ND AND SR ISOTOPE GEOCHEMISTRY OF GRENVILLIAN MASSIF-TYPE ANORTHOSITE FROM THE WESTERN SIERRAS PAMPEANAS (ARGENTINA): MAGMA SOURCES AND GEOTECTONIC IMPLICATIONS.

CASQUET , C.; GALINDO, C.; RAPELA, C.W.; PANKHURST, R.J.; BALDO, E.G.; DAHLQUIST, J.A.; FANNING, C.M.; GONZÁLEZ CASADO, J.; SAAVEDRA, J.

Punta del Este, Rep. Oriental del Uruguay.

Simposio; V SOUTH AMERICAN SYMPOSIUM ON ISOTOPE GEOLOGY.; 2006

Facultad de Ciencias y Facultad de Agronomia. Universidad de la Republica Oriental del Uruguay.

Determined values of eNd1070 from the anorthosite massifs in Sierra de Maz vary between +3.4 y -1.2, mainly positive or slightly negative, which suggests a source impoverished in lithophile elements for the primary anorthosite parent magma. The negative values point to the possible presence of crustal components in the anorthosite, whereas the range implies a mixture between depleted mantle and continental crustal material enriched in lithophile elements. These conclusions are supported by the isotopic composition of Sr (87Sr/86Sr 1070 = 0.7041 – 0.7059) which are more radiogenic than that of Bulk Earth at the same age (=0.7032). Nevertheless, we do not observe a close correlation between the isotopic compositions of Sr and Nd such as might be expected. This could reflect isotopic heterogeneity in the depleted source and/or the crustal component, or even perhaps, post-crystallisation modification (e.g. during metamorphism). It is probably significant that the rocks with the highest K2O contents are those that show the highest values of eSr1070. Thus we suggest that metamorphism was the main cause of the observed dispersion of isotopic compositions. The rock with the most positive eNd1070 value (MAZ-7216), and consequently the least contaminated by crustal material, has a TDM of 1.18 Ga, close to the age of crystallization, which suggests that that depleted source was formed at essentially the same time as the anorthosite magma. These conclusions are compatible with a model of an anorogenic mantle plume following the Grenville orogeny, with which the initial break-up of Rodinia began. The 1106–1122 Ma large igneous province (LIP), recognised between the Kalahari and Laurentia cratons, could be genetically related to the anorthosite event.