Alejandro J. Toselli

Petrology and geochemistry of the orbicular granitoid of Sierra de Velasco (NW Argentina) and implications for the origin of orbicular rocks

The Velasco orbicular granitoid is a small (65 × 15 m), irregularly-shaped body that crops out within the Huaco granite, central Sierra de Velasco, NW Argentina. It consists of ellipsoid-shaped orbicules of 3 to 15 cm length immersed in an aplitic to pegmatitic matrix. The orbicules are formed by a core made up of a K-feldspar megacryst, partially to totally replaced by plagioclase, an inner shell of radial and equant plagioclase crystals, a layer of tangentially oriented biotite laths, and an outer shell of plumose plagioclase crystals, containing diffuse rings of tangentially oriented biotite. The orbicular granitoid formed in situ in a pocket of evolved and volatile-rich melt segregated from the surrounding partially crystallized Huaco granite, possibly via a filter pressing mechanism. The segregated melt entrained relatively few K-feldspar megacrysts into the pocket, leaving behind a concentration of megacrysts around the pocket. High water concentration caused effective superheating of the melt and destruction of nuclei, with only the large megacrysts surviving as solids. Sudden water-pressure loss and exsolution of the volatile phase, perhaps related to a volcanic eruption or fracturing of the surrounding granite, caused rapid undercooling of the melt. The orbicules grew in the undercooled melt by heterogeneous nucleation on the megacrysts, which acted as nucleation seeds, and crystallization of reversely zoned radial plagioclase and sporadic crystallization of tangential biotite rings according to fluctuations in its saturation. Orbicular growth gave way to crystallization of the equiaxial inter-orbicular matrix in two stages, when sufficient polymerization of the melt was attained. The time scale of formation of the orbicular granitoid was fast, possibly a matter of a few weeks or months.