Geoquímica de feldespatos y biotitas del granito orbicular de la Sierra de Velasco, La Rioja, Argentina

GROSSE, P., ROSSI, J. & TOSELLI, A.

Buenos Aires

Congreso; VIII Congreso de Mineralogía y Metalogenia; 2006

The orbicular granite of the Sierra de Velasco is one of only two orbicular rocks described in Argentina. It forms an irregular body of around 65 x 15 m bounded by the Huaco Granite (GH) and consists of ovoid-shaped orbicules immersed in an aplitic to pegmatitic matrix.  The orbicules have an internal concentric structure consisting of a central K-feldspar megacryst, partially replaced by plagioclase, surrounded by shells of radially oriented plumose plagioclase and tangentially oriented biotite. Microprobe chemical analyses of plagioclase and biotite from the orbicules and the matrix were carried out. Plumose plagioclase shows inverse zoning from An9-14 to An18-20. This zoning can be explained by sudden undercooling (possibly due to pressure loss) followed by isothermal crystallization, which provokes non-equilibrium conditions and high growth but low nucleation rates. Biotites from different zones of the orbicules have similar compositions, supporting rapid isothermal crystallization conditions. The Fe/(Fe+Mg) ratio of the biotites increases from those of the GH, to those of the orbicules, to those of the orbicular matrix, suggesting increasing differentiation. Orbicular rocks are very rare because their formation seems to require the unlikely combination of various processes. The orbicular granite probably formed close to the roof of the GH.