonite zone that separates two distinct structural packages: 1) the lower plate, a complexly deformed, high PT migmatitic gneiss complex and 2) the upper plate, a mylonitic package of lower grade metasediments. To determine relative metamorphic grades across the shear zone we conducted an electron microprobe study of porphyroblasts in mafic bodies from the three structural units. Upper-plate and shear zone HBL and GRT compositions record prograde metamorphic conditions, suggesting the shear zone juxtaposed hot lower plate rocks against the cooler upper plate. The break in metamorphic grade across the shear zone, and heating trend of the upper plate, combined with field evidence for top to the SE motion suggests the shear zone records an extensional event that unroofed the gneiss complex. SHRIMP analysis of zircons from a MUS orthogneiss in the heart of the shear zone produces two populations of zircon ages at 461+/- 5 Ma, and high U rims at 401 +/- 8 Ma. These orthogneiss dates represent either Devonian deformation of an Ordovician igneous body, or, less likely, locally derived melt injected into the shear zone with older ages representing inherited components. SHRIMP dates on zircons in a shear zone boudin neck record crystallization of locally derived melt at 405+/- 2Ma. ⁴⁰Ar/³⁹Ar step heating of shear zone muscovite yields a plateau age of 402.7 +/- 1.8Ma, suggesting extension occurred synchronously with melt generation in the lower plate.